Symbiodinium - Intracellular Symbionts

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 (Fig. 2A). 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 (Fig. 2B). 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.