Dinoflagellates representing at least eight genera in four (or five) classical dinoflagellate orders occur as endosymbionts in marine invertebrates and protists (Baker 2003). These eight genera, which are not all closely related, form mutualistic (mutually beneficial) symbioses with a wide range of hosts. Particularly well studied and of especially far-reaching ecological importance are the symbiotic relationships between certain dinoflagellates and their scleratinian coral hosts. These endosymbiont dinoflagellates are photosynthetic and the energy they capture through photosynthesis and transfer to their hosts is critical to the maintenance and growth of coral reefs.
A healthy coral reef might easily contain >1010 algal symbionts per m2 (Baker 2003). Despite their tremendous abundance, however, as a result of their tiny size the total dinoflagellate biomass is very small relative to the entire biomass of a coral reef community. Thus, given the critical importance of these dinoflagellate symbionts to the health of the corals and, by extension, to the well being of the diverse invertebrates, fish, and other organisms dependent on the coral, Baker (2003) suggests that symbiotic dinoflagellates in coral reefs are "keystone species" (i.e., species that have an impact on an ecological community that is extremely large relative to their fraction of the total biomass of the community).
The best studied of the symbiotic dinoflagellates are those in the genus Symbiodinium, which are commonly (but not exclusively) found in shallow water tropical and subtropical cnidarians and in this context are often referred to as zooxanthellae ("little yellow animals", a reference to their typically golden-brown color). Among the diverse cnidarians known to host Symbiodinium are representatives of the class Anthozoa (including anemones, scleractinian corals, zoanthids, corallimorphs, blue corals, alcyonacean corals, and sea fans) and several representatives from the classes Scyphozoa (including rhizostome and coronate jellyfish) and Hydrozoa (including milleporine fire corals) (Baker 2003). Symbiodinium have also been identified from some non-cnidarians, including some gastropod and bivalve mollusks, foraminiferans, sponges, and a giant heterotrich ciliate (Baker 2003 and references therein). Associations between particular Symbiodinium zooxanthellae and particular hosts are clearly nonrandom--i.e., there is some specialization of particular hosts on particular Symbiodinium species and specialization of particular Symbiodinium on particular host species. However, considerable flexibility is evident. It now appears that many (perhaps even most or all) hosts are able to associate with more than one type of Symbiodinium, and Symbiodinium appear to be even less specific than their hosts (i.e., a single Symbiodinium type has the potential to associate with a variety of hosts) (Baker 2003 and references therein). The ability of a particular host species to associate with different Symbiodinium, which may perform differently in different ecological settings (e.g., functioning more efficiently in corals in shallow, high-light situations versus deep water low-light conditions) may allow host species to thrive in a much broader range of ecological conditions than would be possible if they were limited to associating with a single dinoflagellate species (Baker 2003 and references therein).
Dinoflagellates may swim toward or away from various stimuli, including light (phototaxis), particular chemicals (chemotaxis), and gravitational force of the Earth (geotaxis).
Many dinoflagellates are bioluminescent. The bioluminescence system consists of the enzyme luciferase, its substrate luciferin, and a protein that binds luciferin. Most bioluminescent organisms in the ocean, including dinoflagellates, emit light with a peak wavelength near 490 nm. This wavelength, in the blue-green range, is minimally attenuated in water and maximally visible to most marine animals (Hackett et al. 2004). The function of dinoflagellate bioluminescence is not entirely clear, but a range of studies have supported the hypothesis that it serves as a defense against nocturnal grazers such as copepods (Buskey and Swift 1985 and references therein).
Many dinoflagellates exhibit diel vertical migration, moving up and down the water column on a 24-hour cycle (Hackett et al. 2004).
Nash et al. (2008) discuss the many unusual features of the mitochondrial genome and their evolutionary implications.
Dinoflagellates account for about 75% (45-60 taxa) of all algal species forming harmful algal blooms, or HABs (Smayda 1997). HABs have often been referred to as "red tides". This term can be quite misleading, however, given that many toxic blooms occur when waters are not discolored and, conversely, blooms may occur in which the high biomass and pigments of the dinoflagellates turn the water red yet they are not toxic (Hackett et al. 2004). Dinoflagellate toxins may have serious negative impacts on a wide variety of animals exposed to them.
