Inhibition of vacuolar membrane fusion by intracellular symbiotic algae in Hydra viridis (Florida strain).

Hydra viridis (Florida strain) forms a stable symbiotic association with unicellular Chlorella-like algae. Algae are phagocytized by hydra phagocytes and maintained individually within vacuoles of the host-cell. The purpose of the studies presented in this paper was to determine how symbiotic algae avoid host-cell digestive processes. Viable symbiotic algae were found to inhibit the fusion of the algal vacuolar membrane and vacuoles labeled with thorium dioxide. By contrast, thorium-labeled vacuoles did fuse with vacuoles containing nonviable algae. Further, the absence of acid phosphatase activity within vacuoles containing established symbiotic algae implies that inhibition of membrane fusion by algae in hydras prevents the conversion of the algal vacuoles into phagolysosomes.