Bacillariophyta

Diatome of kieselwiere (klas Bacillariophyceae) is eensellige wiere wat gewoonlik 'n geelbruin tot groengeel kleur het. Die wier se kieselagtige selwand bestaan uit twee helftes wat op mekaar pas soos 'n deksel op 'n pildosie. As die selinhoud van die diatoom doodgaan, sink die diatoomskelet na die meer- of seebodem. Deur die eeue heen het daar op hierdie manier 'n baie dik laag diatoomgrond op die seebodem gevorm. Tans word meer as 10 000 soorte diatome onderskei, wat nie net in die see en in varswater voorkom nie, maar ook in vogtige plekke op land.

Bou

Die sowat 10 000 diatome kan van alle ander eensellige wiere onderskei word op grond van hul tweedelige selwand. Die boonste doppie van die selwand pas soos 'n deksel op die ietwat kleiner onderste doppie. Die twee helftes van die selwand word deur 'n bandvormige ring, die gordel, bymekaar gehou. Die harde, kieselagtige wand van die diatoom bevat geen kalkbestanddele soos dikwels met die harde dele van ander organismes die geval is nie, maar wel silikonverbindings, soos kieselsure en silikaat.

Daar word twee ordes diatome onderskei, naamlik die ronde Centrales en die ellips- of bootvormige Pennales. Die Centrales het dikwels 'n growwe oppervlak met fyn groefies. Daarbenewens het die meeste soorte Centrales lang hare of stekelhare, wat die dryfvermoë van die sel verbeter. Die Pennales het 'n gladde selwand met 'n veervormige patroon van porieë. In die sitoplasma van die sel is korrel- of plaatvormige kleurstofdraers (chromatofore) waarin daar chlorofiel en ook geel en bruin pigmente (karoteen, karotenoïed en xantofil) voorkom. Die chromatofore gee aan die diatoom 'n kenmerkende bruingeel tot groengeel kleur.

Voorkoms

Diatome word oral op aarde aangetref. Die meeste soorte lewe in die see of in varswater, maar ook in 'n verskeidenheid van vogtige woonplekke op land, soos byvoorbeeld in boomstamme of onder klippe. Die meeste Centrales lewe in die see, waar hulle 'n belangrike bestanddeel van die plantaardige plankton vorm en 'n groot rol in suurstofproduksie speel. Daarenteen is die Pennales hoofsaaklik varswaterbewoners. Hulle kom gewoonlik voor in kettingvormige of vertakte kolonies wat aan die een of ander bodem vasgeheg is. Die selle word aan mekaar vasgeheg met behulp van 'n klewerige stof wat deur die sitoplasma afgeskei word.

Voortplanting

Die diatome plant gewoonlik voort deur middel van seldeling. Met die seldeling erf albei dogterselle een van die doppies van die moedersel se tweedelige wand en bou dan self die ander helfte om daarby aan te pas. Die doppies wat van die moedersel verkry word, vorm altyd die deksel van die nuwe dogtersel. Die onderste deel word altyd self deur die dogtersel gevorm. Dit lei daartoe dat die dogtersel wat die grootste doppie (die moederseldeksel) erf, altyd so groot soos die moedersel word, terwyl die ander dogtersel baie kleiner is. Hierdie proses van seldeling duur 'n paar generasies voort, en met elke seldeling ontstaan 'n klein en 'n groot nakomeling. Later is die kleinste selle só klein dat hulle nie meer lewensvatbaar is nie. In sulke gevalle vorm die diatoom spore (ouksospore).

By die Centrales deel die selkern 'n paar keer; die protoplasma wring hom uit die sel na buite en neem normale afmetinge aan. Hierná word twee volledige nuwe doppies gevorm.

In die geval van die Pennales word geslagselle gevorm. Hierdie geslagselle verlaat die oorspronklike doppies en versmelt met mekaar. Die sigote hou aan met hulle groei totdat hulle volwassenheid bereik het en kort daarna deel hulle en vorm nuwe doppies. In ongunstige omstandighede vorm die selle russpore, wat weer sal ontwikkel as die sel se lewensomstandighede verbeter.

Nut

Die diatome speel nie net 'n belangrike rol in suurstofvoorsiening nie, maar ook in voedselverskaffing vir planktondiertjies, soos vislarwes en mikroklein krefies.

Wanneer die selinhoud van 'n diatoom doodgaan, of die ou doppies verlaat, sink die diatoomskelet na die bodem van die see of die meer. As gevolg hiervan het daar deur die eeue heen 'n dik laag diatoomgrond op die seebodem gevorm. Hierdie diatoomgrond, wat dikwels kieselguhr genoem word, is van groot belang vir die nywerheid. Nadat Alfred Nobel dinamiet uitgevind het, het hy diatoomgrond gebruik om die gevaarlike nitrogliserien te absorbeer. Dit het die waarskynlikheid van onbeheerde ontploffings baie verminder. Diatoomgrond is vroeër baie gebruik as 'n poleermiddel in tandepasta.

Bronnelys

• Wêreldspektrum, Vol. 5, 30-31, ISBN 0 908409 46 X

Les diatomees (taxón Diatomea, Diatomeae o Bacillariophyceae sensu llato), ye un grupu d'algues unicelulares que constitúi un de los tipos más comunes de fitoplancton. Contién anguaño unes 20.000 especies vives que son importantes productores dientro de la cadena alimenticia.^[3] Munches diatomees son unicelulares, anque dalgunes d'elles pueden esistir como colonies en forma de filamentos o cintes (y.g. Fragillaria), abanicos (y.g. Meridion), zigzags (y.g. Tabellaria) o colonies estrellaes (y.g. Asterionella). Una carauterística especial d'esti tipu d'algues ye que se topen arrodiaes por una paré celular única fecha de xil opalino (dióxidu de siliciu hidratao) llamada frústula. Estes frústules amuesen una amplia variedá na so forma, pero xeneralmente consisten en dos partes asimétriques con una división ente elles, carauterística que-y da nome al grupu. La evidencia fósil suxer que les diatomees aniciáronse mientres o dempués del periodu Xurásicu inferior, anque los primeros restos corpóreos son del Paleógeno. Les comunidaes de diatomees son una ferramienta usada recurrentemente pa la vixilancia de les condiciones medioambientales, de la calidá de l'agua y nel estudiu de los cambeos climáticos.

Ecoloxía

Diatomees al microscopiu

Les diatomees son organismos fotosintetizadores que formen parte del plancton (fitoplancton). Anguaño conócense más de 200 xéneros de diatomees, 20.000 especies vives y envalórase qu'hai alrodíu de 100.000 especies extintes.^[4][5] Como colonizadores, les diatomees destremanse por atopase en cualesquier tipu d'ambiente yá seya marín, dulceacuícola, terrestre o tamién sobre superficies húmedes. Otres atópense n'ambientes onde esisten condiciones estremes de temperatura o salín y d'igual forma atopamosles interautuando con otros organismos como ye'l casu coles cianofícees filamentoses onde esiste un epifitismo per parte de les diatomees. La mayoría son peláxiques (viven n'agües llibres), dalgunes son bentóniques (sobre'l fondu marín), ya inclusive otres viven so condiciones de mugor atmosférico. Son especialmente importantes nos océanos, onde se calcula qu'apurren hasta un 45 % del total de la producción primaria oceánica.^[6] La distribución espacial del fitoplancton marín ye acutada tantu horizontal como verticalmente. Les diatomees viven en tolos océanos dende los polos hasta los trópicos; les rexones polar y subpolar contienen relativamente poques especies en contraste cola biota templada.^[7] Anque les rexones tropicales esiben la mayor cantidá d'especies, les mayores poblaciones de diatomees alcuentrense ente les rexones polar y templao.

Carauterístiques

Imáxenes al microscopiu electrónicu de barríu: A Biddulphia reticulata, B Diploneis, C Eupodiscus radiatus, D Melosira varians.

Terpsinoe musica vista nel microscopiu ópticu

Les diatomees pertenecen a un gran grupu llamáu Heterokontophyta, que inclúi especies tantu autótrofes (y.g algues pardes) como heterótrofes (y.g. oomicetos). Los cloroplastos mariellu-marrones de les diatomees son típicos de los heteroncontos, con cuatro membranes, clorofiles a, c1 y c2 y pigmentos tales como β-caroteno, fucoxantina, diatoxantina y diadinoxantina. Les reserves d'alimentu almacénense como carbohidratos o aceites, qu'amás de sirvir de reserva, contribúin a la so flotabilidá.

Los sos individuos usualmente escarecen de flaxelu, pero tán presentes en gametos y usualmente presenten una estructura heteroconta, sacante en qu'escarecen de vellosidaes (mastigonemas) carauterísticos d'otros grupos. Munches diatomees nun tienen movimientu, anque delles otres tienen movimientu flaxeláu. Por cuenta de la so relativamente pesada paré celular afondien con facilidá, les formes planctónices n'agües abiertes polo xeneral dependen de la turbulencia oceánica producida pol vientu nes capes cimeres pa caltenese suspendíes nes agües superficiales allumaes pol Sol. Delles especies regulen viviegamente la so flotabilidá colos lípidos intracelulares pa faer frente al fundimientu.

A pesar de ser xeneralmente microscópiques, delles especies de diatomees pueden algamar los 2 milímetros de llargor. Les diatomees tán conteníes dientro d'una única paré celular de silicatu (frústula) compuesta de dos valves separaes. El xil biogénica de la que la paré celular componse ye sintetizada intracelularmente por polimerización de monómeros d'ácidu silícico. Esti material ye depués secretado escontra l'esterior de la célula onde participa na conformanza de la paré celular. Les cáscares de les diatomees se superponen una a otra como los dos metaes d'una placa de Petri (esto ye, como una caxa y la so tapadoria). El frústulo apaez delicadamente afatáu con relieves que formen dibuxos variaos y perfeutamente simétricos. Dando llugar a dos tipos de diatomees, les que tienen simetría radial y les de simetría llateral. Los frústulos de les diatomees posense por gravedá cuando ye dixerida o muerre la célula, dando orixe a roques sedimentaries como les diatomites y moronites.

Reproducción

Ciclu de vida d'una diatomea céntrica (oogamia)

Ciclu de vida d'una diatomea pennada (isogamia morfolóxica, anisogamia fisiolóxica)

Les diatomees alternen ente la reproducción asexual por división celular, que ye la más frecuente, y la reproducción sexual.^[8] Na reproducción por división celular, cada célula fía recibe una de les frústules de la célula padre y utilizala como frústula mayor (o epiteca), reconstruyendo una frústula menor (o hipoteca). De resultes d'esti procesu, la célula que recibió la frústula menor resulta nuna diatomea de menor tamañu que la orixinal. Per otra parte, la frústula nun pudi crecer. D'esta forma, na primer xeneración el 50% de les diatomees son de menor tamañu que la orixinal y vanse faciendo cada vez más pequeñes en cada xeneración. Reparóse sicasí, que delles especies pueden estremase ensin causar un amenorgamientu nel tamañu de célula.^[9] El procesu sigue hasta que les célules algamen una tercer parte del so tamañu máximu.^[10] Dempués d'un determináu númberu de xeneraciones, les diatomees reproducir de forma sexual, produciendo gametos ensin frústules que se funden formando una auxospora. Esti mecanismu ayuda a restablecer el tamañu orixinal de les diatomees porque'l cigotu crez muncho primero de producir una nueva frústula.

Les célules vexetatives de les diatomees son diploides (2N) y la meiosis xenera gametos machos y fema qu'entós se funden pa formar el cigotu. El cigotu lliberase de la so cubierta y crez na forma d'una célula esférica cubierta por una membrana orgánica (auxospora). Cuando la auxospora algama la so midida máxima (la de diatomea inicial), forma nel so interior a una diatomea que da empiezu a una nueva xeneración. Tamién pueden formase espores como respuesta a condiciones medioambientales desfavorables, produciéndose la guañada cuándo les condiciones ameyoren.^[11]

Les diatomees son mayoritariamente non móviles, anque la espelma de delles especies puede ser flaxeláu, con movimientu de normal llindáu al deslizamiento.^[12] Nes diatomees céntriques, los gametos machos son pequeños y tienen un flaxelu, ente que los gametos fema son grandes ya inmóviles (oogamia). Otra manera, nes diatomees pennades dambos gametos escarecen de flaxelos (isogamia).^[10] Haise documentáu qu'una especie del grupu de les diatomees pennades ensin rafe ye anisógama y, por tanto, considerase una etapa transicional ente les diatomees céntriques y les diatomees pennades con rafe.^[13]

Clasificación y filoxenia

Diatomea pennada y diatomea centrada. Dibuxos d'Ernst Haeckel.

Les diatomees, Diatomeae (Dumortier, 1821) o Bacillariophyceae (Haeckel, 1878), son clasificaes según la distribución de los sos poros y ornamentación. Si les frústules tienen una simetría radial o trímera nomense diatomees centraes, ente que si tienen una simetría billateral y forma allargada nomenanse pennadas. El primer tipu ye parafiléticu con respectu al segundu. Delles diatomees pennades tamién presenten una fisura a lo llargo de la exa llonxitudinal, denominada rafe, que ta implicada nos movimientos realizaos pola diatomea. Al grupu Bolidophyceae, apocayá descubiertu, considérase-y una clase estreme. La clasificación más recién considera cuatro grupos de diatomees:^[14]

Coscinodiscophyceae (P): Centrales radiales.

Mediophyceae (P) : Centrales polares.

Fragilariophyceae (P): Pennales sin rafe.

Bacillariophyceae: Pennales con rafe.

Aplicaciones

La tierra diatomea ye un material constituyíu poles frústules de diatomees fosilizaes, aplicáu como fertilizante ya inseuticida en tierres pa cultivu, al ser un productu natural, ye inocuo y nun presenta riesgos pa la salú o contaminación. La tierra diatomea aprove micronutrientes al suelu que son de gran importancia pa la crecedera de les plantes, pudiendo amontar la fertilidá del suelu, actuando sinérgicamente con calciu y magnesiu, amás amenorga la lixiviación de fósforu, nitróxenu y potasiu y favorez la so absorción nes plantes. La tierra diatomea tamién actúa como reconstituyente en tierres contaminaes por metales pesaos o hidrocarburos, amás neutraliza la tosicidá del aluminiu en suelos acedos y amenorga l'absorción de fierro y manganesu.^[15]

Les propiedaes d'esos materiales, formaos por partícules microscópiques, entrevesgaes y bien regulares en tamañu, facenlos curiosos pa diversos usos, como la fabricación de la dinamita, onde la nitroglicerina ye enfiñida, amenorgando la probabilidá d'una esplosión accidental.

El polvu de diatomees emplégase como inseuticida n'animales y plantes. Actúa deshidratando a inseutos hasta matalos, amás curtia y fura el exoesqueleto, mancándolos y esaniciándolos de forma progresiva y efeutiva. Les frústules de les diatomees son d'orixe natural, polo cual son inocues p'animales y plantes. A diferencia d'inseuticides tóxicos, el polvu de diatomees nun puede ingresar nos texíos animales por cuenta del so tamañu.^[16]

Les diatomees pertenecen a les microalges oleaxinoses por cuenta de que presenten fracciones lipídices del 25 % (condiciones normales) al 45 % (condiciones de estrés), cultivables en fotobioreautores (FBR). La producción de biodiésel a partir de diatomees dase por mediu de transesterificación del aceite preveniente de les microalgas. La producción de biodiésel basase na producción y captación de biomasa de diatomees, que ye deshidratada y sometida a ultrasoníos por que llibere los sos componentes, darréu los lípidos son dixebraos de carbohidratos y proteínes. L'aceite llográu ye sometíu a transesterificación alcalina, aceda o enzimática pa producir glicerol y biodiésel.

Los aceites provenientes de diatomees son principalmente triglicéridos, que xeneren amiestos de ésteres d'arriendo al convertise en biocombustible. El cracking térmicu o pirólisis, ye un procesu alternu a la transesterificación que tresforma triglicéridos n'otros compuestos orgánicos simples.^[17]

Determinóse que les diatomees tienen la capacidá de producir ácidos grasos poliinsaturados (AGPI) n'altes concentraciones, como por casu la producción de diatomees del xéneru Nitzschia d'ácidu eicosapentanoico (GUO). Nitzschia tien ventayes como la resistencia a temperatures d'hasta -6 °C y ambientes altamente salobres, amás el so aceite algama'l 50 % del pesu secu de la biomasa.^[18]

Les comunidaes de diatomees pueden utilizase pa la determinación de les condiciones ambientales tantu de presente como del pasáu y del cambéu climáticu. Tamién pueden utilizase pa determinar la calidá de l'agua.

La demostración de la presencia de Diatomees dientro de los cuévanos cardiacos o nel migollu óseu de cadabres ye utilizada en melicina forense como fuerte niciu de muerte por sumersión (afogamientu).^[19]

Galería

Centralesː

• Biddulphia pulchella

• Skeltomema potamos

• Cocconeis krammerii

• Melosira sp.

• Cyclotella meneghiniana

• Triceratium dubium

• Aulacodiscus

Pennalesː

• Amphoraovalis

• Phaeodactylum tricornutum

• Surirela sublinearis

• Cymbella sp.

• Licmophora sp.

Referencies

Enllaces esternos

Diatom yosunlar (lat. Bacillariophyta). Bu tipə tək, nadir hallarda qonur rəngli (piqmenti - fukoksantin) mikroskopik (0,75 mkm – 2 mm) kolonial orqanizmlər daxildir. Diatom yosunlar planktonda geniş, daha az miqdarda dəniz və şorsu hövzələrinin bentosunda, bəziləri torpaqda məskunlaşırlar; müasir diatom yosunlar qütb dairəsinə yaxın və əsasən arktik hövzələrdə yayılmışdır. Hüceyrə ikitaylı silisium qabıq içərisində yerləşir, qabıq iki elementdən ibarətdir; bunlardan kiçiyi qapaq kimi həmişə digər tayı örtür. Sonuncunun içərisində dairəvi lövhə (tayın özülü) və kəmərcik (yan tərəflər) ayrılır.

Lövhənin formasına görə diatom yosunlar tipi iki sinifə ayrılır: pennatlar və sentriklər. Pennat diatom yosunlar ikitərəfli simmetriyalı olub, formaları uzunsov oval (yumurtavari), iynəşəkilli olur, adətən, qabığın ortasında yarıqşəkilli dəlik (tikiş xətti) yerləşir. Sentrik diatom yosunlar radial simmetriya ilə səciyyələnir, for­maca dairəvi, üçbucaq, ulduz şəkillidir. Bü­tün diatomeaların kəmərcik tərəfdən gö­rü­nüşü çubuqşəkillidir (uzunsov-düzbu­caq­lı) ki, bu da tipin ikinci adını – Bacillariop­hyta (lat. bacillus - çubuq) təyin etmişdir. Qabığın taylarında çoxsaylı hamar və ya qabarıq nazik haşiyəli iri və xırda dəliklər vardır.

