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Symbiodinium
The genus ''Symbiodinium'' encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellular algal protists commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where the products of their photosynthetic processing are exchanged in the host for inorganic molecules. They are also harbored by various species of sponge, flatworms, mollusks such as the giant clams, foraminifera ''(soritids)'', and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment. The exception is in most mollusks, where ''Symbiodinium'' are intercellular (between the cells). Cnidarians that are associated with ''Symbiodinium'' occur mostly in warm oligotrophic (nutrient-poor), marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems.
''Symbiodinium'' are colloquially called "zooxanthellae" (or "zoox"), and animals symbiotic with algae in this genus are said to be "zooxanthellate". The term was loosely used to refer to any golden-brown endosymbionts, including diatoms and other dinoflagellates. Continued use of the term in the scientific literature should be discouraged because of the confusion caused by overly generalizing taxonomically diverse symbiotic relationships.
==Intracellular symbionts==
''Symbiodinium'' are known primarily for their role as mutualistic endosymbionts. In hosts, they usually occur in high densities, ranging from hundreds of thousands to millions per square centimeter. The successful culturing of swimming gymnodinioid cells from coral led to the discovery that "zooxanthellae" were actually dinoflagellates. Each ''Symbiodinium'' cell is coccoid ''in hospite'' (living in a host cell) and surrounded by a membrane that originates from the host cell plasmalemma during phagocytosis (Figs 2B and 3). This membrane probably undergoes some modification to its protein content, which functions to limit or prevent phago-lysosome fusion. The vacuole structure containing the symbiont is therefore termed the symbiosome, and only a single symbiont cell is found within each symbiosome. It is unclear how this membrane expands to accommodate a dividing symbiont cell. Under normal conditions, symbiont and host cells exchange organic and inorganic molecules that enable the growth and proliferation of both partners.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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