HOST ENHANCEMENT OF SYMBIONT PHOTOSYNTHESIS IN THE HYDRA-ALGAE SYMBIOSIS

¹ School of Life Sciences, University of Nebraska, Lincoln, Nebraska 68588

Photosynthesis by the Chlorella-like algal symbiont of the green hydra, Hydra viridis, was determined for algae in situ and in vitro by measuring photosynthetic oxygen evolution in a modified polarographic electrode. Effects of light intensity, environmental oxygen concentration, and bicarbonate ion concentration on photosynthetic oxygen evolution were measured, with the following results: (1) Bicarbonate ion increased photosynthesis by algal symbionts in situ with up to 1 mM bicarbonate added. (2) Based on light intensity/photosynthesis data, photosynthetic oxygen evolution in symbionts in situ was greater than in those in vitro, especially at ambient oxygen concentrations. Oxygen severely inhibited photosynthetic oxygen evolution by symbionts in vitro but had little or no effect on algae in situ up to ambient oxygen concentrations. These data suggest that hydra symbionts gain a significant photosynthetic advantage, especially at ambient (= 8 mg 1^-1) oxygen concentrations, when they are associated with their hosts. The role of the host-symbiont relationship in contributing to this advantage is discussed.