Lysosomes fuse with one half of alga‐bearing phagosomes during the reestablishment of the European green hydra symbiosis

Sorting of native Chlorella algae during the reestablishment of the European green hydra symbiosis occurs after phagocytosis by host digestive cells, when approximately half the algae subsequently disappear, either by exocytosis or by digestion. Using electron microscopical and cytochemical techniques we have assessed the morphological integrity of algae after phagocytosis and characterized fusion between alga‐bearing phagosomes and host‐digestive cell lysosomes. Five hours after initiation of phagocytosis of algae, approximately 45% were associated with lead reaction product of the lysosomal enzyme acid phosphatase. Some individual alga‐bearing phagosomes had fused, forming large phagolysosomes which contained numbers of algae at various stages of digestive degradation. Thus, sorting may result from the failure of about half of the algae phagocytosed by European digestive cells to prevent lysosome fusion with their phagosomes, and hence their digestion by lysosomal enzymes. Acid phosphatase reaction product was not associated with vacuoles containing established algal or bacterial symbionts. Pretreatment of algae with cationic agents increased the number of algae sorted for digestion, and it also perturbed algal release of photosynthate. It is suggested that algae prevent lysosomal fusion through release of photosynthate, and that sorting is a result either of a rate competition between lysosomal fusion and its inhibition by algae, or of only a proportion of algae releasing photosynthate above the threshold level needed to prevent lysosomal fusion.