Zooxanthellae
zooxanthella) is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs.
[4][a] Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.
[7][8] Zooxanthellae can be grouped in the classes of Bacillariophyceae, Cryptophyceae, Dinophyceae, and Rhodophycaeae and of the genera Amphidinium, Gymnodinium, Aureodinium, Gyrodinium, Prorocentrum, Scrippsiella, Gloeodinium, and most commonly, Symbiodinium.
[15] Cysts possess a thick cell wall yet retain the composition of the cytoplasm and constitute the majority of clustered zooxanthellae in host tissues.
Degenerate cysts are present in clusters, though rare, and lose much of their mutualistic benefit to the host they reside in due to a decrease in photosynthetic efficiency.
[15] In moving forward, the organism rotates on the posterior flagellum’s axis whilst simultaneously propelling through the water column.
[15] Zooxanthellae are particularly associated with reef-building corals but they also inhabit other invertebrates and protists; their hosts include many sea anemones, jellyfish, nudibranchs, certain bivalve molluscs like the giant clam Tridacna, sponges and flatworms as well as some species of radiolarians and foraminiferans.
[2] A juvenile organism or newly established colony can acquire its zooxanthellae via sexual reproduction or directly from the environment.
The egg from which the individual developed may have already been infected by zooxanthellae at the time of fertilization, or cells of the symbiont may have been transferred from the mother in a period during which the larva was brooded by its parent.
Alternatively, the new individual may acquire the zooxanthellae direct from sea water in which the dinoflagellates freely live at some stages of their life cycle.
[17] Zooxanthellae provide nutrients to their host cnidarians in the form of sugars, glycerol, and amino acids and in return gain carbon dioxide, phosphates, and nitrogen compounds.
[17] Variations in salinity, light intensity, temperature, pollution, sedimentation, and disease can all impact the photosynthetic efficiency of zooxanthellae or result in expulsion from their mutualistic relationships.
[17] The physiological mechanisms behind endosymbiont expulsion remain under research but are speculated to involve various means of detachment of zooxanthellae or gastrodermal cells from host corals.
[17] Coral is not the only aquatic organism to be affected by bleaching and the expulsion of zooxanthellae; clams have also been found to undergo a similar process when temperatures become too high.
[23] The relationship between jellyfish and zooxanthellae is affected a little differently than coral in terms of climate change despite both of them being a part of the cnidaria family.
[23] One study suggested that certain species of jellyfish and their symbiotic zooxanthellae may have some type of resistance to decreasing pH caused by climate change to a certain point.
[23] This may be what helps jellyfish survive climate change and bleaching as they could switch feeding methods rather than attempting to recover lost zooxanthellae quickly.