Diatom yosunlar 300-ə qədər cinsi və 1200-dən çox müasir və fossil növləri məlumdur. Cinsi və qeyri-cinsi yolla artırlar. Süxur əmələgətirən orqanizmlərdir; bitkilər məhv olduqdan sonra dibə çökərək (çox vaxt bir-birindən ayrılmış tayalar şəklində) silisium lillərini, qazıntı halında isə silisium süxurlarını (diatomitlər, trepellər, opokalar və b.) əmələ gətirirlər. Diatom yosunlar - sentriklər təbaşirdə (yura tapıntıları haqqında məlumatlar mübahisəlidir), pennatlar isə paleogendə meydana gəlmişdir; orta paleogenin şirinsu hövzələrində yaşamağa uyğunlaşmışdır, buna qədər isə təmamilə dənizlərdə məskunlaşmışlar. Cavan çöküntülərin (neogendən başlayaraq) zonal bölgülərinin əsaslandırılmasında diatom yosunlar bö­yük əhəmiyyəti vardır. Fatsiya göstəricisi (göl, çay və b.) kimi diatom yosunlar olduqca əhəmiyyətlidir. Təbaşir - müasir.

Xəzərdə 292 növü qeyd edilmişdir, onlardan planktonda 165 növ, bentosda isə 129 növü yaşayır. Chaetoceros, Thalassiosira,Coscinodiscus və Rhizosolenia cinsləri Xəzərin planktonunda və fitoplanktonunda mühüm rol oynayır.

Sıraları

• Thalassiosirales
• Coscinodiscales
• Melosirales
• Chaetocerotales
• Fragilariales
• Tabellariales
• Achnanthales
• Cymbellales
• Naviculales
• Bacillariales
• Surirellales

Həmçinin bax

• Kolonial orqanizmlər
• Plankton
• Bentos
• Bitkilər
• Təbaşir dövrü
• Yura dövrü
• Paleogen dovrü
• Neogen dövrü

Mənbə

Geologiya terminlərinin izahlı lüğəti. — Bakı: Nafta-Press, 2006. — Səhifələrin sayı: 679.

Les diatomees (Bacillariophyceae) són una classe d'algues unicel·lulars microscòpiques (encara que n'existeixen algunes que formen colònies), que s'enquadra dintre del fílum Heterokontophyta, superfílum Chromista, regne protoctist, domini Eukarya. El nom científic de la classe és Bacillariophyceae (o Diatomeae) i es relaciona filogenèticament amb la classe Chrysophyceae i d'altres del grup Chromista.

Les diatomees són organismes fotosintetitzadors que viuen en aigua dolça o marina i constitueixen una part molt important del fitoplàncton.

Un dels trets característics de les cèl·lules de diatomees és la presència d'una coberta de sílice (diòxid de silici hidratat), anomenat frústul. Els frústuls mostren una gran diversitat de formes, algunes de molt belles i ornamentades, i generalment consten de dues parts asimètriques o valves amb una divisió entre si, d'aquí el nom del grup. Moltes espècies apareixen formant encadenaments o altres agregats ordenats. L'evidència fòssil suggereix que es van originar durant o abans del període Juràssic primerenc.

Ecologia

Actualment, es coneixen més de 200 gèneres vivents de diatomees i s'estima que hi ha al voltant de 100.000 espècies extintes (%[1] %[2]). Com a colonitzadores, les diatomees es distingeixen per trobar-se en qualsevol tipus d'ambient, ja sigui marí o d'aigua dolça. També es troben en ambients on existeixen condicions extremes de temperatura o salinitat i, de la mateixa forma, les trobem interaccionant amb altres organismes com és el cas de cianofícies filamentoses, en què existeix un epifitisme per part de les diatomees.

La majoria són pelàgiques (viuen en aigües lliures), encara que algunes són bentòniques (sobre el fons marí), o fins i tot poden viure sobre superfícies humides. Són especialment importants en els oceans, on es calcula que proporcionen fins a un 45% del total de la producció primària oceànica (%[3]). Encara que són generalment microscòpiques, algunes espècies de diatomees poden arribar fins a 2 mil·límetres de longitud. Els frústuls de les diatomees se sedimenten per gravetat quan és digerida o mor la cèl·lula, i donen origen a roques sedimentàries, com les diatomites i moronites.

Usos

Les propietats d'aquests materials, formats per partícules microscòpiques, intricades i molt regulars en grandària, els han fets atractius per a diversos usos, com la fabricació de la dinamita, en què la nitroglicerina és embeguda, reduint la probabilitat d'una explosió accidental.

Els tàxons més rics en lípids es fan servir per desenvolupar el biocombustible d'alga.

Galeria

• Amphora ovalis

• Biddulphia pulchella

• Cocconeis krammerii

• Cyclotella meneghiniana

• Phaeodactylum tricornutum

• Skeletonema potamos

• Surirella sublinearis

• Triceratium dubium

• Diatomees. Microscopi òptic. 300X. Mètode de la gota penjant

• 

Phaeodactylum tricornutum

Phaeodactylum tricornutum és una alga diatomea i és l'única espècie del gènere Phaeodactylum. Aquesta alga adopta diferents tipus morfològics, fet que la fa útil per a estudiar alteracions en la forma cel·lular, que poden ser estimulats per canvis en les condicions ambientals. A més a més, pot créixer en absència de silicona, fet que proporciona noves eines en el camp de la fabricació de nanoinstruments de silicona.

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• Fragilariopsis cylindrus.

Rozsivky (Diatomeae, syn Bacillariophyceae) jsou velkou skupinou jednobuněčných fotosyntetizujících organismů s dvojdílnou křemičitou schránkou, tradičně řazenou mezi hnědé řasy.

Základní informace

Rozsivky jsou jednobuněčné řasy s dvojdílnou křemitou schránkou. Skupinu tvoří 285 rodů obsahujících 10 až 12 tisíc druhů (Round et al. 1990). Jejich fotosyntetickými barvivy jsou chlorofyly a a c, fukoxanthin. Chloroplasty mají čtyři obalné membrány a tylakoidy jsou uspořádány v trojicích. Zásobními látkami jsou chrysolaminaran, olej a volutin.

Schéma schránky rozsivek

Schránka rozsivek se nazývá frustula. Je tvořena polymerizovaným oxidem křemičitým, který je proti korozi chráněn vrstvou kyselého polysacharidu diatotepinu. Schránku si buňka vytváří aktivním vychytáváním kyseliny křemičité z prostředí rychlostí až 18 molekul za sekundu. Frustula se skládá ze dvou částí (jako krabice s víkem či Petriho miska) – epithéky a hypothéky, z nichž každá má svou plochu (valvu) a boční stěnu (pleuru či cingulum; viz obrázek). Během nepohlavního rozmnožování získá každá z dceřiných buněk jednu část mateřské frustuly a druhou, vždy tu menší, si dotvoří. V případě nedostatku křemíku v prostředí se nepohlavní rozmnožování zastavuje. Podle tvaru frustuly se rozsivky dělí na dvě hlavní skupiny: centrické, radiálně souměrné a penátní, dvoustranně souměrné.

Rozmnožování

Při nepohlavním rozmnožování probíhá rovina dělení rovnoběžně s rovinou valv. Dělení frustuly viz výše. Při zmenšení frustuly pod určitou mez dochází ke smrti buňky či k pohlavnímu rozmnožení.

Centrické rozsivky se pohlavně rozmnožují oogamicky. Z jedné buňky vznikne oogonium s oosférou z další pak antheridium, v němž vzniknou čtyři spermatozoidy s jedním bičíkem. Po splynutí spermatozoidu s oosférou vzniká auxospora, velká buňka s polysacharidickou stěnou, na které se časem začne usazovat stěna křemičitá.

Gyrosigma sp.

Penátní rozsivky se pohlavně rozmnožují izogamicky nebo anizogamicky. Dvě rozsivky se k sobě přiblíží a vytvoří společný slizový obal. Následně v každé z buněk proběhne meióza a jedna ze vzniklých gamet améboidním pohybem pronikne do druhé frustuly, splyne s jednou z gamet v ní za vzniku auxospory (viz výše).

Ekologie

Rozsivky jsou velmi významnými primárními producenty. Podle některých pramenů připadá na mořské rozsivky 20–25 % roční celkové primární produkce Země. Podle některých pramenů se výrazně, spolu se sinicemi, podílely na vzniku kyslíkaté atmosféry na Zemi. Jsou dominantní skupinou mořského planktonu, zvláště v temperátních a chladných mořích. Významně jsou zastoupeny také ve sladkovodním planktonu a bentosu. Mohou žít také přisedle na pevných podkladech ve vodě, jako jsou kameny, dřevo, rostliny či živočichové. Některé rozsivky žijí i v půdě.

Historie

Nejstarší rozsivky pocházejí z období spodní křídy. Evolučně starší jsou rozsivky centrické. Záznamy o nejmladších penátních jsou z období před 70 mil. let. V rámci hnědých řas jsou monofyletickou skupinou.

Využití

Schránky odumřelých rozsivek tvoří horninu diatomit (křemelinu), který se těží (v ČR například u Borovan u Českých Budějovic) a využívá se jako filtrační či sorpční materiál. V některých oblastech se rozsivky podílely na vzniku ložisek ropy. Vzhledem ke specifickým nárokům některých rozsivek se tyto využívají jako indikační organismy pro určení kvality vody, a to i zpětně v archeologických vykopávkách.

Příklady rodů rozsivek

• Centrické rozsivky:
  • Stephanodiscus
  • Asteromphalus
  • Chaetoceros
  • Cyclotella

• Penátní rozsivky:
  • Diatoma
  • Gyrosigma
  • Navicula
  • Nitzschia

Taxonomické poznámky

Moderní taxonomové rozsivky oddělují zcela od rostlin a řadí je do samostatné, páté říše živých organismů, Chromista. Zdůvodňuje se to tím, že svými vlastnostmi leží na rozhraní mezi rostlinami a živočichy. V této říši tvoří část jedné ze dvou podříší (Chromobiota), do níž jsou zařazeny jako samostatná třída kmene (oddělení) Ochrophyta.

Třetí možné dělení je řadí do oddělení Heterokontophyta a spolu s ním do čtvrté říše, Protista.

Třída Bacillariophyceae se v současných systémech dělí obvykle na dvě velké podtřídy, a to Eunotiophycidae a Bacillariophycidae.

Externí odkazy

• Obrázky, zvuky či videa k tématu Rozsivky ve Wikimedia Commons
  • Lokální šablona odkazuje na jinou kategorii Commons než přiřazená položka Wikidat:
    • Lokální odkaz: Diatoms
    • Wikidata: Bacillariophyta
• Bacillariophyceae - rozsivky. - Server „Sinice a řasy“, Jihočeská univerzita
• TŘÍDA: BACILLARIOPHYCEAE - rozsivky. - Mendelova zemědělská a lesnická univerzita
• Analýza rozsivek. - Jihočeská univerzita – diatomární analýzy v archeologii

Algedække på strand.

Underrækken Diatomeae kiselalger – tidligere rækken Bacillariophyta – også kaldet diatoméer, hører til planteplanktonet og er levende organismer. De indeholder klorofyl, et grønt stof, der bruges i omdannelsen af solens lys til energi. Algerne har to skaller dannet af kisel, der sidder sammen som låget sidder på en madkasse. Skallerne har små huller, som algen optager næring og udskiller affaldsstoffer igennem. Nogle af disse organismer har små børster og pigge som de bruger til at holde sig svævende i vandet med.

Disse alger blev før i tiden kun inddelt i to grupper: de centriske, der hovedsagelig lever i de frie vandmasser, og de pennate, som især er bundlevende. Algerne er brune eller brungule, de pennate alger kan af og til farve havbunden helt gulbrun, hvis de forekommer i store mængder, f.eks. på de vanddækkede lavvandede, områder i Vadehavet.

• Række kiselalger (Bacillariophyta) eller diatoméer
  • Fragilariophyceae
    • Fragilariophycidae
  • Bacillariophyceae
    • Centriske kiselagler Centrales
      • Eunotiophycidae
    • Bacillariales
      • Bacillariophycidae
    • Pennate kiselagler Pennales
  • Coscinodiscophyceae
    • Thalassiosirophycidae
    • Coscinodiscophycidae
    • Biddulphiophycidae
    • Lithodesmiophycidae
    • Corethrophycidae
    • Cymatosirophycidae
    • Rhizosoleniophycidae
    • Chaetocerotophycidae

Se også

• Mikroorganisme
• Moler

Kilder/Henvisninger

• Hjemmesiden: http://www.naturligvis.u-net.dk/
• Systema Naturae 2000: Phylum Bacillariophyta
• Systema naturae 2000 (classification) – Taxon: Subphylum Diatomeae

Kieselalgen bilden oft Überzüge und Matten am Gewässergrund aus, die auch makroskopisch erkennbar sind

Die Kieselalgen oder Diatomeen (Bacillariophyta) bilden ein Taxon von Photosynthese betreibenden Protisten (Protista) und werden in die Gruppe der Stramenopilen (Stramenopiles) eingeordnet.

Oft wird die Gruppe mit dem synonymen Namen Diatomea Dumortier bezeichnet, alternativ sind auch die synonymen Namen Fragilariophyceae, Diatomophyceae in Verwendung. Einige Autoren nennen die Kieselalgen Bacillariophyceae, sie ordnen sie also als Klasse in die, dann als Phylum aufgefassten photosynthetischen Vertreter der Stramenopiles (Ochrophyta) ein, diese Auffassung wird etwa in den Datenbanken DiatomBase^[1] und WoRMS vertreten. Diese Verwendung ist allerdings missverständlich, da andere Taxonomen eine enger abgegrenzte Klasse Bacillariophyceae, als eine von drei Klassen innerhalb der Diatomeen aufführen. Bei Verwendung dieses Namens ist also die jeweilige Auffassung zu kontrollieren, da es sonst zu Missverständnissen kommt.

Man unterscheidet heute rund 6000 Arten. Es wird jedoch angenommen, dass insgesamt bis zu 100.000 Arten existieren.^[2] Eine Arbeitsstelle für Diatomeen-Forschung (mit umfangreicher Sammlung und Online-Katalog), begründet von Friedrich Hustedt, befindet sich im Alfred-Wegener-Institut.^[3]

Merkmale

Pennate und zentrische Kieselalge;
Aus Kunstformen der Natur (Ausschnitt)

Ihren deutschen Trivialnamen verdanken die Kieselalgen der Zellen­hülle (Frustel), die überwiegend aus Siliziumdioxid (Summenformel: SiO₂) besteht, dem Anhydrid der Kieselsäure (vereinfachte Summenformel: SiO₂ · n H₂O). Die Kieselsäure wird jedoch im deutschen Sprachraum oft fälschlich mit ihrem Anhydrid gleichgesetzt. Das Siliziumdioxid gewinnt der Organismus aus der Monokieselsäure Si(OH)₄.

Diese sekundär­elektronen­mikro­skopischen Aufnahmen zeigen die Schalen- und Gürtelbänder, bei B sind zudem die schlitzförmigen Raphen erkennbar

Die Frustel ist schachtelförmig und besteht aus zwei schalenförmigen Teilen unterschiedlicher Größe, von denen die eine („Epitheka“) mit ihrer Öffnung über die Öffnung der anderen („Hypotheka“) greift. Die Schalen sind in charakteristischen Mustern strukturiert. Aufgrund der Schalengeometrie werden zwei Typen von Kieselalgen unterschieden: Zentrische Kieselalgen (Centrales) haben zumeist runde, bisweilen auch dreieckige Schalen, während pennate Kieselalgen (Pennales) stab- oder schiffchenförmige, mitunter auch bogen- oder S-förmig gekrümmte Gehäuse ausbilden. Die Schalen werden mittels spezieller Peptide angelegt, die als Silaffine bezeichnet werden. Die Silaffine ermöglichen die Ausfällung des Siliziumdioxids in kleine globuläre Siliziumdioxidaggregate, den sogenannten „Nanospheren“. Diese haben einen Durchmesser von 30 bis 50 nm und bilden in ihrer Gesamtheit die eigentlichen Schalen aus.^[4][5] Viele pennate Kieselalgen können auf einer festen Unterlage mit Hilfe einer Raphe kriechen. Die Geschwindigkeit beträgt bis zu 20 μm/s.

Kieselalgen sind einzellig und fast stets unbegeißelt. Nur bei einigen Arten besitzen die männlichen Gameten eine Geißel, eine nach vorn gerichtete Flimmergeißel. Die durch sekundäre Endosymbiose mit einer Rotalge entstandenen Plastiden^[6] sind braun gefärbt, da das Xanthophyll Fucoxanthin die Farbe der Chlorophylle (Chlorophyll a und c) überdeckt. Als Reservestoff dient Chrysolaminarin.

In der Regel sind Kieselalgen mikroskopisch klein. Zum Beispiel erreicht die Achnanthes eine Länge von 40 Mikrometer. Einige Arten können jedoch bis zu 2 Millimeter lang werden.

Vermehrung

Auxosporen von Melosira varians

Die Diatomeen sind diploid und vermehren sich hauptsächlich ungeschlechtlich durch Zellteilung. Die Tochterzellen erhalten jeweils einen Schalenteil und bilden den anderen Teil neu; hiervon leitet sich auch die Bezeichnung „Diatomee“ (altgriechisch διατέμνειν (diatemnein) = spalten) ab. Der neue Schalenteil ist stets die kleinere Hypotheka, so dass im Generationenverlauf die Zellgröße fast aller Nachkommen fortlaufend schwindet, nur die Tochterzelllinie der Ausgangs-Epitheka behält die ursprüngliche maximale Größe bei. Wird eine Minimalgröße unterschritten, stirbt das Individuum. Bevor eine Minimalgröße erreicht wird, können jedoch Sexualvorgänge stattfinden. Aus den Zellen bilden sich durch Meiose haploide Gameten. Bei zentrischen Kieselalgen wurde Oogamie nachgewiesen: Die Gameten werden frei, nach Verschmelzen eines weiblichen mit einem männlichen Gameten bildet sich aus der Zygote unter Größenwachstum eine Dauerform, eine sogenannte Auxospore. Bei pennaten Kieselalgen wurde Konjugation beobachtet: Zwei Partner legen sich aneinander und bilden eine gemeinsame Cytoplasmabrücke („Konjugationskanal“), in die jeweils ein haploider Kern und ein Chloroplast der beiden Partner einwandern. Aus der so gebildeten Zygote bildet sich eine Auxospore, in der die Kernverschmelzung (Karyogamie) stattfindet. Aus den Auxosporen der zentrischen und pennaten Kieselalgen wird jeweils eine größere neue Kieselalge mit einer neuen zweiteiligen Schale gebildet.

