The Green Hydra Symbiosis. VII. Conservation of the Host Cell Habitat by the Symbiotic Algae

Freshly isolated `European' algae phagocytosed by digestive cells of `European' green hydra were distinguished from the pre-existing population of algae by prestaining with the fluorescent agent Calcofluor White. Only a small number of phagocytosed `European' algae or algae cultured from Paramecium bursaria avoided lysosomal degradation and were transported to the cell base in symbiotic digestive cells, although in aposymbionts up to 50% of phagocytosed algae were transported. Degradation of almost all phagocytosed algae also occurred in digestive cells of hydra containing only half the normal complement of algae, and in those of hydra symbiotic with algae cultured from Paramecium. The presence of algae at the bases of digestive cells appears to negate the mechanism by which potentially symbiotic algae normally avoid lysosomal attack. This protects the host cell and its symbionts from invasion by `foreign' algae and suggests that once established the green hydra symbiosis is conservative in nature.