Welcome back to our coral adventure!
As mentioned in the previous blog post, zooxanthellae are small single celled algal bodies that live in symbioses with cnidarian species. The specific symbiont we will be looking at is one that resides inside the Coral and its polyps. Whenever you hear the word algae, you probably think something that looks like this:
And you wouldn’t be completely wrong! Maybe you’ve experienced the little icky green film that forms on your grandma’s pool. Zooxanthellae that live in coral just exist in a much smaller scale. Zooxanthellae can range anywhere from 5-12 micrometers in size.
Symbiodinium, commonly known as zooxanthellae, is a type of dinoflagellate that lives in correlation with a coral. They live inside the coral tissue and preform photosynthesis to provide the coral with oxygen and nutrients. In return, the coral provides protection, carbon dioxide, and a home for the zooxanthellae.
What does the zooxanthellae look like? There are two different life forms that the zooxanthellae displays: one of them more common than the other. There is the motile masticate phase and the stationary phase. The motile phase is before the zooxanthellae finds its little coral soulmate to live with. This phase is characterized as an algal body with a flagella wrapped around its middle and continues into free space to help with motility. The stationary phase loses the flagellum and resides inside the coral tissue.
Pictured below is both the motile phase and the stationary phase.
There are different species of zooxanthellae and each species is specific to the coral they inhabit. Each different kind of species is called a clade. There are 9 different clade’s named via alphabetic order (clade A through I). Each clade has their own specific genome. Extensive research has not been done on the specific genomes of each clade, but we do know that the genomes help with temperature resistance. The general size of the genomes of dinoflagellates is ridiculously large in variation, ranging anywhere from ∼3–245 giga base pairs. Dinoflagellate nuclear DNA is extensively methylated. “Previous studies showed that 12–70% of thymine is replaced by 5-hydroxymethyluracil and varying levels of methylation occur to cytosine”. (https://www.sciencedirect.com/science/article/pii/S0923250811000684).
As you can see pictured above, all clades except for clade H are represented in phylum Cnidaria.
The cellular structure of the symbiodinium is simple yet adequate. The cells contain a nucleus that holds super coiled DNA, chloroplasts containing pigments such as pigments a and c, a strange structure called the accumulation body whose contents are unknown, and a cell wall filled with glycosidic proteins. Chloroplasts preforms the photosynthesis so important for this relationship.
Thanks for coming on this zooxanthellae journey!