Vorkommen

Kieselalgen kommen hauptsächlich im Meer und in Süßgewässern planktisch oder benthisch vor, oder sie sind auf Steinen oder Wasserpflanzen (Epiphyten) angesiedelt. Manche Arten brauchen reines und kaum verschmutztes Wasser und sind aus diesem Grunde auch Zeigerorganismen für unbelastete Gewässer. Andere Arten wiederum, die im engl. auch als agricultural guild bezeichnet werden, sind typisch für Gewässer, die durch landwirtschaftliche Einträge, bspw. durch Überdüngung, besonders belastet sind. Zu diesen werden u. a. Navicula radiosa, Melosira varians, Nitzschia palea, Diatoma vulgare oder Amphora perpusilla gezählt.^[7] Auch terrestrische Arten finden sich unter den Diatomeen; diese besiedeln Böden, in tropischen Gebieten auch Blätter von Bäumen.

Systematik

Illustration: Kieselalgen Süßwasser-Pennales

Illustration: Zentrische Süßwasser-Kieselalgen

Die Kieselalgen wurden traditionell in die radiärsymmetrischen Centrales und die bilateralsymmetrischen Pennales gegliedert. Die Centrales sind jedoch paraphyletisch, eine stabile Systematik auf molekulargenetischer Grundlage hat sich noch nicht etabliert. Nach der taxonomischen Datenbank Diatombase^[1] existiert derzeit keine allgemein akzeptierte Gliederung der Kieselalgen in Subtaxa.

Eine weit verbreitete, aber nicht von allen Taxonomen anerkannte Gliederung könnte so aussehen^[8], die höhere Gliederung wurde in vergleichbarer Form in die weithin verwendete Klassifikation der Eukaryoten durch Sina Adl und Kollegen übernommen.^[9]

Odontella aurita

Fluoreszenzmikroskopie einer Kette von Asterionellopsis sp., einer marinen Gattung (Balken 5 µm).

• Unterabteilung Coscinodiscophytina
  • Klasse Coscinodiscophyceae
    • Ordnung Asterolamprales
    • Ordnung Arachnoidiscales
    • Ordnung Aulacoseirales
      • Gattung Aulacoseira
    • Ordnung Chrysanthemodiscales
    • Ordnung Corethrales
    • Ordnung Coscinodiscales
    • Ordnung Ethmodiscales
    • Ordnung Melosirales
      • Gattung Melosira
    • Ordnung Orthoseirales
    • Ordnung Rhizosoleniales
      • Gattung Rhizosolenia
    • Ordnung Stictocyclales
    • Ordnung Stictodiscales
    • Ordnung Leptocylindrales
• Unterabteilung Bacillariophytina
  • Klasse Mediophyceae
    • Ordnung Anaulales
    • Ordnung Biddulphiales
    • Ordnung Chaetocerotales
      • Gattung Chaetoceros
    • Ordnung Cymatosirales
    • Ordnung Eupodiscales (Tricertiales)
      • Gattung Pleurosira
    • Ordnung Hemiaulales
    • Ordnung Lithodesmiales
    • Ordnung Thalassiosirales
      • Gattung Cyclotella
    • Ordnung Toxariales
  • Klasse Bacillariophyceae (entspricht weitgehend den früheren Pennales)
    • Ordnung Raphoneidales
    • Ordnung Rhabdonematales
    • Ordnung Cocconeidales
      • Gattung Algenläuse (Cocconeis)
    • Ordnung Fragilariales (vermutlich paraphyletisch)
      • Gattung Asterionella
      • Gattung Diatoma
      • Gattung Fragilaria
      • Gattung Meridion
      • Gattung Synedra
    • Ordnung Tabellariales
      • Gattung Tabellaria
    • Ordnung Licmophorales
    • Ordnung Thalassionematales
    • Ordnung Eunotiales
      • Gattung Eunotia
    • Ordnung Lyrellales
    • Ordnung Mastogloiales
    • Ordnung Dictyoneidales
    • Ordnung Cymbellales
      • Gattung Cymbella
      • Gattung Didymosphenia (u. a. Didymosphenia geminata)
      • Gattung Gomphonema
      • Gattung Rhoicosphenia
    • Ordnung Achnanthales
      • Gattung Achnanthes
    • Ordnung Naviculales
      • Gattung Diploneis
      • Gattung Gyrosigma
      • Gattung Navicula
      • Gattung Pinnularia
      • Gattung Stauroneis
    • Ordnung Thalassiophysales
      • Gattung Amphora
    • Ordnung Bacillariales
      • Gattung Bacillaria
      • Gattung Nitzschia
    • Ordnung Rhopalodiales
      • Gattung Epithemia
    • Ordnung Surirellales
      • Gattung Cymatopleura
      • Gattung Surirella

Bedeutung

Untersuchung einer kommerziellen Diatomeenerde in Wasserfiltern für Schwimmbecken

Präparat verschiedener Diatomeen unter dem Rasterelektronenmikroskop

Die Kieselalgen sind Hauptbestandteil des Meeresphytoplanktons und sind die Haupt-Primärproduzenten organischer Stoffe, bilden also einen wesentlichen Teil der Basis der Nahrungspyramide. Als Sauerstoff produzierende (oxygene) Phototrophe erzeugen sie auch einen großen Teil des Sauerstoffs in der Erdatmosphäre.

Aus der relativen Arten-Zusammensetzung der Kieselalgenpopulation eines Gewässers kann recht exakt dessen Trophiegrad abgeleitet werden (Diatomeenindex), sowie weitere Gewässerparameter wie pH-Wert, Salinität, Saprobie etc. Diese Verfahren können auch auf Sedimente oder auf Öllagerstätten angewandt werden und geben dann Aufschluss über die ehemals herrschenden Lebensbedingungen.

Zur Identifizierung der Arten wird eine Aufwuchsprobe mit Kieselalgen mit Schwefelsäure, Wasserstoffperoxid, Kaliumdichromat oder einem anderen Oxidationsmittel behandelt und so werden alle organischen Bestandteile der Probe aufgelöst. Es bleiben nur noch die reinen Siliziumdioxid-Schalen übrig. Diese werden in einem Einschlussmedium mit hohem optischem Brechungsindex (z. B. Naphrax™) eingebettet und lichtmikroskopisch mit dem Phasenkontrast-Verfahren bei ca. 1000-facher Vergrößerung identifiziert.

Sterben die Zellen, sinken sie auf den Grund des Gewässers ab, die organischen Bestandteile werden abgebaut und die Siliziumdioxid-Schalen bilden eine Ablagerung, die sogenannte Kieselgur (Diatomeenerde). Dieser Prozess ist, insbesondere im Marinen, erst unterhalb der CCD (Calcit-Kompensationstiefe) effizient genug, um große Vorkommen zu bilden. Die entstehende Kieselgur wird in Technik und Medizin angewendet. Diatomeenschalen finden unter anderem Verwendung als Filter, zur Herstellung von Dynamit, in Zahnpasta als Putzkörper, als giftfreies Insektenbekämpfungsmittel in Form von Ungezieferpuder, sowie als reflektierendes Material in der Farbe, die für Fahrbahnmarkierungen im Straßenbau verwendet wird. Außerdem finden Kieselalgen in der forensischen Medizin Verwendung (Diatomeennachweis). Ihre Aussagekraft für den Nachweis eines Ertrinkungstodes wird jedoch kontrovers diskutiert.^[10][11]

Insbesondere im 19. Jahrhundert dienten Diatomeen zur Anfertigung ästhetischer mikroskopischer Präparate, deren gemeinsame Betrachtung in Salons ein beliebter Zeitvertreib in höheren gesellschaftlichen Kreisen war. Zur höchsten Perfektion in der handwerklich extrem schwierigen Anfertigung solcher Präparate hat es Johann Diedrich Möller aus Wedel bei Hamburg gebracht.

Marine Diatomeen können zu Vergiftungen bei Mensch und Tier führen, da einige Arten, insbesondere Pseudo-nitzschia, Nitzschia oder Amphora Domoinsäure produzieren.^[12] In filtirierende Meerwasser-Organismen, wie z. B. Muscheln, oder in sich von diesen Tieren ernährende Arten, wie z. B. Fischen, können sich diese Diatomeen akkumulieren. Kommt es zum Verzehr solcher mit Domoinsäure angereicherten Organismen durch den Menschen, treten Vergiftungserscheinungen auf, die als Amnesic Shellfish Poisoning (ASP) bezeichnet werden. Als Symptome treten insbesondere Gedächtnisverlust, Übelkeit, Krämpfe, Durchfall, Kopfschmerz und Atembeschwerden auf.^[13]

Literatur

Aus Ernst Haeckels Kunstformen der Natur (1904), Tafel 84

Diatomeen-Kreispräparat aus dem Jahr 1903 von Watson & Sons, London

• Friedrich Hustedt: Bacillariophyta (Diatomeae). 2. Auflage. Fischer, Jena 1930.
• Friedrich Hustedt: Kieselalgen (Diatomeen). 3. Auflage. Franckh, Stuttgart 1965.
• Kurt Krammer: Kieselalgen. Franck, Stuttgart 1986.
• Kurt Krammer, Horst Lange-Bertalot: Bacillariophyceae in Süsswasserflora von Mitteleuropa 1986–2000. Fischer, Stuttgart und Spektrum, Akademischer Verlag, Heidelberg.
  • Band 1: Naviculaceae. 1986.
  • Band 2: Bacillariaceae, Epithemiaceae, Surirellaceae. 1988.
  • Band 3: Centrales, Fragilariaceae, Eunotiaceae. 1991.
  • Band 4: Achnanthaceae. 1991.
  • Band 5: English and French translations and additions. 2000.

• Diatoms of Europe : diatoms of the European inland waters and comparable habitats. Gantner, Rugell.
  • Band 1: Kurt Krammer: The genus Pinnularia. 2000.
  • Band 2: Horst Lange-Bertalot: Navicula sensu stricto. 2001.
  • Band 3: Kurt Krammer: The genus Cymbella. 2002.
  • Band 4: Kurt Krammer: The genera Cymbopleura, Delicata, Navicymbula, Gomphocymbellopsis and Afrocymbella. 2003.

• F. E. Ross, R. M. Crawford, D. G. Mann: The Diatom. Biology and Morphology of The genera. Cambridge Univ. Press, 1990.

Einzelnachweise

1. ↑ ^{a b} Class Bacillariophyceae. in Kociolek, J.P.; Balasubramanian, K.; Blanco, S.; Coste, M.; Ector, L.; Liu, Y.; Kulikovskiy, M.; Lundholm, N.; Ludwig, T.; Potapova, M.; Rimet, F.; Sabbe, K.; Sala, S.; Sar, E.; Taylor, J.; Van de Vijver, B.; Wetzel, C.E.; Williams, D.M.; Witkowski, A.; Witkowski, J. (2017). DiatomBase abgerufen am 4. Dezember 2017
2. ↑ T. A. Norton, M. Melkonian, R. A. Andersen: Algal biodiversity. In: Phycologia. 35, S. 308–326.
3. ↑ Hustedt Zentrum für Diatomeenforschung, Alfred Wegener Institut Bremerhaven
4. ↑ N. Kroger: The sweetness of diatom molecular engineering. In: Journal of Phycology. Volume 37, 2001, S. 93–112.
5. ↑ N. Kroger, M. Sumper: The molecular basis of diatom biosilica formation. In: E. Baeuerlein (Hrsg.): Biomineralization: Progress in Biology, Molecular Biology and Application. 2004, S. 137–158, Wiley-VCH, Weinheim
6. ↑ Secondary Endosymbiosis Exposed. In: The Scientist. Juni 2005, abgerufen am 1. Juni 2016.
7. ↑ J. L. Richardson, N. S. Mody, M. E. Stacey: Diatoms and water quality in Lancaster County (PA) streams: a 45 year perspective. In: Jornal - Pennsylvania Academy of Sciences. Volume 70, 1996, S. 30–39.
8. ↑ Linda K. Metlin (2016): Evolution of the diatoms: major steps in their evolution and a review of the supporting molecular and morphological evidence. Phycologia 55 (1): 79–103. doi:10.2216/15-105.1
9. ↑ Adl, S. M., Simpson, A. G. B., Lane, C. E., Lukeš, J., Bass, D., Bowser, S. S., Brown, M. W., Burki, F., Dunthorn, M., Hampl, V., Heiss, A., Hoppenrath, M., Lara, E., le Gall, L., Lynn, D. H., McManus, H., Mitchell, E. A. D., Mozley-Stanridge, S. E., Parfrey, L. W., Pawlowski, J., Rueckert, S., Shadwick, L., Schoch, C. L., Smirnov, A. and Spiegel, F. W. (2012): The Revised Classification of Eukaryotes. Journal of Eukaryotic Microbiology 59: 429–514. doi:10.1111/j.1550-7408.2012.00644.x
10. ↑ B. Madea: Praxis Rechtsmedizin: Befunderhebung, Rekonstruktion, Begutachtung. 2., aktualisierte Auflage. Springer, Berlin/ Heidelberg/ New York 2007.
11. ↑ P. Lunetta, J. H. Modell: Macropathological, Microscopical, and Laboratory Findings in Drowning Victims. In: M. Tsokos (Hrsg.): Forensic Pathology Reviews. Vol. 3, Humana Press, Totowa, NJ 2005, S. 4–77.
12. ↑ S. S. Bates: Domoic-acid-producing diatoms: another genus added. In: Journal of Phycology. Volume 36, 2000, S. 978–985.
13. ↑ Gesundheitliche Folgen toxischer Algenblüten (Memento des Originals vom 9. Februar 2007 im Internet Archive) Info: Der Archivlink wurde automatisch eingesetzt und noch nicht geprüft. Bitte prüfe Original- und Archivlink gemäß Anleitung und entferne dann diesen Hinweis.@1@2Vorlage:Webachiv/IABot/www.gesundes-reisen.de

Ang mga diatomea (di-tom-os 'gupitin sa kalahati', mula sa diá, 'sa pamamagitan ng' o 'hiwalay' at ang ugat ng tém-n-ō, 'Pinutol ko') ay isang pangunahing pangkat ng algae, partikular na microalgae, na matatagpuan sa mga karagatan, mga daluyan ng tubig at mga soils ng mundo. Ang mga numero ng buhay na diatoms sa trillions: bumubuo sila ng halos 20 porsiyento ng oksiheno na ginawa sa planeta bawat taon, tumagal ng higit sa 6.7 bilyon metrikong toneladang silikon bawat taon mula sa tubig kung saan sila nakatira, at nag-ambag ng halos kalahati ng organiko na materyal na natagpuan sa mga karagatan.

Ang lathalaing ito ay isang usbong. Makatutulong ka sa Wikipedia sa nito.

Diatoms (diá-tom-os 'cut in hauf', frae diá, 'throu' or 'apairt'; an the ruit o tém-n-ō, 'A cut'.) ^[10] are a major group o algae,^[11] specifically microalgae, foond in the oceans, watterweys an siles o the warld.

References

Diatom suvoʻtlar (yun. diatomos -teng ikkiga boʻlingan) — kremniyli suvoʻtlar (Bacillariophyta); suv-oʻtlar boʻlimiga mansub. 20 mingga yaqin turi bor. D. s.ning mikroskopik (0,75— 1500 mkm), bir hujayrali, yakka yoki kolonial formalari mavjud. D. s. hujayralari kremniyli ikki pallali qattiq qobiq bilan oʻralgan. Qobiq devorida tashqi muhit bilan moddalar almashinuvi sodir boʻladigan tirqishlar bor. D. s.ning hujayralarida bir yoki bir nechta yadrochali yadro ustida va bir yoki bir nechta sariq-qoʻngʻir xromatoforalar va b. qismlar boʻladi. D. s. boʻlinib koʻpayadi: har bir qiz hujayra ona qobigʻining bir pallasini oladi, boshqasi esa qaytadan oʻsadi, bunda eski yarim palla oʻz chekkasi bilan yangi yarim pallani tutib turadi. D. s koʻpayish meʼyoriga qarab sekin-asta maydalashadi. Maydalashgan hujayralardan ikkitasi bir-biriga yaqinlashib, pallalari ochiladi va ular oʻrtasida konʼyugatsiyaga oʻxshash jarayon sodir boʻladi yoki anizogamiya (baʼzan ooga-miya) orqali jinsiy koʻpayish sodir boʻladi. Zigotasi autosporaga aylanib oʻsadi. Bunday zigota oʻsib, dastlabki hujayralarga nisbatan bir necha marta yirik hujayrani hosil qiladi. Baʼzi turlari tinim davrida spora hosil qiladi. D. s. diploidli, ularning gametalari esa gaploidli. D. s. suv-oʻtlarning tabiatda keng tarqalgan guruhi boʻlib, chuchuk suv va dengizlarda, ayniqsa dengiz tubidagi loyqada, suv oʻsimliklari va toshlar, nam va b. joylarda oʻsadi. Hayvonlarga oziq boʻladi.

Adabiyot

• OʻzME. Birinchi jild. Toshkent, 2000-yil

Diatomea es un taxon.

Classification

Rango taxinomic

• classe

Supertaxones

• Stramenopiles, Sar

Diatomeen (Bacillariophyceae of Bacillariophyta) san algen faan di stam Ochrophyta. Daalang san amanbi 6.000 slacher bekäänd, man diar san wel son 100.000 slacher noch goorei fünjen wurden.

