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Pediastrum ist eine Gattung koloniebildender Grünalgen aus der Klasse der Chlorophyceae.
Die Einzelzellen von 8–32 µm Durchmesser bilden einen typischen, flachen, runden und scheibenförmigen Aggregationsverband, ein Coenobium. Dieses besteht aus 4 bis 128 Einzelzellen mit einem Gesamtdurchmesser von bis zu 450 µm. Die Zellen besitzen einen einzelnen Zellkern, sowie einen einzelnen Chloroplasten mit einem Pyrenoid. Die Form der Zellen ist polygonal mit konkav oder konvex gewölbten Seiten. Die Randzellen haben im Gegensatz zu denen in der Mitte nach außen gerichtete Fortsätze.
Die ungeschlechtliche Fortpflanzung erfolgt durch die Bildung von 2n zweigeißeligen Zoosporen oder unbeweglichen Aplanosporen. Diese verlassen die Mutterzelle in einer Blase und bilden innerhalb dieser Blase eine Tochterkolonie. Die geschlechtliche Fortpflanzung erfolgt durch Isogamie. Zellen der Kolonie bilden zweigeißelige Gameten, die zu einer Hypnozygote verschmelzen. Aus dieser entstehen nach Reduktionsteilung bewegliche Meiosporen. Die Meiosporen wandeln sich in polyedrische, unbewegliche Zellen um. In diesen entstehen Zoosporen, die wie bei der ungeschlechtlichen Vermehrung in einer Blase das Sporangium verlassen.
Pediastrum kommt im Süßwasser-Plankton von arktischen bis tropischen Klimazonen vor. Sie kann Wasserblüten bilden. Sie kommt auch im Aufwuchs eutropher stehender Gewässer vor. Sie tritt ganzjährig auf.
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Pediastrum ist eine Gattung koloniebildender Grünalgen aus der Klasse der Chlorophyceae.
Pediastrum is a genus of green algae, in the family Hydrodictyaceae.[1] It is a photoautotrophic, nonmotile coenobial green alga that inhabits freshwater environments.
Pediastrum develops colonies with a fixed number of cells, termed coenobia. In this case, the coenobia are composed of between 22 and 27 cells, which are orderly arranged in a flat disk. The diameter of a single coenobium ranges from 20 to 80 μm, making them microalgae. Cells in Pediastrum are dimorphic, consisting of interior cells and peripheral cells, distinguished by their position in the colony and by their shape. Some species have inter-cellular spaces between their interior cells. The peripheral cells surround the interior cells, and they usually possess bristles, V-like cutting edges, or wavy projections.
Pediastrum shows lots of morphological diversity, both between and within species. Although most species produce flat, circular coenobia, others produce coenobia that are more oval-shaped, or curved and bowl-shaped. For species identification, the shape of the cells (particularly marginal ones), presence or absence of intercellular gaps, and ornamentation of the cell wall are all important distinguishing characteristics.[2]
The genus of Pediastrum belongs to the Hydrodictyaceae family, along with Pseudopediastrum, Tetraedron, Hydrodictyon and so on. Pediastrum spp. on the phylogenetic tree can be divided into Group I, Group II, Group III.[3] Group I, which includes Pediastrum duplex, are mainly distributed in North America and Europe. Group II is mostly from Australia, containing among others P. angulosum and P. alternans. Group I, Group II form a monophyletic group (clade) in the phylogenetic tree together with Hydrodictyon spp. lastly, Group III forms a sister group with Monactinus spp.
For the morphological characteristics of these groups, Group I has intracellular spaces and V-like incisions in the cells along the outside, while some species in Group II lack intercellular spaces, and their peripheral cells are wavy. Although there are significant differences between the appearance of Pediastrum and Hydrodictyon, it is still related more closely to it than several genera that look more alike, such as Pseudopediastrum, Monactinus and Stauridium.
