It was found that the genome contains unidirectionally aligned genes and that these genes form a cluster-like arrangement. The Symbiodinium genome was very recently sequenced. The zooxanthellae’s chloroplast has three membranes, and the thylakoid membranes differ between species. Mitosis occurs on the coccoid cells as well, which are surrounded by a cell wall of glycoproteins and proteins, and only one species of zooxanthellae is known to have surface projections (13). This division in log phase is about every one to three days, but in culture division slows during stationary phase and fewer motile cells are produced. During reproduction, the chromosomal and nuclear division occurs in darkness, while the cellular division into two flagellated cells (cytokinesis) occurs in exposure to light. In the coccoid stage, the stage in which zooxanthellae are more commonly found, the zooxanthellae are intracellular symbionts within the coral and do not keep their flagella. In the motile phase, the zooxanthellae retain their flagella and are free-living. Their life cycle is in two stages: the coccoid stage and the motile masticate phase (Figure 2).
Zooxanthellae undergo asexual reproduction by division, and most of their energy comes from performing photosynthesis using the byproducts of cellular respiration produced from the host coral. Zooxanthellae are unicellular and spherical with two flagella that fall off once they are incorporated within a host. The tissue has two layers, the epidermis and the gastrodermis, where the zooxanthellae live (36). Polyps are live coral tissue extensions that cover the calcium carbonate structure, and are usually only a few millimeters thick. Zooxanthellae usually occur in extremely high densities on their host, enhancing the constant exchange of nutrients between them and their host (Figure 1).Ĭorals are usually colonies of polyps. It is an algal protist that is best known for its symbiotic relationship with marine coral. Zooxanthellae is a term for any dinoflagellate that participates in symbiosis with sponges, coral, clams, mollusks, flatworms, jellyfish, etc (1,2). Zooxanthellae is the brown-yellow algae that lives in coral’s gastrodermis, and is the common name of the broader Symbiodinium genus (3). A picture showing where the zooxanthellae reside in coral
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