Onerklasen

• Bacillariophycidae
• Coscinodiscophycidae
• Fragilariophycidae

Dijatomeje (lat. Diatomeae, Bacillariophyceae) su alge kremenjašice iz carstva Protisti. Ćelijska membrana im je inkrustrirana (prožeta) kremenom ili silicij dioksidom , a označava se kao frustulum. Ovi jednoćelijski organizmi sastavljeni su od dvije valve (poklopca): gornje (veće) i donje, koje su međusobno bočno vezane pleurom.^[5]

Razmnožavanje

Kremene alge se razmnožavaju vegetativno i spolno. Vegetativnog se odvija tako da se valve razmaknu, protoplazma se podijeli, a zatim i jedro, u procesu mitoze. Obje novonatale ćelije dobiju po jednu roditeljsku valvu, koja postaje gornja ili veća valva, dok manju valvu same obnove. Ovim putem postepeno dolazi do smanjivanja veličine ćelije. Kada to dode do veličine kod koje se ne može dalje dijeliti, nastupa posebni način dijeljenja tzv. auksosporulacija i spolni proces. Auksosporulacija rezultira stvaranjem zigot u vidu velike auksospore koja zatim luči veći oklop. Zapaženo je i spolno razmnožavanje muškim spolnim ćelijama koje imaju po jedan bič. U nepovoljnim uvjetima, stvaraju trajne spore sa tvrdom silificiranom ljušturicom koja kod raznih vrsta ima karakterističe nastavke. Trajne spore padnu na morsko dno gdje mogu preživjeti nepovoljnnu sezonu i uvjete i aktivirati se kada nastupe povoljne okolnosti.

Ekologija i rasprostranjenje

Diatomeje se hrane autotrofno. Sadrže pigmente hlorofil a i c, karotenoidi i ksantofil (fukoksantin). Proizvod fotosinteze je hrizolaminarin, polimer glukoze.

Procjenjuje se da danas na Zemlji ima oko 100 miliona vrsta algi kremenjašica, koje se razvrstavaju u 250 rodova.

Staništa su im razna: na kopnu, u stajaćim i tekućim vodama, u slobodnoj vodi (plankton) ili na dnu (bentos). Mogu i obrastati potopljene predmete različitog porijekla. Općenito naseljavaju hladnija mora, a u toplijim su ćešće zastupljene u hladnijem dijelu godine.

Podjela

Prema strukturi ljušturice dijele se na

• Centricae – koje imaju zrakastu (radijalnu) strukturu u odnosu na centralnu tačku, i
• Pennatae – koje su bilateralno izdužene. Prve su pretežno planktonske dok druge uglavnom bentoske.

Uginuli organizmi, sa ljušturama padaju na dno, gdje se tokom miliona godina na stvara dijatomejska zemlja. Alge kremenjašice upotrebljavaju se za proizvodnju izolacijskih materijala, izradu paste za zube i brušenje metala.

Također pogledajte

• Alga
• Kremen

Reference

Diatomeo esas familio de algi feofices, od algi bruna.

Κάποια θαλάσσια διάτομα, μια ομάδα - κλειδί του φυτοπλαγκτόν.

Τα διάτομα (ομοταξία: Βακιλλαριοφύκη (Bacillariophyceae)) αποτελούν μια πολύ σημαντική ομάδα φυτοπλαγκτικών οργανισμών και είναι άφθονα τόσο στα θαλάσσια οικοσυστήματα όσο και στα εσωτερικά νερά. Πρόκειται για μονοκύτταρα φύκη με κυτταρικό τοίχωμα που εμφανίστηκαν την Ιουρασική περίοδο, πριν από περίπου 185 εκατομμύρια χρόνια. Ο αριθμός των ειδών που ανήκουν στα διάτομα δεν είναι απόλυτα γνωστός, αλλά πιστεύεται ότι σήμερα υπάρχουν περίπου 100.000 είδη, τα μισά από τα οποία είναι θαλάσσια. Είναι μονοκύτταροι ευκαρυωτικοί οργανισμοί, αν και πολλά από αυτά σχηματίζουν αποικίες σε μορφή αλυσίδας ή σε αστερόμορφους και άλλους σχηματισμούς. Ιδιαίτερο χαρακτηριστικό της ομάδας είναι το κυτταρικό τοίχωμα, με βασικό του συστατικό το διοξείδιο του πυριτίου. Οι κοινωνίες των διατόμων συχνά χρησιμοποιούνται ως βιολογικοί δείκτες σε προγράμματα παρακολούθησης της οικολογικής κατάστασης των υδάτων.

Μορφολογία

Το μέγεθος των οργανισμών κυμαίνεται από 2 έως 200 μm. Τα είδη της ομοταξίας έχουν χρώμα από κίτρινο έως καφέ και οφείλεται στην ύπαρξη των κίτρινων – φαιών καροτινοειδών χρωστικών τους. Οι φωτοσυνθετικές χρωστικές των διατόμων γενικά είναι οι χλωροφύλλες a, c₁ και c₂, οι β- και ε-καροτίνες και από τις ξανθοφύλλες η φουκοξανθίνη (που δίνει στα διάτομα το χαρακτηριστικό τους χρώμα), η διατοξανθίνη και η διαδινοξανθίνη.

Το χαρακτηριστικό της ομοταξίας είναι το ανάγλυφο κυτταρικό τους τοίχωμα ή θήκη, όπως συχνά ονομάζεται, με κύριο συστατικό του το διοξείδιο του πυριτίου (SiO₂). Αποτελείται από μια εσωτερική συνεχή μεμβράνη από πηκτινικές ουσίες και εξωτερικά από δύο κελυφοειδείς θυρίδες οι οποίες εφαρμόζουν μεταξύ τους και η μία είναι ελαφρώς μεγαλύτερη από την άλλη. Η θήκη δεν είναι απλή στη μορφή αλλά διάτρητη και μπορεί να φέρει περίπλοκες δομές όπως ραβδώσεις, αγκάθια, πόρους και προεξοχές. Η θήκη των διατόμων είναι διαφανής και διαπερατή από το φως, ώστε οι οργανισμοί να μπορούν να δεσμεύουν την ηλιακή ενέργεια απαραίτητη για τη φωτοσύνθεση, ενώ οι πόροι επιτρέπουν τη διέλευση των διαλυμένων στο νερό αερίων και των θρεπτικών συστατικών μέσα και έξω από τα κύτταρα. Η περίπλοκη δομή του τοιχώματος των διατόμων αποτελεί σημαντικό διαγνωστικό χαρακτηριστικό για την ταξινόμησή τους σε γένη και είδη.

Οικολογία

Τα διάτομα είναι άφθονα στις εύκρατες και πολικές περιοχές, τόσο στα παράκτια όσο και στα ωκεάνια νερά. Γενικά είναι κοινά σε νερά πλούσια σε θρεπτικά συστατικά. Η ομάδα των διατόμων περιλαμβάνει είδη της θάλασσας και των εσωτερικών υδάτων (λίμνες και ποτάμια). Τα περισσότερα διάτομα είναι πλαγκτικά, δηλαδή προσαρμοσμένα να ζουν σε αιώρηση μέσα στο νερό, αλλά πολλά είδη διαθέτουν προσαρμογές και εξαρτήματα που τους επιτρέπουν να προσκολλώνται σε σκληρές επιφάνειες όπως βράχους και σημαδούρες.

Τα διάτομα είναι σημαντικοί πρωτογενείς παραγωγοί στα υδάτινα οικοσυστήματα. Συγκεκριμένα υπολογίζεται ότι το 20 – 25% της συνολικής δέσμευσης του διοξειδίου του άνθρακα πραγματοποιείται από αυτά, ενώ θεωρούνται οι σημαντικότεροι μη προσκολλημένοι πρωτογενείς παραγωγοί της ανοικτής θάλασσας στις εύκρατες και πολικές περιοχές. Λίγα μόνο είδη είναι άχρωμα και ζουν προσκολλημένα πάνω στις επιφάνειες των φυκών ως ετερότροφοι οργανισμοί.

Διάτομα όπως το Biddulphia tridens αποτελούν βασικό παράγοντα που οξυγονώνει την γήινη ατμόσφαιρα ενώ παράλληλα αποτελούν τροφή για πολλά θαλάσσια πλάσματα. Οι βιολόγοι χρησιμοποιούν τα διάτομα όπως το Triceratium castelliferum ως βιοδείκτες για την ποιότητα του νερού μια και είναι ιδιαίτερα ευαίσθητα στις αλλαγές του περιβάλλοντος.

Τα διάτομα συμμετέχουν συχνά σε φαινόμενα «άνθισης του νερού» (ή φυτοπλαγκτικής «άνθισης») - ο αντίστοιχος αγγλικός όρος είναι «water bloom», δηλαδή φαινόμενα κατά τα οποία ορισμένοι φυτοπλαγκτικοί οργανισμοί αυξάνονται ανά περιόδους σε αφθονία και βιομάζα, εξ’ αιτίας της επικράτησης ευνοϊκών συνθηκών στο νερό (διαθεσιμότητα θρεπτικών συστατικών και άλλοι παράγοντες) και το νερό χρωματίζεται ανάλογα με τις φωτοσυνθετικές χρωστικές των οργανισμών που επικρατούν. Τα διάτομα εμφανίζουν φαινόμενα άνθισης τόσο σε λίμνες όσο και σε παράκτιες αλλά και ωκεάνιες περιοχές (για παράδειγμα στον Ατλαντικό ωκεανό) κατά την άνοιξη.

Κάτω από δυσμενείς συνθήκες τα διάτομα έχουν την ικανότητα να σχηματίζουν ανθεκτικές δομές γνωστές ως έμμονα σπόρια, τα οποία διατηρούνται αδρανή όσο οι συνθήκες είναι δυσμενείς, ενώ όταν αυτές γίνουν ξανά ευνοϊκές προκύπτουν πάλι διάτομα από τα σπόρια.

Στο βυθό των θαλασσών σε διάφορες ωκεάνιες περιοχές οι θήκες των νεκρών διατόμων καθιζάνουν, συσσωρεύονται και σχηματίζουν μεγάλες αποθέσεις, γνωστές ως γη των διατόμων. Τα ιζήματα αυτά έχουν πολλές εμπορικές εφαρμογές, όπως κατασκευή φίλτρων, ηχομονωτικών και θερμομονωτικών υλικών, καθαριστικών, αποσμητικών, λειαντικών και πολλών άλλων υλικών.

Ταξινόμηση

Η ομοταξία των διατόμων χωρίζεται σε δύο τάξεις:

• τα κεντρικά διάτομα (Biddulphiales ή Centrales) τα οποία έχουν ακτινωτή συμμετρία και κυλινδρικές θυρίδες, και
• τα πτεροειδή διάτομα (Bacillariales ή Pennales) τα οποία έχουν κυρίως αμφίπλευρη συμμετρία και βαρκόμορφες θυρίδες.

Μια ακόμη διαφορά ανάμεσα στις δύο κλάσεις αφορά στην εγγενή αναπαραγωγή των οργανισμών: τα κεντρικά διάτομα παράγουν αρσενικούς γαμέτες που φέρουν μαστίγιο και κινούνται, ενώ ο θηλυκός γαμέτης δε φέρει μαστίγιο. Αντίθετα τα πτεροειδή διάτομα παράγουν γαμέτες που δε φέρουν μαστίγια.

Αναπαραγωγή

Ο πιο κοινός τρόπος αναπαραγωγής για τα διάτομα είναι η αγενής αναπαραγωγή με κυτταρική διαίρεση. Κατά τη διαδικασία αυτή το κάθε νέο κύτταρο φέρει μία θυρίδα από το αρχικό κύτταρο και εκκρίνει μια δεύτερη, μικρότερη θυρίδα. Επαναλαμβανόμενες κυτταρικές διαιρέσεις κατά τις περιόδους ταχείας αναπαραγωγής (γνωστή ως φάση ακμής) με ευνοϊκές συνθήκες στο περιβάλλον (θερμοκρασία, διαθεσιμότητα θρεπτικών κ.α.) οδηγούν σε σταδιακά μικρότερο μέγεθος των διατόμων, κυρίως επειδή το διαλυμένο στο νερό πυρίτιο εξαντλείται κατά την κατασκευή των νέων κυτταρικών τοιχωμάτων, αλλά και επειδή τα κελύφη τους δεν έχουν τη δυνατότητα να μαγαλώσουν. Όταν τα κύτταρα φτάσουν σε ένα ελάχιστο κρίσιμο μέγεθος σχηματίζονται τα αυξοσπόρια (στάδια ήπιας ανακαμπτικής ανάπτυξης), τα οποία σταδιακά αυξάνουν το μέγεθός τους και ανακτάται το κανονικό μέγεθος, ή λαμβάνει χώρα εγγενής αναπαραγωγή, σε συνδυασμό με την επικράτηση ορισμένων απαραίτητων ευνοϊκών συνθηκών στο περιβάλλον, όσον αφορά τη θερμοκρασία, τα θρεπτικά, τα ιχνοστοιχεία και άλλους παράγοντες. Με τη συγχώνευση των δύο γαμετών αναπτύσσεται ένα αυξοσπόριο και το μέγεθος αποκαθίσταται όπως περιγράφηκε προηγουμένως.

Τοξίνες Διατόμων

Η σημαντικότερη και πιο ισχυρή τοξίνη που παράγουν πολλά διάτομα είναι το Δομοϊκό Οξύ (Domoic Acid, DA). Το δομοϊκό οξύ είναι μια ισχυρή νευροτοξίνη φυκών που παράγεται με φυσικό τρόπο από αρκετά Διάτομα. Συγκεκριμένα, παράγεται από 12 είδη του γένους Pseudo-nitzschia καθώς επίσης και από τα είδη: Amphora coffeaeformis και Nitzschia navis-varingica. Το DA είναι ένα υδατοδιαλυτό, πολικό αμινοξύ που συνδέεται δομικά με το καϊνικό οξύ. Δρα αγωνιστικά με το γλουταμινικό οξύ και έχει εξωτοξική δράση στο Κεντρικό Νευρικό Σύστημα αλλά και στα όργανα που είναι πλούσια σε υποδοχείς του γλουταμινικού στα σπονδυλωτά. Η τοξίνη αυτή είναι υπεύθυνη για μια ασθένεια που προκαλεί στους ανθρώπους, γνωστή ως αμνησιακή δηλητηρίαση οστρακοειδών (Amnesic Shellfish Poisoning, ASP). Αυτή η ασθένεια ταυτοποιήθηκε για πρώτη φορά το 1987, όταν 143 άνθρωποι αρρώστησαν και τέσσερις πέθαναν έπειτα από την κατανάλωση μολυσμένων με δομοϊκό οξύ μυδιών, τα οποία είχαν συλλεχθεί από ειδικές εγκαταστάσεις καλλιέργειάς τους στην ανατολική ακτή του Prince Edward Island στον Καναδά. Αν και έχουν αναγνωριστεί πολλές πηγές του DA και σε Μακροφύκη, τη σημαντικότερη απειλή στην ανθρώπινη υγεία προβάλλουν τα τοξικά είδη των διατόμων εξαιτίας της συσσώρευσης του DA στο σύστημα φιλτραρίσματος της τροφής που διαθέτουν κάποιοι θαλάσσιοι οργανισμοί. Τα κλινικά συμπτώματα της ASP στους ανθρώπους περιλαμβάνουν γαστρεντερική διαταραχή, σύγχυση, αποπροσανατολισμό, επιληπτικές κρίσεις, μόνιμη απώλεια της βραχύχρονης μνήμης και σε πιο ακραίες περιπτώσεις θάνατο.

Οι πληθυσμιακές εκρήξεις των τοξικών διατόμων είναι ένα παγκόσμιο πρόβλημα και φαίνεται να παρουσιάζουν αύξηση τόσο στην συχνότητα όσο και στην τοξικότητα απειλώντας την ανθρώπινη υγεία και την ασφάλεια των θαλασσινών τροφίμων. Εξαιτίας αυτού, έχουν εγκαθιδρυθεί πολλά προγράμματα έρευνας και ελέγχου στην δυτική ακτή των Η.Π.Α. με σκοπό τον εντοπισμό του DA σε οστρακοειδή και παράκτια νερά. Στην Ελλάδα γίνονται κάποια παρόμοια προγράμματα, μικρότερης όμως κλίμακας, κυρίως για ακαδημαϊκούς σκοπούς. Την περίοδο Αυγούστου-Οκτωβρίου 2008, έξι στελέχη του είδους Pseudo-nitzschia pseudodelicatissima απομονώθηκαν από τον Θερμαϊκό κόλπο. Τα στελέχη καλλιεργήθηκαν και υποβλήθηκαν σε ελέγχους για να διαπιστωθεί εάν παρήγαγαν DA. Τελικά εντοπίστηκε η παραγωγή του και στα έξι στελέχη και η μελέτη αυτή επιβεβαίωσε την παραγωγή του δομοϊκού οξέος από το P. pseudodelicatissima. Στη λήψη μέτρων για τον έλεγχο της ανθρώπινης έκθεσης στο DA, ηγετικό ρόλο παίζει ο Καναδάς και ακολουθούν οι: Νορβηγία, Σκωτία, Γαλλία, Ιρλανδία, Πορτογαλία, Ισπανία, Ιαπωνία, Αυστραλία και Νέα Ζηλανδία.

Σε πολλές περιοχές έχουν εδραιωθεί προγράμματα ελέγχου της τοξίνης και η λήψη μέτρων και εκτέλεσή τους, έχει συνδράμει στην αποφυγή νέου –καταγεγραμμένου- περιστατικού από αυτό του 1987. Αν και τα προγράμματα αυτά δείχνουν να είναι αποτελεσματικά στο να αποτρέπουν δηλητηριάσεις, είναι πιθανό να υπάρχουν επιπτώσεις από την επαναλαμβανόμενη μακρόχρονη έκθεση σε μικρή ποσότητα DA που δεν έχουν ακόμα διερευνηθεί. Έχει π.χ. διαπιστωθεί ένα σύνδρομο χρόνιας τοξικότητας του DA σε θαλάσσιους λέοντες οι οποίοι χρησιμοποιούνται ως πειραματικό είδος για την ανίχνευση των πιθανών κινδύνων σε ανθρώπους από την τοξίνη. Επιπλέον, βρίσκεται σε εξέλιξη η δημιουργία ενός σπονδυλωτού μοντέλου που θα επιτρέπει την αξιολόγηση των επιπτώσεων από την επαναλαμβανόμενη μακρόχρονη έκθεση σε χαμηλά επίπεδα DA.

Όπως αναφέρθηκε, ο μεγαλύτερος κίνδυνος έκθεσης σε DA για τους ανθρώπους και τη θαλάσσια πανίδα είναι η διατροφική κατανάλωση από μολυσμένα με τοξίνη θαλασσινά, όπως οστρακοειδή και κάποια είδη ψαριών. Επιπλέον, έχει διαπιστωθεί ότι και οι βενθικοί οργανισμοί στους οποίους συσσωρεύεται το δομοϊκό οξύ μπορούν να δράσουν σημαντικά ως τοξικοί φορείς στην πελαγική τροφική αλυσίδα. Με σκοπό την ενημέρωση των καταναλωτών έχουν διαμορφωθεί πίνακες που αναφέρουν τα είδη των σπονδυλωτών ζώων που μπορεί να είναι φορείς DA εξαιτίας της διατροφής τους. Κάποια ενδεικτικά παραδείγματα τέτοιων οργανισμών είναι: Balaenoptera musculus (μπλε φάλαινα), γένος Delphinus (κοινό δελφίνι), Engraulis mordax (γαύρος), Sardinops sagax (σαρδέλα), Psettichthys melanostictus (γλώσσα) κ.ά.