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Pediastrum
Stauridium tetras
Tetraedron
Pseudopediastrum boryanum
Monactinus simplex
Hydrodictyon reticulatum
Pediastrum boryanum
Colony of Pediastrum algae
Cell walls of Pediastrum form a “globular network” with interconnected nodes.[4] These walls are difficult to break down due to the presence of sporopollenin[2] and silica,[5] which the algae uses to defend itself against infection. This also coincidentally makes it very resistant to diagenesis. Because of this, Pediastrum is known to have existed since the Early Cretaceous.[6] In 2005, several studies on the fossil record of Pediastrum were compiled, and 10 species in the genus are now recognized between the Late Cretaceous and the Quaternary in southern South America alone.[7]
The genus Pediastrum is present all around the world, and they are usually found in sediments of freshwater lakes or wetlands. The strong cell walls of Pediastrum make them preserve very well in sediments. The cell clusters retain their morphology, some organelles inside, and genetic materials for thousands of years. The cell walls can survive the harsh chemical treatments used to prepare samples in palynology, allowing them to still be identified to a species level in pollen samples, which is uncommon in that field of study. These special characteristics make Pediastrum useful as a bioindicator. Therefore, Pediastrum is very useful in the fields of paleoecology and paleolimnology.[8][9] For example, Pediastrum is absent or decreased when less aquatic plants are present, allowing one to determine the health of an aquatic ecosystem even when no direct information about aquatic plant life is present in a sample. Dissolved organic carbon (DOC) and pH value are the main environmental variables that affect the distribution of Pediastrum. Because of this, sediment-preserved Pediastrum spp. helps correlate community changes and nutrient availability, and can be a useful indicator for the reconstruction of paleoenvironments.[10]
Pediastrum possesses a haplontic life cycle, and can be divided into three types.[11] The most commonly seen is the first kind of asexual life cycle (ALC1), which generates autocolonies. In ALC1 the zoospores aggregate into an orderly flat disk, with concentric rings (central cell, six cells, and then nine cells.) Afterwards,the peripheral cells grow in one or two spines from the external wall. The second asexual life cycle (ALC2,) is a single celled version of the former. The vesicle that contains the zoospores breaks down immediately after formation, and individual cells grow until maturity, forming motile zoospores. Each cell can then go on to generate a daughter colony with exactly the same number and arrangement of cells as the parent colony. As for the Sexual life cycle in Pediastrum, the cell forms two bi-flagellate gametes (isogamy), these then fuse into a zygote and develop further through the asexual lifecycle as described before.
The second asexual and sexual life cycles are rarely observed in Pediastrum, and only occur in very bright conditions or in high temperatures.
Pediastrum is a genus of green algae, in the family Hydrodictyaceae. It is a photoautotrophic, nonmotile coenobial green alga that inhabits freshwater environments.
Pediastrum est un genre d'algues vertes de la famille des Hydrodictyaceae.
Selon AlgaeBase (12 nov. 2010)[1] :
Pediastrum est un genre d'algues vertes de la famille des Hydrodictyaceae.
Pediastrum Meyen, 1829 è un genere di alghe verdi della famiglia Hydrodictyaceae.[1]
Si trova sia in un habitat di acque stagnanti che in habitat ad acque correnti. Sono privi di movimenti autonomi quindi vengono spostati dalla corrente. Pur formando colonie di grandi dimensioni rimangono comunque di dimensioni microscopiche, costituiscono una gran parte del fitoplancton d'acqua dolce.
Pediastrum Meyen, 1829 è un genere di alghe verdi della famiglia Hydrodictyaceae.
Si trova sia in un habitat di acque stagnanti che in habitat ad acque correnti. Sono privi di movimenti autonomi quindi vengono spostati dalla corrente. Pur formando colonie di grandi dimensioni rimangono comunque di dimensioni microscopiche, costituiscono una gran parte del fitoplancton d'acqua dolce.
