The green hydra symbiosis. V. Stages in the intracellular recognition of algal symbionts by digestive cells

Aposymbiotic (alga-free) green hydra may be reinfected by injecting a suspension of Chlorella symbionts into their coelenterons. Digestive cells could phagocytose a wide range of Chlorella types, but transport to the cell base, where the symbionts normally reside, was limited to algae that release detectable amounts of maltose. Transport of their own symbionts was inhibited when maltose release was curtailed by prolonged pretreatment with a photosynthetic inhibitor. After phagocytosis, only about half of their own symbionts were transported, the rest remaining at the digestive cell apex where they disintegrated. This phenomenon was termed sorting. It was not due to damage of algae during isolation, nor to saturation of the transport mechanism. A further stage of discrimination was observed to take place up to 5 days after injection; some symbionts that had been transported to the cell bases were removed to the apices and disintegrated or were ejected (re-sorting). It is concluded that recognition of suitable algae is unlikely to involve identification of a single algal character by the European digestive cells. The establishment of the symbiosis may depend upon a number of algal properties and interactions within the host cell.