Μετά το επεισόδιο του 1987 πολυάριθμες έρευνες τοξικότητας διενεργήθηκαν με σκοπό να προσδιοριστεί η δραστικότητα του DA σε διάφορα σπονδυλωτά είδη. Τα μέχρι τώρα δεδομένα δείχνουν ότι οι άνθρωποι είναι πιο ευαίσθητοι στο DA και εμφανίζουν δυσμενή συμπτώματα σε αρκετά χαμηλότερα επίπεδα της τοξίνης έναντι των τρωκτικών και των ψαριών.

Προκειμένου να προστατευτούν οι καταναλωτές των θαλασσινών, έπειτα από το περιστατικό του ASP στον Καναδά το 1987, οι αρχές θέσπισαν ένα όριο για το επίπεδο του DA που ανέρχεται στα 20 μg DA/g ιστού οστρακοειδών. Αν τα επίπεδα του DA υπερβαίνουν αυτό το όριο, τότε είναι δυνατόν αυτό να προκαλέσει την απομόνωση της μολυσμένης παραλίας ή περιοχή καλλιέργειας οστρακοειδών. Το παραπάνω όριο έχει υιοθετηθεί πλέον και από άλλες χώρες και εφαρμόζεται υποχρεωτικά στις Η.Π.Α., Ε.Ε., Νέα Ζηλανδία και Αυστραλία για μια ποικιλία οστρακοειδών όπως τα μύδια, χτένια και στρείδια.

Βιοτεχνολογία

Η μεγάλη αύξηση του ανθρώπινου πληθυσμού και η ανάπτυξη των πόλεων συμβάλλουν στην εξάντληση των φυσικών πόρων, στην αύξηση του κόστους παραγωγής τους και συντελούν στο φαινόμενο της κλιματικής αλλαγής. Για να ξεπεραστούν οι δυσκολίες στον εφοδιασμό του πληθυσμού και να μειωθεί το κόστος των πόρων η επιστημονική κοινότητα στράφηκε σε εναλλακτικές πηγές, όπως τα μικροφύκη (που συμπεριλαμβάνουν και τα διάτομα) χρησιμοποιούν διοξείδιο του άνθρακα (CO₂) για να παράγουν βιομάζα ή πολύτιμες ενώσεις.

Τα διάτομα έχουν διαδραματίσει καθοριστικό ρόλο στο οικοσύστημα για εκατομμύρια χρόνια ως μία σημαντική ομάδα παραγωγής οξυγόνου στη γη και ως μία από τις σημαντικότερες πηγές βιομάζας στους ωκεανούς. Τα φύκη καλλιεργούνται εδώ και πολλά χρόνια, αλλά μόλις πρόσφατα συνειδητοποιήθηκε ότι ο ωκεανός είναι μία σχετικά αναξιοποίητη και ανεξερεύνητη πηγή βιομάζας. Χάρη στην αποτελεσματική τους ικανότητα για φωτοσύνθεση, μετατρέπουν την φωτεινή ενέργεια σε χημική ενέργεια και σε οργανικά μόρια όπως οι υδατάνθρακες και τα λιπίδια. Μέχρι πριν από κάποια χρόνια τα διάτομα είχαν περιοριστεί σχεδόν αποκλειστικά στην βασική έρευνα με ελάχιστη εκτίμηση των πρακτικών τους χρήσεων πέρα από τις πιο υποτυπώδεις εφαρμογές. Έχουν καταβληθεί προσπάθειες για την καθιέρωσή τους ως αποφασιστικά χρήσιμες εμπορικές και βιομηχανικές εφαρμογές, αλλά και νανοτεχνολογίας.

Εφαρμογές

Οι βιοτεχνολογικές εφαρμογές είναι πολλές και ιδιαίτερα προσοδοφόρες, κάποιες από αυτές είναι:

Βιομηχανική χρήση: λιπάσματα, υδατάνθρακες για παραγωγή αιθανόλης μέσω ζύμωσης, πρωτεΐνες για παραγωγή μεθανίου μέσω αναερόβιας αεριοποίησης και φυσικά έλαια για παραγωγή βιοντίζελ. Το μεγάλο πλεονέκτημα που διαθέτουν τα διάτομα είναι οι διατροφικές τους συνήθειες, απαιτούν διοξείδιο του άνθρακα, νερό, ανόργανα άλατα και φως για να αναπτυχθούν. Επιπλέον, οι χώροι παραγωγής τους μπορούν να βρίσκονται οπουδήποτε, με αποτέλεσμα να μη στερούν καλλιεργήσιμη γη.

Νανοτεχνολογία: σήμερα τα διάτομα χρησιμοποιούνται ως συστατικά φίλτρων σε διαδικασίες καθαρισμού DNA και απορρόφησης βαρέων μετάλλων. Λόγω του ιδιαίτερου κυτταρικού τοιχώματος θα μπορούσαν μελλοντικά να χρησιμοποιηθούν και σε τσιπ υπολογιστών.

Φαρμακευτική και ιατρική χρήση: εμβόλια αντισώματα, ορμόνες, ένζυμα σκόνη ψύλλων, καλλυντικά και συστατικά οδοντόπαστας.

Διατροφική χρήση: βιταμίνες υψηλής ποιότητας, καθώς είναι πλούσια σε ακόρεστα λιπαρά οξέα και αμινοξέα, επίσης λόγω των υδατανθράκων και των λιπιδίων που παράγουν, μπορούν να αξιοποιηθούν ως τρόφιμα αλλά και ως ζωοτροφές.

Με τα τεχνολογικά άλματα των τελευταίων ετών αλλά και με την μεγάλη ανεκτικότητα σε ποικίλα περιβάλλοντα σε συνδυασμό με την οικονομική αποδοτικότητα, τα διάτομα προσφέρουν ευκαιρίες επαγγελματικές τόσο σε επιστημονικό όσο και σε επιχειρηματικό επίπεδο.

Πηγές

• Castro, P., Huber, Μ.Ε., (1992). Marine Biology. Mosby-Year Book, Inc, St. Louis.CS1 maint: Πολλαπλές ονομασίες: authors list (link)

• Lee, R. E., (2008). Phycology, fourth ed. Cambridge University Press, Cambridge.CS1 maint: Πολλαπλές ονομασίες: authors list (link)

• Μουστάκα – Γούνη, Μ., (1997). Ωκεανογραφία. Μια Βιολογική προσέγγιση. Εκδόσεις EXIN, Θεσσαλονίκη.CS1 maint: Πολλαπλές ονομασίες: authors list (link)

• Reynolds, C. S., (2006). Ecology of phytoplankton. Ecology, Biodiversity and Conservation. Cambridge University Press, Cambridge.CS1 maint: Πολλαπλές ονομασίες: authors list (link)

• Wetzel, R. G., (2001). Limnology: Lake and River Ecosystems, third ed. Academic Press, London.CS1 maint: Πολλαπλές ονομασίες: authors list (link)

• Lefebvre, K.A., Robertson, A., (2010). Domoic acid and human exposure risks: A review. Toxicon 56, 218-230.
• Moschandreou, K.K., Papaefthimiou, D., Katikou, P., Kalopesa, E., Panou, A., Nikolaidis, G., (2010). Morphology, phylogeny and toxin analysis of Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae) isolated from the Thermaikos Gulf, Greece. Phycologia 49, Issue 3, 260-273.
• Armbrust, E. (2009). The life of diatoms in the world's oceans. Nature, 459(7244), pp.185-192.
• Bozarth, A., Maier, U. and Zauner, S. (2008). Diatoms in biotechnology: modern tools and applications. Applied Microbiology and Biotechnology, 82(2), pp.195-201.
• Pulz, O. and Gross, W. (2004). Valuable products from biotechnology of microalgae. Applied Microbiology and Biotechnology, 65(6), pp.635-648.
• Mohammed, J. (2015). Micro- and nanotechnologies in plankton research. Progress in Oceanography, 134, pp.451-473.
• Vinayak, V., Manoylov, K., Gateau, H., Blanckaert, V., Hérault, J., Pencréac'h, G., Marchand, J., Gordon, R. and Schoefs, B. (2015). Diatom Milking: A Review and New Approaches. Marine Drugs, 13(5), pp.2629-2665.

Дијатомеите^[1] или силикатните алги (латински: Bacillariophyceae) се голема група на еукариотски алги со космоплитско распространување. Најголем дел од дијатомеите се едноклеточни, но има и такви кои формираат различни типови на колонии. Имињата на групата доаѓаат од морфологијата и градбата на нивните клеточни ѕидови кои се силикатни и се организирани во форма на кутија и поклопец. При микроскопските анализи вообичаено овие два дела се одвојуваат еден од друг па оттука и нивното име: грчки: διά = "низ" + τέμνειν = "да исечеш", односно "исечи на половина".

Биологија

Дијатомеите претставуваат многу разнообразна група на организми. Се смета дека постојат над 200 дијатомејски родови и над 100000 рецентни видови.^[2][3][4][5] Распространети се вo сите типови на слатководни и морски екосистеми како и на влажни почви. Вообичаено го населуваат пелагијалот на стагнантните води како дел од планктонската заедница (биоценоза) или литоралната зона каде формираат биофилмови на површината на седиментот, таканаречени бентосни заедници. Нивното големо значење особено се изразува во океаните каде се смета дека се одговорни за 45% од примарната продукција.

Дијатомеите се вклучени во групата на хетероконтните алги. Оваа голема подгрупа од протистите (Protista) е дефинирана со присуството на два нееднакви по градба флагелуми, поставени на предната и задната страна на клетката. Иако во адултна форма дијатомеите не поседуваат флагелуми овие структури се вообичаено застапени кај машките гамети кај оние дијатомеи кои се размножуваат со оогамен полов процес. Клетите содржат еден или поголем број жолто-кафеави хлоропласти обвиткани со четири мембрани кои се продукт на секундарна ендосимбиоза на хлоропластот (види Ендосимбиотска теорија). Основните хлорофилни пигменти се a и c, додека најзначаен додатен пигмент е фукоксантинот кој е кафеаво обоен. Резервните хранливи материи се депонираат во форма на хризоламинарин, покрај кој можат да се сретнат и масни капки или волутински гранули.

Клетките ги содржат вообичаените органели типични за останатите еукариотски организми. За нив специфична е везикулата за депозиција на силикат во која се врши синтеза на компонентите на силикатниот клеточен ѕид. Оваа органела е типична и за некои од другите хетероконтни алги кои имаат способност за синтеза на силикатни клеточни ѕидови (на пример, некои групи од златните алги Chysophyceae).

Клеточниот ѕид уште наречен и фрустула е типично граден и по својата комплексност и организација единствен во живиот свет. Составен е од два дела наречени теки кои се однесуваат како кутија и поклопец (хипо- и епитека). Секоја од теките е понатаму поделена на лицев дел или валва и страничен дел или плеура. Самата фрустула со сите компоненти е нераздвојливо споена и клетката е заробена во неа, единствениот момент во кој теките се раздвојуваат е при клеточна делба.

Дијатомеите се неподвижни во адултна форма. Способноста за движење е овозможена со специфичната структура наречена рафа присутна кај одредени групи на пенатни дијатомеи (редот Pennales). Движењето со помош на оваа структура (пукнатина на клеточниот ѕид) е лизгачко по површината на супстратот. Планктонските дијатомеи кои живеат слободно во водениот столб во основа се неподвижни и зависат од водените струи, миксијата и ветровите на површината за нивно придвижување. Покрај рафата на површината на клеточниот ѕид се забележуваат и безброј комплексни перфорации (пори, ареоли, ...) кои овозможуваат прекрасен орнаметиран изглед на клеточниот ѕид и енормна морфолошка варијабилност.

Животен циклус

Дијатомеите се диплонтски организми. Во целиот тек на нивниот живот поседуваат два комплети на хромозоми кој број се редуцира само при продукција на гамети.

Вегетативна делба

Имајќи в предвид дека теките се нераздвојливи, клетката во својата стаклена (силикат) кутија има ограничен раст. Со акумулација на резервни материи и самиот раст во фрустулата доаѓа зголемување на тургорниот притисок (тургор) со што се врши силен внатрешен притисок кој условува отварање на теките. Митотската делба се одвива докрај по што двете новодобиени клетки заджуваат една од теките на мајката. Онаа ќерка клетка која ја задржала мајкината епитеката (поголемата тека) формира хипотека, додека пак онаа ќерка клетка која ја задржала мајкината хипотека формира сопствена хипотека (мајкината хипотека во случајот претставува епитека). На ваков начин доаѓа до прогресивно намалување на димензиите на клетките во популацијата, односно како како популацијата расте (старее) така популацијата ја намалува својата големина. Овој процес е познат како правилото на MacDonald и Pfitzer. По поголем број на делби најголемиот дел од клетките во популацијата значајно ги намалуваат своите димензии до состојба кога во рамките клетoчниот ѕид нема повеќе простор за нормална функција. Овој момент во животниот циклус, односно оваа големина на клетките се нарекува долен праг на делба по чиешто достигнување клетката мора да премина на процес на полово размножување. Долниот праг на делба е видово специфичен и различен кај различни видови од истиот род.

Полово размножување

Половиот процес настапува кога клетката е на ниво на долниот праг. Вегетативна делба и намалување на димензиите под овој праг вообичаено предизвикуваат губење на способноста за полово размножување. Клетките кои се наоѓаат на овој праг на големина можат да преминат во гаметангиуми односно клетки кои ќе продуцираат гамети. Кај дијатомеите се застапени трите основни типови на полово размножување и тоа оогамија, анизогамија и изогамија при што се продуцираат изогамети, хетерогамети (различни по големина) како и јајце клетка и сперматозоиди кај оогамниот процес. Бројот на гаметите исто така може да варира и тоа од 1 до 8. По завршениот полов процес се добива зигот кој кај дијатомеите има типична градба и функција. Зиготот се нарекува ауксоспора и негова најзначајна функција е враќање на првобитната големина на популацијата од конкретниот вид. Ауксоспората е обвиткана со перизониум, еластична, силикатна обвивка составена од повеќе напречни ленти во форма на прстени и овозможува раст на зиготот во должина. Максималната должина која ја достигнува ауксоспората претставува горниот праг на големина и таа е исто така видово специфична. Зиготот е повторно доплоиден со што самиот вид се враќа кон двојниот број на хромозоми по клетка. По завршувањето на зиготната експанзија се формираат првите, ригидни валви под перизониумот. На ваков начин се добиваат првите адултни единки во популацијата кои се наречени иницијални клетки кои вообичаено имаат чуден, некарактеристичен изглед.

Класификација

Постојат повеќе различни системи за класификацијата на дијатомеите. Нивниот статус како група (Тип, Класа) се менува често во последно време како ресултат на новите сознанија од молекуларната биологија. Историски, најчесто беа класифицирани како Тип Bacillariophyta, но денес претставуваат класа Bacillariohyceae од Типот хетероконтни алги (Heterokontophyta) во кој се вклучени и на пример Кафеавите алги (Phaeophyceae) и Златните алги (Chrysophyceae).

Без разлика на повисоката класификација, дијатомеите секогаш се поделени врз база на симетријата на нивниот клеточен ѕид. Оттука ќе имаме дијатомеи со радијална симетрија, симетрија во однос на точка, кои се наречени центрични дијатомеи (Centrophyceae или Centrales) и дијатомеи со билатерална симетрија, во однос на права, наречени пенатни дијатомеи (Pennatophyceae или Pennales). Дали центричните или пенатните дијатомеи ќе имаат статус на Класа (ceae) или Ред (ales) зависи од повисоките систематски категории (погл. погоре).

Поврзано

• Алги
• Сликикати
• Силикатни минерали

Наводи

डायटम (Diatoms)^[6] सूक्ष्मशैवालों (microalgae) का प्रमुख समूह है जो सबसे आम पादपप्लवक (phytoplankton) भी हैं।

सन्दर्भ

இருகலப்பாசிகள் (இலங்கை வழக்கு: தயற்றம், ஆங்கிலம்: Diatoms) என்னும் சொல் இரண்டு எனப்பொருள் தரும் கிரேக்க மொழிச்சொற்களில் இருந்து உருவானது:: διά (dia, ட'யா) = "through" ("ஊடே")+ τέμνειν (temnein, டெம்னைன்) = "to cut" ("வெட்டு"), அதாவது "பாதியாய் பகுப்பது" ("cut in half" ) பாசிகளிலேயே மிகவும் தனித்தன்மை கொண்டவை. இவற்றின் அமைப்பு சலவைக் கட்டிகளை இட்டுவைக்கும் டப்பிக்களைப் போல, மேலே ஒரு கலமும் கீழே ஒரு கலமுமாக இருக்கும். இதன் கலங்கள் சிலிக்கா செல்களால் அமைந்தவை. ஒவ்வொறு சிற்றினமும் தங்களுக்கே உரிய பல்வேறு வேலைப்பாடுகள் மிகுந்த கல மேற்கூரையைக் கொண்டிருக்கும். இந்த வேலைப்பாடுகளே ஒரு சிற்றினத்திலிருந்து மற்றொன்றைக் கண்டுபிடிக்க வகைப்பாட்டியலில் உதவுகிறது.இருகலப் பாசிகளில் மட்டும் ஒரு லட்சத்துக்கும் மேற்பட்ட சிற்றினங்கள் உள்ளன.

இருகலப்பாசிகளின் இருப்பு அதனை சுற்றியுள்ள சுற்றுப்புறச்சூழலை பொருத்ததாகும். ஒவ்வொரு சிற்றினமும் தங்களுக்குரிய சூழ்நிலைக்கூறுகளுக்குள் (Ecological Niche) மட்டும் வாழும். இருகலப்பாசிகளின் இத்தகைய பண்புகள் இவற்றை மிகவும் சிறந்த உயிர் சுட்டிக்காட்டியாக (Bioindicator) உபயோகிக்க உதவுகின்றது. இருகல பாசிகளின் கல அமைப்பு சிலிகாவாலனது, அவை நைட்ரிக் அமிலத்தையும் எதிர்த்து நிற்க கூடியது. ஒவ்வொரு நீர் நிலையில் உள்ள இருகல பாசியின் வடிவமும் தனி தன்மை உடையது. அது மட்டுமின்றி ஒரே நீர் நிலையில் வெவ்வேறு கால நிலைகளில் வெவ்வேறு வடிவ இருகலப்பாசிகள் காணப்படும்.