Consumption of seafood (shellfish or fish) contaminated by algal toxins may result in a variety of seafood poisoning syndromes in humans, including among others paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP), diarrheic shellfish poisoning (DSP), ciguatera fish poisoning (CFP) and azaspiracid shellfish poisoning (ASP) (Hackett et al. 2004; Wang 2008). Most of these poisonings are caused by neurotoxins which present themselves with highly specific effects on the nervous systems of animals, including humans, by interfering with nerve impulse transmission.
PSP, which is caused by blooms of dinoflagellates belonging to several different genera, is probably the most widespread of the HAB poisoning syndromes. It may result in human illness and death, loss of seafood resources, reduced tourism and recreational activities, alteration of local marine ecosystems, and death of marine mammals, fish, and seabirds (Hackett et al. 2004).
Although the impact of HABs is mainly negative, Camacho et al (2007) review some of the potential medical applications of dinoflagellate toxins, as well as the challenges in pursuing research on them.
Based on recent molecular phylogenetic analyses by several researchers, it appears that the sister group to the dinoflagellates (i.e., the group with which the dinoflagellates share a most recent common evolutionary ancestor) is the Apicomplexa (Hackett et al. 2004), a group of protists that includes some taxa well known as parasites of humans and other animals. Among these familiar apicomplexans are Plasmodium (the cause of malaria), Cryptosporidium (the cause of cryptosporidiosis), Babesia (the cause of babesiosis), and Toxoplasma gondii (the cause of toxoplasmosis).
The dinoflagellates are an important group of phytoplankton (microscopic free-floating photosynthetic organisms) in both marine and fresh waters. They may occur as swimming, solitary cells or as nonmotile symbionts of various invertertebrates such as corals. Many are photosynthetic, but many others are not. Most of the photosynthetic species share certain types of pigment, including several pigments apparently found only in dinoflagellates. Numerous other aspects of the cell biology and genetics of dinoflagellates are unusual as well (reviewed in Hackett et al. 2004 and Wong and Kwok 2005). Some dinoflagellates produce toxins that may harm a wide variety of vertebrates and invertebrates (see Relevance). A large group of photosynthetic dinoflagellates are endosymbionts on which many corals and other invertebrates depend for their survival (see Associations).
Coral reef ecosystems are particularly sensitive to climate change. Since the 1980s, coral reef bleaching, caused by unusually high sea temperatures, has had devastating and widespread effects worldwide (Baker et al. 2008 and references therein). Environmental extremes, such as high or low temperatures or high irradiance, trigger a cacade of physiological and biochemical changes that lead to eventual cellular damage in the dinoflagellate symbionts and/or their coral hosts, and can lead to the expulsion of symbionts and the eventual breakdown of the symbiosis (Lesser 2004, 2006; Baker et al. 2008 and references therein). The loss of zooxanthellae (and/or a reduction in their pigment concentrations) as a result of this process is known as “bleaching”. In extreme cases, bleaching leads to the visible paling of the host organism, as the yellow-brown pigmentation of the symbionts is lost (Baker et al. 2008).
These episodes of mass coral bleaching and mortality have raised concerns about the long-term survival of coral reef ecosystems. There is increasing evidence that under "normal" conditions dinoflagellate communities in coral reefs are in flux, with species densities and species composition shifting in response to numerous factors including changes in environmental conditions, such as warming of the seas. Some researchers are hopeful that mass bleaching events, which involve large-scale losses of endosymbiotic zooxanthellae, are simply an extreme example of a "normal" transition in the dinoflagellate community composition and that given time some reef recovery is possible (Baker 2003 and references therein). (Some researchers have even argued that bleaching is an adaptive response to extreme environmental changes that allows corals to rapidly change their dinoflagellate associates to species better suited to the new environmental conditions.) Most ecological communities are quite resilient, but as is is often the case with extreme environmental perturbations, the question of whether long-term persistence of coral reefs is possible turns largely on questions of scale: Is the change too rapid for corals and their dinoflagellate associates to adapt? Might persistence be possible on a large geographic scale even if local extinctions are inevitable? If recovery is possible, how long might it take and how similar would the recovered reef systems be to those that preceded them?
Many dinoflagellates are photosynthetic, many others ingest other phytoplankton (such as diatoms or other dinoflagellates), and still others combine these two trophic modes (Hackett 2004).
