Zooxanthellae

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. Zooxanthellae of genus Symbiodinium belong to a total of eight phylogenetic clades A-H, differentiated via their nuclear ribosomal DNA and chloroplast DNA. Zooxanthellae are autotrophs containing chloroplasts composed of thylakoids present in clusters of three. The DNA possesses ribosomal RNA (rRNA) that is folded and of similar morphology to rRNA in archaeobacteria. This indicates that RNA is important for DNA packaging in zooxanthellae. Zooxanthellae, in addition to all other dinoflagellates, possess 5-hydroxymethyluracil and thymidine in their genomes, unlike any other eukaryotic genome. ==Life history==

Zooxanthellae alternate between life phases expressed as cysts and as motile organisms in the water column. In zooxanthellae of the genus Gymnodinium, one possible life cycle of the cell begins as an immature cyst which reaches maturity then divides to form an immature cyst once more. Once growing into an older cell, it becomes no longer useful. In the life cycle of a motile zooxanthellae cell, its youngest stage is known as a zoosporangium, which matures into a zoospore capable of motility. This motile cell produces and releases gametes for reproduction. Vegetative phase The vegetative phase in the life cycle of a zooxanthellae is the predominant form of the organism. In this form, the single-celled organism has a thin cell wall. As opposed to the zoospore, the zooxanthellae contains numerous chloroplasts. Once the cell continues growing, however, chloroplasts decrease in abundance. The vegetative cell will either divide into two separate daughter cells or transition into a cyst stage. Cyst stages The most common phases in the life history of zooxanthellae following the vegetative phase are cysts, dividing cysts, and degenerate cysts. Cysts possess a thick cell wall yet retain the composition of the cytoplasm and constitute the majority of clustered zooxanthellae in host tissues. This stage of the cell provides the host with a reddish-brown hue. Dividing cysts make up a fourth of the composition of zooxanthellae clusters in host tissues and are expressed as cell stages where two daughter cells remain adjoined but possess individual cell walls. 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. The young zoosporangium and motile zoospore stages, though seen in zooxanthellae life cycles, are much rarer amongst clades. The zoospore resides in the zoosporangium until the cell wall of the cyst bursts. Zooxanthellae is only motile if it originates as a zoospore. Motility Zooxanthellae in the zoospore stage exhibit motility as forward movement or gyratory movement. In moving forward, the organism rotates on the posterior flagellum's axis whilst simultaneously propelling through the water column. The zoospore gyrates through the water column via attachment of the posterior flagellum to a substrate. ==Ecology==

Endosymbiont acquisition 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. Many different species of zooxanthellae are present in host organisms, each species with its own adaptive capabilities and degree of tolerance of varying environmental factors. 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. However, clams discard zooxanthellae that are still alive and have been observed being able to recover them. They are also found in the eyes of clams like Tridacna where they act as a lens. Different clades of zooxanthellae have an impact on clam morphology. Clade E1 of zooxanthellae seems to influence or favor smaller offspring from clams when compared to clams harboring other clades and all five clades appear to be needed in order for larval settlement to occur. Many different types of zooxanthellae have been observed forming relationships with jellyfish across many different phylogenetic branches, and the roles they play will change throughout the jellyfish's life cycle. Although, jellyfish bleaching events have been documented during extreme heat events. Light availability can affect the lipid production of zooxanthellae that the jellyfish then utilize. To maximize their light uptake, jellyfish will both swim near the surface and do very specific migrations. The migration patterns also assist with helping the zooxanthellae access specific nutrients. Many of these jellyfish appear to be mixotrophic consuming both live prey and utilizing phototrophy. 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. There are many unknowns in when it comes to the relationship between zooxanthellae and jellyfish that scientists look to answer. ==References==