இந்தியாவில் இருகலப்பாசிகள் ஆராய்ச்சி

இந்தியாவில் பாசிகளை பற்றிய ஆராய்ச்சியை தொடங்கியவர் சென்னையை சேர்ந்த எம்.ஓ.பி. ஐயங்கார். இவர் இந்திய பாசியியல் துறையின் தந்தை எனப் போற்றப்படுபவர். இவர் அனைத்து வகையான பாசிகளைப் பற்றிய ஆராய்ச்சியை தொடங்கினாலும். "இருகலப் பாசிகள்" பற்றிய ஆராய்ச்சியை இவரது மாணவர்கள் தொடங்கி வைத்தனர். இவரை தொடர்ந்து டி.வி. தேசிகாச்சாரி, குசராத்தை சேர்ந்த எச்.பி. காந்தி, மகாராட்டிராவை சேர்ந்த பி.டி. சரோட் மற்றும் என். டி. காமத் என்பவர்கள் இந்தியாவில் காணப்படும் இருகலப் பாசிகளை பற்றிய ஆராய்ச்சியை தொடர்ந்தார்கள்.

தடயவியலில் இருகலப்பாசிகள்

ஒருவர் நீரில் மூழ்கி இறக்கும் போது இருகலப் பாசிகள் நுரையீரலில் உள்ள காற்றுப் பைகள் வெடிப்பதன் மூலம் குருதி ஓட்டத்திற் கலந்து உடலின் பல்வேறு திசுக்களை அடைகின்றன. குறிப்பாக, என்பு மச்சையில் இவற்றின் இருப்பை தடயவியல் வல்லுநர்கள் பரிசோதிப்பர். ஒருவரை வேறு ஏதேனும் வழியிற் கொன்று விட்டு நீரிற் தூக்கிப் போட்டிருப்பின், அவரது எலும்பு மச்சையில் இருகலப்பாசி இருக்காது. ஏனென்றால் இருகலப்பாசி என்பு மச்சையை அடைய உயிருள்ள குருதி ஓட்டம் தேவை.

நன்னீரிற் காணப்படும் பல்வேறு இருகலப்பாசிச் சிற்றினங்கள்

மேற்கோள்களும் அடிக்குறிப்புகளும்

ಡಯಾಟಮ್ ಒಂದು ಪ್ರಮುಖ ಯುಕಾರ್ಯೊಟ (ಕೋಶಬೀಜ ಇರುವ ಜೀವ) ಶೈವಲ ಹಾಗೂ ಸಾಮಾನ್ಯ ಸಸ್ಯ ಪ್ಲವಕ ದಲ್ಲಿ ಒಂದು ವಿಧ. ಪ್ರಮುಖವಾಗಿ ಎಲ್ಲಾ ಡಯಾಟಮ್‍ಗಳು ಏಕಕೋಶ ಜೀವಿಗಳಾಗಿದ್ದರೂ, ತಾಮ್ಡೆಯಲಲ್ಲಿ ಕಂಡುಬರುತ್ತವೆ. ತಾಮ್ಡಯ ಆಕಾರವು ಎಳೆ ಅಥವಾ ಪಟ್ಟಿ (ಉ.ಹ. Fragillaria), ಬೀಸಣಿಗೆ (Meridion), ವಕ್ರವಾದ (Tabellaria), ಅಥವಾ ನಕ್ಷತ್ರಾಕಾರದಲ್ಲಿ (Asterionella) ಕಂಡುಬರುತ್ತದೆ. ಈ ಸಸ್ಯಗಳು ದ್ಯುತಿ ಸಂಶ್ಲೇಷಣೆಯಿಂದ ಆಹಾರವನ್ನು ತಯಾರಿಸುತ್ತವೆ. ಡಯಾಟಮ್ ಕೋಶಬಿತ್ತಿಯು ಎರಡು ಕವಾಟಗಳಿಂದ ಮಾಡಲ್ಪಟ್ಟಿದೆ. ಕವಾಟಗಳು ಸಿಲಿಕಾದಿಂದ ಕೂಡಿದ್ದು, ಒಂದು ಇನ್ನೊಂದನ್ನು ಮುಚ್ಚಿರುತ್ತದೆ. ಇವು ಡಬ್ಬಿಯಂತೆ ಕಾಣುತ್ತವೆ.ಈ ಕವಾಟಗಳಿಗೆ ಪ್ರೊಸ್ಥುಲ್ ಗಳೆಂದು ಕರೆಯುತ್ತಾರೆ. ಡಯಾಟಮ್ ಗಳು ಪ್ರಪಂಚದ ಎಲ್ಲಾ ಭಾಗಗಳಲ್ಲಿಯೂ ಕಂಡುಬರುತ್ತವೆ. ಸಿಹಿನೀರು ಮತ್ತು ಸಾಗರ ವ್ಯೆವಸ್ಥೆಗಳೆರಡರಲ್ಲೂ ಜೀವಿಸುತ್ತವೆ. ಡಯಾಟಮ್ ಗಳು ಜಲ ಆಹಾರ ಸರಪಣಿಯ ಅತಿ ಮುಖ್ಯ ಕೊಂಡಿಗಳು.

Several species of fresh-water diatoms.

ನೋಡಿ

• ಜೀವ ವಿಕಾಸವಾದ

ඩයටම් බැසිලාරියෝෆීටා වංශයට අයත් වේ. කරදිය ප්‍රාථමික නිපැයුම් ප්‍රජාවේ ප්‍රධාන ස්ථානයක් ගනී. 7/10ක් කරදිය වේ. මිරිදිය, කරදිය මතුපිට ස්ථරවල ඒක සෛලීය ප්‍රභා ස‍්වයංපෝෂී වේ. සෛල බිත්තිය සෙලියුලෝස්, පෙක්ටීන් හා ප්‍රධාන වශයෙන් සිලිකන් ඇත. දර්ශීය ජීවියා පිනියුලේරියා වේ.

ඩයටමයක්. Numbered graduations are 10 micrometres apart

ව්‍යුහය

මොවුන්ගේ විශේෂිත ම ලක්‍ෂණය සෛල වටා ඇති අලංකාර පාරදෘෂ්‍ය සිලිකන් කවචය යි. එක මත එක පිහිටි දෙපියනකින් යුතු ය. කවචයේ ඇති තන්තු ආධාරයෙන් චලනය විය හැක. ක්ලෝරොෆීල් A, B, හා කැරොටිනොයිඩ ඇත. ප්‍රභාසංස්ලේෂක වර්ණය ෆියුකොසැන්තීන් වේ. සංචිත ආකාරය ක්‍රිසොලැමිනරීන් වේ.

ප්‍රජනනය

අලිංගිකව ද්විඛණ්ඩනයෙන් ප්‍රජනනය කරන අතර ලිංගිකව ඌනන විභාජනයෙන් කශිකා සහිත සචල පුංජන්මාණුවක් හා අචල ඩිම්බ සාදයි. පුං ජන්මාණුවට තනි කශිකාවක් ඇත.

මූලාශ්‍ර

• E. Virginia Armbrust, et al. The Genome of the Diatom Thalassiosira Pseudonana: Ecology, Evolution, and Metabolism. Science 306:79-86. 2004.
• Ben Waggoner. University of California, Berkeley. Diatoms: Life, History and Ecology.
• Christopher D. Clack, Eileen Cox, Katie Bantley.. Diatom Morphogenesis. 1994.
• Doe Joint Genome Institute. Diatom Genome Reveals key Role in Biosphere's Carabon Cycle. Sept 30, 2004.
• E. Fourtanier, J.P. Kociolek, J. Demouthe California Academy of Sciences. Department of Inverebrate Zoology and Geology. Diatom Collection. Bacillariophyta.
• J. La Claire.University of Texas. Selected handouts For Biology 324. Diatom Life Cycle and History. January 2005.
• Jacob Feldman. Bacillariophytes - Diatoms.
• Jan Parmentier
• Marine Diatoms. Microscopy UK.
• The Micropaleontology Society. Diatoms: Algae with Animal Traits. Science, 1 Oct., 2004.
• NCBI. Taxonomy browser.
• Owen Davis. University of Arizona. Micropaleontology. April 2003.
• Staff, Star Tribune. Species list. June 11, 2004.
• Urban Rivers Awareness. Diatoms.

A diatom (Neo-Latin diatoma)^[a] is any member of a large group comprising several genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year,^[10][11] take in over 6.7 billion tonnes of silicon each year from the waters in which they live,^[12] and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile (800 m) deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara, much of it from the Bodélé Depression, which was once made up of a system of fresh-water lakes.^[13][14]

Diatoms are unicellular organisms: they occur either as solitary cells or in colonies, which can take the shape of ribbons, fans, zigzags, or stars. Individual cells range in size from 2 to 200 micrometers.^[15] In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles approximately every 24 hours by asexual multiple fission; the maximum life span of individual cells is about six days.^[16] Diatoms have two distinct shapes: a few (centric diatoms) are radially symmetric, while most (pennate diatoms) are broadly bilaterally symmetric. A unique feature of diatom anatomy is that they are surrounded by a cell wall made of silica (hydrated silicon dioxide), called a frustule.^[17] These frustules have structural coloration due to their photonic nanostructure, prompting them to be described as "jewels of the sea" and "living opals". Movement in diatoms primarily occurs passively as a result of both ocean currents and wind-induced water turbulence; however, male gametes of centric diatoms have flagella, permitting active movement to seek female gametes. Similar to plants, diatoms convert light energy to chemical energy by photosynthesis, but their chloroplasts were acquired in different ways.^[18]

Unusually for autotrophic organisms, diatoms possess a urea cycle, a feature that they share with animals, although this cycle is used to different metabolic ends in diatoms. The family Rhopalodiaceae also possess a cyanobacterial endosymbiont called a spheroid body. This endosymbiont has lost its photosynthetic properties, but has kept its ability to perform nitrogen fixation, allowing the diatom to fix atmospheric nitrogen.^[19] Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera Hemiaulus, Rhizosolenia and Chaetoceros.^[20]

Dinotoms are diatoms that have become endosymbionts inside dinoflagellates. Research on the dinoflagellates Durinskia baltica and Glenodinium foliaceum have shown that the endosymbiont event happened so recently, evolutionarily speaking, that their organelles and genome are still intact with minimum to no gene loss. The main difference between these and free living diatoms is that they have lost their cell wall of silica, making them the only known shell-less diatoms.^[21]

The study of diatoms is a branch of phycology. Diatoms are classified as eukaryotes, organisms with a nuclear envelope-bound cell nucleus, that separates them from the prokaryotes archaea and bacteria. Diatoms are a type of plankton called phytoplankton, the most common of the plankton types. Diatoms also grow attached to benthic substrates, floating debris, and on macrophytes. They comprise an integral component of the periphyton community.^[22] Another classification divides plankton into eight types based on size: in this scheme, diatoms are classed as microalgae. Several systems for classifying the individual diatom species exist.

Fossil evidence suggests that diatoms originated during or before the early Jurassic period, which was about 150 to 200 million years ago. The oldest fossil evidence for diatoms is a specimen of extant genus Hemiaulus in Late Jurassic aged amber from Thailand.^[23]

Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatomaceous earth (diatomite) is a collection of diatom shells found in the earth's crust. They are soft, silica-containing sedimentary rocks which are easily crumbled into a fine powder and typically have a particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, and as a dynamite stabilizer.

Overview

Diatoms are protists that form massive annual spring and fall blooms in aquatic environments and are estimated to be responsible for about half of photosynthesis in the global oceans.^[27] This predictable annual bloom dynamic fuels higher trophic levels and initiates delivery of carbon into the deep ocean biome. Diatoms have complex life history strategies that are presumed to have contributed to their rapid genetic diversification into ~200,000 species ^[28] that are distributed between the two major diatom groups: centrics and pennates.^[29][30]

Morphology

Diatoms are generally 2 to 200 micrometers in size,^[15] with a few larger species. Their yellowish-brown chloroplasts, the site of photosynthesis, are typical of heterokonts, having four cell membranes and containing pigments such as the carotenoid fucoxanthin. Individuals usually lack flagella, but they are present in male gametes of the centric diatoms and have the usual heterokont structure, including the hairs (mastigonemes) characteristic in other groups.

Diatoms are often referred as "jewels of the sea" or "living opals" due to their optical properties.^[31] The biological function of this structural coloration is not clear, but it is speculated that it may be related to communication, camouflage, thermal exchange and/or UV protection.^[32]

Diatoms build intricate hard but porous cell walls called frustules composed primarily of silica.^{[33]: 25–30} This siliceous wall^[34] can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations; all of which can be used to delineate genera and species.

The cell itself consists of two halves, each containing an essentially flat plate, or valve, and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical. Their distinguishing feature is a hard mineral shell or frustule composed of opal (hydrated, polymerized silicic acid).

Representation of a diatom

1) Nucleus; holds the genetic material
2) Nucleolus; location of the chromosomes
3) Golgi apparatus; modifies proteins and sends them out of the cell
4) Cell wall; outer membrane of the cell
5) Pyrenoid; center of carbon fixation
6) Chromatophore; pigment carrying membrane structure
7) Vacuoles; vesicle of a cell that contains fluid bound by a membrane
8) Cytoplasmic strands; hold the nucleus
9) Mitochondria; create ATP (energy) for the cell
10) Valves/Striae; allow nutrients in, and waste out, of the cell

Intricate structures of the diatom

a) Areolae (hexagonal or polygonal boxlike perforation with a sieve present on the surface of diatom)
b) Striae (pores, punctae, spots or dots in a line on the surface)
c) Raphe (slit in the valves)
d) Central nodule (thickening of wall at the midpoint of raphe)
e) Stigmata (holes through valve surface which looks rounded externally but with a slit like internal)
f) Punctae (spots or small perforations on the surface)
g) Polar nodules (thickening of wall at the distal ends of the raphe) ^[35][36]

Selections from Ernst Haeckel's 1904 Kunstformen der Natur (Art Forms of Nature), showing pennate (left) and centric (right) frustules.

Diatoms are divided into two groups that are distinguished by the shape of the frustule: the centric diatoms and the pennate diatoms.

Pennate diatoms are bilaterally symmetric. Each one of their valves have openings that are slits along the raphes and their shells are typically elongated parallel to these raphes. They generate cell movement through cytoplasm that streams along the raphes, always moving along solid surfaces.

Centric diatoms are radially symmetric. They are composed of upper and lower valves – epitheca and hypotheca – each consisting of a valve and a girdle band that can easily slide underneath each other and expand to increase cell content over the diatoms progression. The cytoplasm of the centric diatom is located along the inner surface of the shell and provides a hollow lining around the large vacuole located in the center of the cell. This large, central vacuole is filled by a fluid known as "cell sap" which is similar to seawater but varies with specific ion content. The cytoplasmic layer is home to several organelles, like the chloroplasts and mitochondria. Before the centric diatom begins to expand, its nucleus is at the center of one of the valves and begins to move towards the center of the cytoplasmic layer before division is complete. Centric diatoms have a variety of shapes and sizes, depending on from which axis the shell extends, and if spines are present.

Shape classification of diatom frustules. The images are 3D models. The actual sizes of the frustules are about 10–80 μm.^[37]

Structure of a centric diatom frustule ^[37]

Silicification

Diatom cells are contained within a unique silica cell wall known as a frustule made up of two valves called thecae, that typically overlap one another.^[38] The biogenic silica composing the cell wall is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is then extruded to the cell exterior and added to the wall. In most species, when a diatom divides to produce two daughter cells, each cell keeps one of the two-halves and grows a smaller half within it. As a result, after each division cycle, the average size of diatom cells in the population gets smaller. Once such cells reach a certain minimum size, rather than simply divide, they reverse this decline by forming an auxospore. This expands in size to give rise to a much larger cell, which then returns to size-diminishing divisions. Auxospore production is almost always linked to meiosis and sexual reproduction.

Pennate diatom from an Arctic meltpond, infected with two chytrid-like [zoo-]sporangium fungal pathogens (in false-colour red). Scale bar = 10 μm.^[39]

Light microscopy of a living diatom. Numbered graduations are 10 micrometres apart

The exact mechanism of transferring silica absorbed by the diatom to the cell wall is unknown. Much of the sequencing of diatom genes comes from the search for the mechanism of silica uptake and deposition in nano-scale patterns in the frustule. The most success in this area has come from two species, Thalassiosira pseudonana, which has become the model species, as the whole genome was sequenced and methods for genetic control were established, and Cylindrotheca fusiformis, in which the important silica deposition proteins silaffins were first discovered.^[40] Silaffins, sets of polycationic peptides, were found in C. fusiformis cell walls and can generate intricate silica structures. These structures demonstrated pores of sizes characteristic to diatom patterns. When T. pseudonana underwent genome analysis it was found that it encoded a urea cycle, including a higher number of polyamines than most genomes, as well as three distinct silica transport genes.^[41] In a phylogenetic study on silica transport genes from 8 diverse groups of diatoms, silica transport was found to generally group with species.^[40] This study also found structural differences between the silica transporters of pennate (bilateral symmetry) and centric (radial symmetry) diatoms. The sequences compared in this study were used to create a diverse background in order to identify residues that differentiate function in the silica deposition process. Additionally, the same study found that a number of the regions were conserved within species, likely the base structure of silica transport.

These silica transport proteins are unique to diatoms, with no homologs found in other species, such as sponges or rice. The divergence of these silica transport genes is also indicative of the structure of the protein evolving from two repeated units composed of five membrane bound segments, which indicates either gene duplication or dimerization.^[40] The silica deposition that takes place from the membrane bound vesicle in diatoms has been hypothesized to be a result of the activity of silaffins and long chain polyamines. This Silica Deposition Vesicle (SDV) has been characterized as an acidic compartment fused with Golgi-derived vesicles.^[42] These two protein structures have been shown to create sheets of patterned silica in-vivo with irregular pores on the scale of diatom frustules. One hypothesis as to how these proteins work to create complex structure is that residues are conserved within the SDV's, which is unfortunately difficult to identify or observe due to the limited number of diverse sequences available. Though the exact mechanism of the highly uniform deposition of silica is as yet unknown, the Thalassiosira pseudonana genes linked to silaffins are being looked to as targets for genetic control of nanoscale silica deposition.

The ability of diatoms to make silica-based cell walls has been the subject of fascination for centuries. It started with a microscopic observation by an anonymous English country nobleman in 1703, who observed an object that looked like a chain of regular parallelograms and debated whether it was just crystals of salt, or a plant.^[43] The viewer decided that it was a plant because the parallelograms didn't separate upon agitation, nor did they vary in appearance when dried or subjected to warm water (in an attempt to dissolve the "salt"). Unknowingly, the viewer's confusion captured the essence of diatoms—mineral utilizing plants. It is not clear when it was determined that diatom cell walls are made of silica, but in 1939 a seminal reference characterized the material as silicic acid in a "subcolloidal" state^[44] Identification of the main chemical component of the cell wall spurred investigations into how it was made. These investigations have involved, and been propelled by, diverse approaches including, microscopy, chemistry, biochemistry, material characterisation, molecular biology, 'omics, and transgenic approaches. The results from this work have given a better understanding of cell wall formation processes, establishing fundamental knowledge which can be used to create models that contextualise current findings and clarify how the process works.^[45]

The process of building a mineral-based cell wall inside the cell, then exporting it outside, is a massive event that must involve large numbers of genes and their protein products. The act of building and exocytosing this large structural object in a short time period, synched with cell cycle progression, necessitates substantial physical movements within the cell as well as dedication of a significant proportion of the cell's biosynthetic capacities.^[45]

The first characterisations of the biochemical processes and components involved in diatom silicification were made in the late 1990s.^[46][47][48] These were followed by insights into how higher order assembly of silica structures might occur.^[49][50][51] More recent reports describe the identification of novel components involved in higher order processes, the dynamics documented through real-time imaging, and the genetic manipulation of silica structure.^[52][53] The approaches established in these recent works provide practical avenues to not only identify the components involved in silica cell wall formation but to elucidate their interactions and spatio-temporal dynamics. This type of holistic understanding will be necessary to achieve a more complete understanding of cell wall synthesis.^[45]

Behaviour

Chaetoceros willei
Gran, 1897

Chaetoceros furcillatus J.W.Bailey, 1856

Most centric and araphid pennate diatoms are nonmotile, and their relatively dense cell walls cause them to readily sink. Planktonic forms in open water usually rely on turbulent mixing of the upper layers of the oceanic waters by the wind to keep them suspended in sunlit surface waters. Many planktonic diatoms have also evolved features that slow their sinking rate, such as spines or the ability to grow in colonial chains.^[54] These adaptations increase their surface area to volume ratio and drag, allowing them to stay suspended in the water column longer. Individual cells may regulate buoyancy via an ionic pump.^[55]

Some pennate diatoms are capable of a type of locomotion called "gliding", which allows them to move across surfaces via adhesive mucilage secreted through a seamlike structure called the raphe.^[56][57] In order for a diatom cell to glide, it must have a solid substrate for the mucilage to adhere to.

Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures; mucilage pads, stalks or tubes; amorphous masses of mucilage; or by threads of chitin (polysaccharide), which are secreted through strutted processes of the cell.

Planktonic diatoms such as Thalassiosira sp. (56-62), Asteromphalus sp. (63), Aulacoseira sp. (64-66), and Chaetoceros (see twin image above) often grow in chains, and have features such as spines which slow sinking rates by increasing drag.

Some Thalassiosira diatoms form chain-like colonies, like these collected near the Antarctic peninsula coast by the schooner of the Tara Oceans Expedition for plankton research.
This projection of a stack of confocal images shows the diatoms' cell wall (cyan), chloroplasts (red), DNA (blue), membranes and organelles (green).

Life cycle

Sexual reproduction

Centric diatom (oogamy)

Pennate diatom (morphological isogamy, physiological anisogamy)

Reproduction and cell size

Reproduction among these organisms is asexual by binary fission, during which the diatom divides into two parts, producing two "new" diatoms with identical genes. Each new organism receives one of the two frustules – one larger, the other smaller – possessed by the parent, which is now called the epitheca; and is used to construct a second, smaller frustule, the hypotheca. The diatom that received the larger frustule becomes the same size as its parent, but the diatom that received the smaller frustule remains smaller than its parent. This causes the average cell size of this diatom population to decrease.^[15] It has been observed, however, that certain taxa have the ability to divide without causing a reduction in cell size.^[58] Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and auxospore formation must occur.^[15]

Cell division

Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form the zygote. The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore. A new diatom cell of maximum size, the initial cell, forms within the auxospore thus beginning a new generation. Resting spores may also be formed as a response to unfavourable environmental conditions with germination occurring when conditions improve.^[33]

A defining characteristic of all diatoms is their restrictive and bipartite silica cell wall that causes them to progressively shrink during asexual cell division. At a critically small cell size and under certain conditions, auxosporulation restitutes cell size and prevents clonal death.^{[59][60][61][62][63]} The entire lifecycles of only a few diatoms have been described and rarely have sexual events been captured in the environment.^[30]

Sperm motility

Diatoms are mostly non-motile; however, sperm found in some species can be flagellated, though motility is usually limited to a gliding motion.^[33] In centric diatoms, the small male gametes have one flagellum while the female gametes are large and non-motile (oogamous). Conversely, in pennate diatoms both gametes lack flagella (isogamous).^[15] Certain araphid species, that is pennate diatoms without a raphe (seam), have been documented as anisogamous and are, therefore, considered to represent a transitional stage between centric and raphid pennate diatoms, diatoms with a raphe.^[58]

Degradation by microbes

Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by using hydrolytic enzymes to break down the organic algal material.^[64][65]

Ecology

• Category

Regions of high abundance of diatoms in the ocean

Diatom dominance (as a percentage of total cell counts)
versus silicate concentration ^[66]

Distribution

Diatoms are a widespread group and can be found in the oceans, in fresh water, in soils, and on damp surfaces. They are one of the dominant components of phytoplankton in nutrient-rich coastal waters and during oceanic spring blooms, since they can divide more rapidly than other groups of phytoplankton.^[67] Most live pelagically in open water, although some live as surface films at the water-sediment interface (benthic), or even under damp atmospheric conditions. They are especially important in oceans, where they contribute an estimated 45% of the total oceanic primary production of organic material.^[68] Spatial distribution of marine phytoplankton species is restricted both horizontally and vertically.^[69][33]

Growth

Planktonic diatoms in freshwater and marine environments typically exhibit a "boom and bust" (or "bloom and bust") lifestyle. When conditions in the upper mixed layer (nutrients and light) are favourable (as at the spring), their competitive edge and rapid growth rate^[67] enables them to dominate phytoplankton communities ("boom" or "bloom"). As such they are often classed as opportunistic r-strategists (i.e. those organisms whose ecology is defined by a high growth rate, r).

Impact

The freshwater diatom Didymosphenia geminata, commonly known as Didymo, causes severe environmental degradation in water-courses where it blooms, producing large quantities of a brown jelly-like material called "brown snot" or "rock snot". This diatom is native to Europe and is an invasive species both in the antipodes and in parts of North America.^[70][71] The problem is most frequently recorded from Australia and New Zealand.^[72]

When conditions turn unfavourable, usually upon depletion of nutrients, diatom cells typically increase in sinking rate and exit the upper mixed layer ("bust"). This sinking is induced by either a loss of buoyancy control, the synthesis of mucilage that sticks diatoms cells together, or the production of heavy resting spores. Sinking out of the upper mixed layer removes diatoms from conditions unfavourable to growth, including grazer populations and higher temperatures (which would otherwise increase cell metabolism). Cells reaching deeper water or the shallow seafloor can then rest until conditions become more favourable again. In the open ocean, many sinking cells are lost to the deep, but refuge populations can persist near the thermocline.

Ultimately, diatom cells in these resting populations re-enter the upper mixed layer when vertical mixing entrains them. In most circumstances, this mixing also replenishes nutrients in the upper mixed layer, setting the scene for the next round of diatom blooms. In the open ocean (away from areas of continuous upwelling^[73]), this cycle of bloom, bust, then return to pre-bloom conditions typically occurs over an annual cycle, with diatoms only being prevalent during the spring and early summer. In some locations, however, an autumn bloom may occur, caused by the breakdown of summer stratification and the entrainment of nutrients while light levels are still sufficient for growth. Since vertical mixing is increasing, and light levels are falling as winter approaches, these blooms are smaller and shorter-lived than their spring equivalents.

In the open ocean, the diatom (spring) bloom is typically ended by a shortage of silicon. Unlike other minerals, the requirement for silicon is unique to diatoms and it is not regenerated in the plankton ecosystem as efficiently as, for instance, nitrogen or phosphorus nutrients. This can be seen in maps of surface nutrient concentrations – as nutrients decline along gradients, silicon is usually the first to be exhausted (followed normally by nitrogen then phosphorus).

Because of this bloom-and-bust cycle, diatoms are believed to play a disproportionately important role in the export of carbon from oceanic surface waters^[73][74] (see also the biological pump). Significantly, they also play a key role in the regulation of the biogeochemical cycle of silicon in the modern ocean.^[68][75]

Reason for success

Diatoms are ecologically successful, and occur in virtually every environment that contains water – not only oceans, seas, lakes, and streams, but also soil and wetlands. The use of silicon by diatoms is believed by many researchers to be the key to this ecological success. Raven (1983)^[76] noted that, relative to organic cell walls, silica frustules require less energy to synthesize (approximately 8% of a comparable organic wall), potentially a significant saving on the overall cell energy budget. In a now classic study, Egge and Aksnes (1992)^[66] found that diatom dominance of mesocosm communities was directly related to the availability of silicic acid – when concentrations were greater than 2 μmol m⁻³, they found that diatoms typically represented more than 70% of the phytoplankton community. Other researchers^[77] have suggested that the biogenic silica in diatom cell walls acts as an effective pH buffering agent, facilitating the conversion of bicarbonate to dissolved CO₂ (which is more readily assimilated). More generally, notwithstanding these possible advantages conferred by their use of silicon, diatoms typically have higher growth rates than other algae of the same corresponding size.^[67]

Sources for collection

Diatoms can be obtained from multiple sources.^[78] Marine diatoms can be collected by direct water sampling, and benthic forms can be secured by scraping barnacles, oyster and other shells. Diatoms are frequently present as a brown, slippery coating on submerged stones and sticks, and may be seen to "stream" with river current. The surface mud of a pond, ditch, or lagoon will almost always yield some diatoms. Living diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. Cladophora is frequently covered with Cocconeis, an elliptically shaped diatom; Vaucheria is often covered with small forms. Since diatoms form an important part of the food of molluscs, tunicates, and fishes, the alimentary tracts of these animals often yield forms that are not easily secured in other ways. Diatoms can be made to emerge by filling a jar with water and mud, wrapping it in black paper and letting direct sunlight fall on the surface of the water. Within a day, the diatoms will come to the top in a scum and can be isolated.^[78]

Biogeochemistry

• 

  The modern oceanic silicon cycle
  Fluxes are in T mol Si y⁻¹ (28 million metric tons of silicon per year)

Silica cycle

The diagram shows the major fluxes of silicon in the current ocean. Most biogenic silica in the ocean (silica produced by biological activity) comes from diatoms. Diatoms extract dissolved silicic acid from surface waters as they grow, and return it to the water column when they die. Inputs of silicon arrive from above via aeolian dust, from the coasts via rivers, and from below via seafloor sediment recycling, weathering, and hydrothermal activity.^[75]

Although diatoms may have existed since the Triassic, the timing of their ascendancy and "take-over" of the silicon cycle occurred more recently. Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle.^[79][80][81] Subsequently, the cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter-feeders primarily on the continental shelves.^[82] Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms.

However, the precise timing of the "take-over" remains unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the displacement of siliceous sponges from the shelves,^[83] suggests that this takeover began in the Cretaceous (146 Ma to 66 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (66 Ma to present).^[84]

• 

  Ocean carbon cycle and diatom carbon dioxide concentration mechanisms ^[85]

Carbon cycle

The diagram depicts some mechanisms by which marine diatoms contribute to the biological carbon pump and influence the ocean carbon cycle. The anthropogenic CO₂ emission to the atmosphere (mainly generated by fossil fuel burning and deforestation) is nearly 11 gigatonne carbon (GtC) per year, of which almost 2.5 GtC is taken up by the surface ocean. In surface seawater (pH 8.1–8.4), bicarbonate (HCO⁻
₃) and carbonate ions (CO²⁻
₃) constitute nearly 90 and <10% of dissolved inorganic carbon (DIC) respectively, while dissolved CO₂ (CO₂ aqueous) contributes <1%. Despite this low level of CO₂ in the ocean and its slow diffusion rate in water, diatoms fix 10–20 GtC annually via photosynthesis thanks to their carbon dioxide concentrating mechanisms, allowing them to sustain marine food chains. In addition, 0.1–1% of this organic material produced in the euphotic layer sinks down as particles, thus transferring the surface carbon toward the deep ocean and sequestering atmospheric CO₂ for thousands of years or longer. The remaining organic matter is remineralized through respiration. Thus, diatoms are one of the main players in this biological carbon pump, which is arguably the most important biological mechanism in the Earth System allowing CO₂ to be removed from the carbon cycle for very long period.^[86][85]

• 

  Mitochondrial urea cycle in a generic diatom cell and the potential fates of urea cycle intermediates ^[87]

Urea cycle

A feature of diatoms is the urea cycle, which links them evolutionarily to animals. In 2011, Allen et al. established that diatoms have a functioning urea cycle. This result was significant, since prior to this, the urea cycle was thought to have originated with the metazoans which appeared several hundreds of millions of years before the diatoms. Their study demonstrated that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionarily linked in such a way that animals and plants are not.^[88]

While often overlooked in photosynthetic organisms, the mitochondria also play critical roles in energy balance. Two nitrogen-related pathways are relevant and they may also change under ammonium (NH⁺
₄) nutrition compared with nitrate (NO⁻
₃) nutrition. First, in diatoms, and likely some other algae, there is a urea cycle.^[89][90][91] The long-known function of the urea cycle in animals is to excrete excess nitrogen produced by amino acid catabolism; like photorespiration, the urea cycle had long been considered a waste pathway. However, in diatoms the urea cycle appears to play a role in exchange of nutrients between the mitochondria and the cytoplasm, and potentially the plastid ^[92] and may help to regulate ammonium metabolism.^[89][90] Because of this cycle, marine diatoms, in contrast to chlorophytes, also have acquired a mitochondrial urea transporter and, in fact, based on bioinformatics, a complete mitochondrial GS-GOGAT cycle has been hypothesised.^[90][87]

Other

Diatoms are mainly photosynthetic; however a few are obligate heterotrophs and can live in the absence of light provided an appropriate organic carbon source is available.^[93][94]

Photosynthetic diatoms that find themselves in an environment absent of oxygen and/or sunlight can switch to anaerobic respiration known as nitrate respiration (DNRA), and stay dormant for up till months and decades.^[95][96]

Major pigments of diatoms are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.^[15]

Taxonomy

Light microscopy of several species of living freshwater diatoms

Centric diatom

Linked diatoms

Thalassiosirales
Stephanodiscus hantzschii

Coscinodiscophyceae
Isthmia nervosaIsthmia nervosa

Coscinodiscophyceae
Odontella aurita

Diatoms belong to a large group of protists, many of which contain plastids rich in chlorophylls a and c. The group has been variously referred to as heterokonts, chrysophytes, chromists or stramenopiles. Many are autotrophs such as golden algae and kelp; and heterotrophs such as water moulds, opalinids, and actinophryid heliozoa. The classification of this area of protists is still unsettled. In terms of rank, they have been treated as a division, phylum, kingdom, or something intermediate to those. Consequently, diatoms are ranked anywhere from a class, usually called Diatomophyceae or Bacillariophyceae, to a division (=phylum), usually called Bacillariophyta, with corresponding changes in the ranks of their subgroups.

Genera and species

An estimated 20,000 extant diatom species are believed to exist, of which around 12,000 have been named to date according to Guiry, 2012^[97] (other sources give a wider range of estimates^{[15][98][99][100]}). Around 1,000–1,300 diatom genera have been described, both extant and fossil,^[101][102] of which some 250–300 exist only as fossils.^[103]

Classes and orders

For many years the diatoms—treated either as a class (Bacillariophyceae) or a phylum (Bacillariophyta)—were divided into just 2 orders, corresponding to the centric and the pennate diatoms (Centrales and Pennales). This classification was extensively overhauled by Round, Crawford and Mann in 1990 who treated the diatoms at a higher rank (division, corresponding to phylum in zoological classification), and promoted the major classification units to classes, maintaining the centric diatoms as a single class Coscinodiscophyceae, but splitting the former pennate diatoms into 2 separate classes, Fragilariophyceae and Bacillariophyceae (the latter older name retained but with an emended definition), between them encompassing 45 orders, the majority of them new.

Today (writing at mid 2020) it is recognised that the 1990 system of Round et al. is in need of revision with the advent of newer molecular work, however the best system to replace it is unclear, and current systems in widespread use such as AlgaeBase, the World Register of Marine Species and its contributing database DiatomBase, and the system for "all life" represented in Ruggiero et al., 2015, all retain the Round et al. treatment as their basis, albeit with diatoms as a whole treated as a class rather than division/phylum, and Round et al.'s classes reduced to subclasses, for better agreement with the treatment of phylogenetically adjacent groups and their containing taxa. (For references refer the individual sections below).

One proposal, by Linda Medlin and co-workers commencing in 2004, is for some of the centric diatom orders considered more closely related to the pennates to be split off as a new class, Mediophyceae, itself more closely aligned with the pennate diatoms than the remaining centrics. This hypothesis—later designated the Coscinodiscophyceae-Mediophyceae-Bacillariophyceae, or Coscinodiscophyceae+(Mediophyceae+Bacillariophyceae) (CMB) hypothesis—has been accepted by D.G. Mann among others, who uses it as the basis for the classification of diatoms as presented in Adl. et al.'s series of syntheses (2005, 2012, 2019), and also in the Bacillariophyta chapter of the 2017 Handbook of the Protists edited by Archibald et al., with some modifications reflecting the apparent non-monophyly of Medlin et al. original "Coscinodiscophyceae". Meanwhile, a group led by E.C. Theriot favours a different hypothesis of phylogeny, which has been termed the structural gradation hypothesis (SGH) and does not recognise the Mediophyceae as a monophyletic group, while another analysis, that of Parks et al., 2018, finds that the radial centric diatoms (Medlin et al.'s Coscinodiscophyceae) are not monophyletic, but supports the monophyly of Mediophyceae minus Attheya, which is an anomalous genus. Discussion of the relative merits of these conflicting schemes continues by the various parties involved.^{[104][105][106][107]}

Adl et al., 2019 treatment

In 2019, Adl et al.^[108] presented the following classification of diatoms, while noting: "This revision reflects numerous advances in the phylogeny of the diatoms over the last decade. Due to our poor taxon sampling outside of the Mediophyceae and pennate diatoms, and the known and anticipated diversity of all diatoms, many clades appear at a high classification level (and the higher level classification is rather flat)." This classification treats diatoms as a phylum (Diatomeae/Bacillariophyta), accepts the class Mediophyceae of Medlin and co-workers, introduces new subphyla and classes for a number of otherwise isolated genera, and re-ranks a number of previously established taxa as subclasses, but does not list orders or families. Inferred ranks have been added for clarity (Adl. et al. do not use ranks, but the intended ones in this portion of the classification are apparent from the choice of endings used, within the system of botanical nomenclature employed).

• Clade Diatomista Derelle et al. 2016, emend. Cavalier-Smith 2017 (diatoms plus a subset of other ochrophyte groups)

• Phylum Diatomeae Dumortier 1821 [= Bacillariophyta Haeckel 1878] (diatoms)

• Subphylum Leptocylindrophytina D.G. Mann in Adl et al. 2019

• Class Leptocylindrophyceae D.G. Mann in Adl et al. 2019 (Leptocylindrus, Tenuicylindrus)
• Class Corethrophyceae D.G. Mann in Adl et al. 2019 (Corethron)

• Subphylum Ellerbeckiophytina D.G. Mann in Adl et al. 2019 (Ellerbeckia)
• Subphylum Probosciophytina D.G. Mann in Adl et al. 2019 (Proboscia)
• Subphylum Melosirophytina D.G. Mann in Adl et al. 2019 (Aulacoseira, Melosira, Hyalodiscus, Stephanopyxis, Paralia, Endictya)
• Subphylum Coscinodiscophytina Medlin & Kaczmarska 2004, emend. (Actinoptychus, Coscinodiscus, Actinocyclus, Asteromphalus, Aulacodiscus, Stellarima)
• Subphylum Rhizosoleniophytina D.G. Mann in Adl et al. 2019 (Guinardia, Rhizosolenia, Pseudosolenia)
• Subphylum Arachnoidiscophytina D.G. Mann in Adl et al. 2019 (Arachnoidiscus)
• Subphylum Bacillariophytina Medlin & Kaczmarska 2004, emend.

• Class Mediophyceae Jouse & Proshkina-Lavrenko in Medlin & Kaczmarska 2004

• Subclass Chaetocerotophycidae Round & R.M. Crawford in Round et al. 1990, emend.
• Subclass Lithodesmiophycidae Round & R.M. Crawford in Round et al. 1990, emend.
• Subclass Thalassiosirophycidae Round & R.M. Crawford in Round et al. 1990
• Subclass Cymatosirophycidae Round & R.M. Crawford in Round et al. 1990
• Subclass Odontellophycidae D.G. Mann in Adl et al. 2019
• Subclass Chrysanthemodiscophycidae D.G. Mann in Adl et al. 2019

• Class Biddulphiophyceae D.G. Mann in Adl et al. 2019

• Subclass Biddulphiophycidae Round and R.M. Crawford in Round et al. 1990, emend.
• Biddulphiophyceae incertae sedis (Attheya)

• Class Bacillariophyceae Haeckel 1878, emend.

• Bacillariophyceae incertae sedis (Striatellaceae)
• Subclass Urneidophycidae Medlin 2016
• Subclass Fragilariophycidae Round in Round, Crawford & Mann 1990, emend.
• Subclass Bacillariophycidae D.G. Mann in Round, Crawford & Mann 1990, emend.

See taxonomy of diatoms for more details.

Gallery

• Scanning electron microscope images
• 

  Diatom Surirella spiralis

• 

  Diatoms Thalassiosira sp. on a membrane filter, pore size 0.4 μm.

• 

  Diatom Paralia sulcata.

• 

  Diatom Achanthes trinodis

• 

  Stand-alone cell of Bacillaria paxillifer

• 

  Colonial group of Bacillaria paxillifer

Three diatom species were sent to the International Space Station, including the huge (6 mm length) diatoms of Antarctica and the exclusive colonial diatom, Bacillaria paradoxa. The cells of Bacillaria moved next to each other in partial but opposite synchrony by a microfluidics method.^[109]

Evolution and fossil record

Origin

Heterokont chloroplasts appear to derive from those of red algae, rather than directly from prokaryotes as occurred in plants. This suggests they had a more recent origin than many other algae. However, fossil evidence is scant, and only with the evolution of the diatoms themselves do the heterokonts make a serious impression on the fossil record.

Earliest fossils

The earliest known fossil diatoms date from the early Jurassic (~185 Ma ago),^[110] although the molecular clock^[110] and sedimentary^[111] evidence suggests an earlier origin. It has been suggested that their origin may be related to the end-Permian mass extinction (~250 Ma), after which many marine niches were opened.^[112] The gap between this event and the time that fossil diatoms first appear may indicate a period when diatoms were unsilicified and their evolution was cryptic.^[113] Since the advent of silicification, diatoms have made a significant impression on the fossil record, with major fossil deposits found as far back as the early Cretaceous, and with some rocks such as diatomaceous earth, being composed almost entirely of them.

Relation to grasslands

The expansion of grassland biomes and the evolutionary radiation of grasses during the Miocene is believed to have increased the flux of soluble silicon to the oceans, and it has been argued that this promoted the diatoms during the Cenozoic era.^[114][115] Recent work suggests that diatom success is decoupled from the evolution of grasses, although both diatom and grassland diversity increased strongly from the middle Miocene.^[116]

Relation to climate

Diatom diversity over the Cenozoic has been very sensitive to global temperature, particularly to the equator-pole temperature gradient. Warmer oceans, particularly warmer polar regions, have in the past been shown to have had substantially lower diatom diversity. Future warm oceans with enhanced polar warming, as projected in global-warming scenarios,^[117] could thus in theory result in a significant loss of diatom diversity, although from current knowledge it is impossible to say if this would occur rapidly or only over many tens of thousands of years.^[116]

Method of investigation

The fossil record of diatoms has largely been established through the recovery of their siliceous frustules in marine and non-marine sediments. Although diatoms have both a marine and non-marine stratigraphic record, diatom biostratigraphy, which is based on time-constrained evolutionary originations and extinctions of unique taxa, is only well developed and widely applicable in marine systems. The duration of diatom species ranges have been documented through the study of ocean cores and rock sequences exposed on land.^[118] Where diatom biozones are well established and calibrated to the geomagnetic polarity time scale (e.g., Southern Ocean, North Pacific, eastern equatorial Pacific), diatom-based age estimates may be resolved to within <100,000 years, although typical age resolution for Cenozoic diatom assemblages is several hundred thousand years.

Diatoms preserved in lake sediments are widely used for paleoenvironmental reconstructions of Quaternary climate, especially for closed-basin lakes which experience fluctuations in water depth and salinity.

Isotope records

Intricate silicate (glass) shell, 32-40 million years old, of a diatom microfossil

When diatoms die their shells (frustules) can settle on the seafloor and become microfossils. Over time, these microfossils become buried as opal deposits in the marine sediment. Paleoclimatology is the study of past climates. Proxy data is used in order to relate elements collected in modern-day sedimentary samples to climatic and oceanic conditions in the past. Paleoclimate proxies refer to preserved or fossilized physical markers which serve as substitutes for direct meteorological or ocean measurements.^[119] An example of proxies is the use of diatom isotope records of δ13C, δ18O, δ30Si (δ13C_diatom, δ18O_diatom, and δ30Si_diatom). In 2015, Swann and Snelling used these isotope records to document historic changes in the photic zone conditions of the north-west Pacific Ocean, including nutrient supply and the efficiency of the soft-tissue biological pump, from the modern day back to marine isotope stage 5e, which coincides with the last interglacial period. Peaks in opal productivity in the marine isotope stage are associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone.^[120]

The initial development of the halocline and stratified water column has been attributed to the onset of major Northern Hemisphere glaciation at 2.73 Ma, which increased the flux of freshwater to the region, via increased monsoonal rainfall and/or glacial meltwater, and sea surface temperatures.^{[121][122][123][124]} The decrease of abyssal water upwelling associated with this may have contributed to the establishment of globally cooler conditions and the expansion of glaciers across the Northern Hemisphere from 2.73 Ma.^[122] While the halocline appears to have prevailed through the late Pliocene and early Quaternary glacial–interglacial cycles,^[125] other studies have shown that the stratification boundary may have broken down in the late Quaternary at glacial terminations and during the early part of interglacials.^{[126][127][128][129][130][120]}

Diversification

The Cretaceous record of diatoms is limited, but recent studies reveal a progressive diversification of diatom types. The Cretaceous–Paleogene extinction event, which in the oceans dramatically affected organisms with calcareous skeletons, appears to have had relatively little impact on diatom evolution.^[131]

Turnover

Although no mass extinctions of marine diatoms have been observed during the Cenozoic, times of relatively rapid evolutionary turnover in marine diatom species assemblages occurred near the Paleocene–Eocene boundary,^[132] and at the Eocene–Oligocene boundary.^[133] Further turnover of assemblages took place at various times between the middle Miocene and late Pliocene,^[134] in response to progressive cooling of polar regions and the development of more endemic diatom assemblages.

A global trend toward more delicate diatom frustules has been noted from the Oligocene to the Quaternary.^[118] This coincides with an increasingly more vigorous circulation of the ocean's surface and deep waters brought about by increasing latitudinal thermal gradients at the onset of major ice sheet expansion on Antarctica and progressive cooling through the Neogene and Quaternary towards a bipolar glaciated world. This caused diatoms to take in less silica for the formation of their frustules. Increased mixing of the oceans renews silica and other nutrients necessary for diatom growth in surface waters, especially in regions of coastal and oceanic upwelling.

Genetics

Phaeodactylum tricornutum
is widely used as a model organism

Expressed sequence tagging

In 2002, the first insights into the properties of the Phaeodactylum tricornutum gene repertoire were described using 1,000 expressed sequence tags (ESTs).^[135] Subsequently, the number of ESTs was extended to 12,000 and the diatom EST database was constructed for functional analyses.^[136] These sequences have been used to make a comparative analysis between P. tricornutum and the putative complete proteomes from the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the diatom Thalassiosira pseudonana.^[137] The diatom EST database now consists of over 200,000 ESTs from P. tricornutum (16 libraries) and T. pseudonana (7 libraries) cells grown in a range of different conditions, many of which correspond to different abiotic stresses.^[138]

Genome sequencing

Thalassiosira pseudonana was the first eukaryotic marine phytoplankton to have its genome sequenced

In 2004, the entire genome of the centric diatom, Thalassiosira pseudonana (32.4 Mb) was sequenced,^[139] followed in 2008 with the sequencing of the pennate diatom, Phaeodactylum tricornutum (27.4 Mb).^[140] Comparisons of the two reveal that the P. tricornutum genome includes fewer genes (10,402 opposed to 11,776) than T. pseudonana; no major synteny (gene order) could be detected between the two genomes. T. pseudonana genes show an average of ~1.52 introns per gene as opposed to 0.79 in P. tricornutum, suggesting recent widespread intron gain in the centric diatom.^[140][141] Despite relatively recent evolutionary divergence (90 million years), the extent of molecular divergence between centrics and pennates indicates rapid evolutionary rates within the Bacillariophyceae compared to other eukaryotic groups.^[140] Comparative genomics also established that a specific class of transposable elements, the Diatom Copia-like retrotransposons (or CoDis), has been significantly amplified in the P. tricornutum genome with respect to T. pseudonana, constituting 5.8 and 1% of the respective genomes.^[142]

Endosymbiotic gene transfer

Diatom genomics brought much information about the extent and dynamics of the endosymbiotic gene transfer (EGT) process. Comparison of the T. pseudonana proteins with homologs in other organisms suggested that hundreds have their closest homologs in the Plantae lineage. EGT towards diatom genomes can be illustrated by the fact that the T. pseudonana genome encodes six proteins which are most closely related to genes encoded by the Guillardia theta (cryptomonad) nucleomorph genome. Four of these genes are also found in red algal plastid genomes, thus demonstrating successive EGT from red algal plastid to red algal nucleus (nucleomorph) to heterokont host nucleus.^[139] More recent phylogenomic analyses of diatom proteomes provided evidence for a prasinophyte-like endosymbiont in the common ancestor of chromalveolates as supported by the fact the 70% of diatom genes of Plantae origin are of green lineage provenance and that such genes are also found in the genome of other stramenopiles. Therefore, it was proposed that chromalveolates are the product of serial secondary endosymbiosis first with a green algae, followed by a second one with a red algae that conserved the genomic footprints of the previous but displaced the green plastid.^[143] However, phylogenomic analyses of diatom proteomes and chromalveolate evolutionary history will likely take advantage of complementary genomic data from under-sequenced lineages such as red algae.

Horizontal gene transfer

In addition to EGT, horizontal gene transfer (HGT) can occur independently of an endosymbiotic event. The publication of the P. tricornutum genome reported that at least 587 P. tricornutum genes appear to be most closely related to bacterial genes, accounting for more than 5% of the P. tricornutum proteome. About half of these are also found in the T. pseudonana genome, attesting their ancient incorporation in the diatom lineage.^[140]

Genetic engineering

To understand the biological mechanisms which underlie the great importance of diatoms in geochemical cycles, scientists have used the Phaeodactylum tricornutum and Thalassiosira spp. species as model organisms since the 90's.^[144] Few molecular biology tools are currently available to generate mutants or transgenic lines : plasmids containing transgenes are inserted into the cells using the biolistic method^[145] or transkingdom bacterial conjugation^[146] (with 10⁻⁶ and 10⁻⁴ yield respectively^[145][146]), and other classical transfection methods such as electroporation or use of PEG have been reported to provide results with lower efficiencies.^[146]

Transfected plasmids can be either randomly integrated into the diatom's chromosomes or maintained as stable circular episomes (thanks to the CEN6-ARSH4-HIS3 yeast centromeric sequence^[146]). The phleomycin/zeocin resistance gene Sh Ble is commonly used as a selection marker,^[144][147] and various transgenes have been successfully introduced and expressed in diatoms with stable transmissions through generations,^[146][147] or with the possibility to remove it.^[147]

Furthermore, these systems now allow the use of the CRISPR-Cas genome edition tool, leading to a fast production of functional knock-out mutants^[147][148] and a more accurate comprehension of the diatoms' cellular processes.

Human uses

• 

  Diatomaceous earth consisting of centric (radially symmetric) and pennate (bilaterally symmetric) diatoms suspended in water.
  (click 3 times to fully enlarge)

Paleontology

Decomposition and decay of diatoms leads to organic and inorganic (in the form of silicates) sediment, the inorganic component of which can lead to a method of analyzing past marine environments by corings of ocean floors or bay muds, since the inorganic matter is embedded in deposition of clays and silts and forms a permanent geological record of such marine strata (see siliceous ooze).

Industrial

Diatoms, and their shells (frustules) as diatomite or diatomaceous earth, are important industrial resources used for fine polishing and liquid filtration. The complex structure of their microscopic shells has been proposed as a material for nanotechnology.^[149]

Diatomite is considered to be a natural nano material and has many uses and applications such as: production of various ceramic products, construction ceramics, refractory ceramics, special oxide ceramics, for production of humidity control materials, used as filtration material, material in the cement production industry, initial material for production of prolonged-release drug carriers, absorption material in an industrial scale, production of porous ceramics, glass industry, used as catalyst support, as a filler in plastics and paints, purification of industrial waters, pesticide holder, as well as for improving the physical and chemical characteristics of certain soils, and other uses.^{[150][151][152]}

Diatoms are also used to help determine the origin of materials containing them, including seawater.

Nanotechnology

The deposition of silica by diatoms may also prove to be of utility to nanotechnology.^[153] Diatom cells repeatedly and reliably manufacture valves of various shapes and sizes, potentially allowing diatoms to manufacture micro- or nano-scale structures which may be of use in a range of devices, including: optical systems; semiconductor nanolithography; and even vehicles for drug delivery. With an appropriate artificial selection procedure, diatoms that produce valves of particular shapes and sizes might be evolved for cultivation in chemostat cultures to mass-produce nanoscale components.^[154] It has also been proposed that diatoms could be used as a component of solar cells by substituting photosensitive titanium dioxide for the silicon dioxide that diatoms normally use to create their cell walls.^[155] Diatom biofuel producing solar panels have also been proposed.^[156]

• Supporting and regulating services provided by marine diatoms and some of their negative impacts
• 

    CNN = cloud condensation nuclei, DMS = dimethylsulphide, DMSP = dimethylsulfoniopropionate, VOCs = volatile organic compounds
   dashed arrow: negative effect, solid arrow: positive effects 

Forensic

The main goal of diatom analysis in forensics is to differentiate a death by submersion from a post-mortem immersion of a body in water. Laboratory tests may reveal the presence of diatoms in the body. Since the silica-based skeletons of diatoms do not readily decay, they can sometimes be detected even in heavily decomposed bodies. As they do not occur naturally in the body, if laboratory tests show diatoms in the corpse that are of the same species found in the water where the body was recovered, then it may be good evidence of drowning as the cause of death. The blend of diatom species found in a corpse may be the same or different from the surrounding water, indicating whether the victim drowned in the same site in which the body was found.^[157]

History of discovery

Tabellaria is a genus of freshwater diatoms, cuboid in shape with frustules (siliceous cell walls) attached at the corners so the colonies assume a zigzag shape.

The first illustrations of diatoms are found in an article from 1703 in Transactions of the Royal Society showing unmistakable drawings of Tabellaria.^[158] Although the publication was authored by an unnamed English gentleman, there is recent evidence that he was Charles King of Staffordshire.^[158][159] It is only 80 years later that we find the first formally identified diatom, the colonial Bacillaria paxillifera, discovered and described in 1783 by Danish naturalist Otto Friedrich Müller.^[158] Like many others after him, he wrongly thought that it was an animal due to its ability to move. Even Charles Darwin saw diatom remains in dust whilst in the Cape Verde Islands, although he was not sure what they were. It was only later that they were identified for him as siliceous polygastrics. The infusoria that Darwin later noted in the face paint of Fueguinos, native inhabitants of Tierra del Fuego in the southern end of South America, were later identified in the same way. During his lifetime, the siliceous polygastrics were clarified as belonging to the Diatomaceae, and Darwin struggled to understand the reasons underpinning their beauty. He exchanged opinions with the noted cryptogamist G. H. K. Thwaites on the topic. In the fourth edition of On the Origin of Species he stated that "Few objects are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the high powers of the microscope"? and reasoned that their exquisite morphologies must have functional underpinnings rather than having been created purely for humans to admire.^[160]

See also

• Highly branched isoprenoid, long-chain alkenes produced by a small number of marine diatoms

Notes

References