Regulation of Respiration and Photosynthesis in Hydra viridis and in Its Separate Cosymbionts: Effect of Nutrients

Respiratory and photosynthetic rates of green and aposymbiotic Hydra viridis (Swiss strain) and of their freshly isolated symbiotic algae (FISA) were compared. The effects of some soluble organic nutrients on these parameters were examined. The respiratory rates of intact green or aposymbiotic hydra and the FISA of a single symbiotic hydranth were essentially equal to each other. Per milligram of protein, the relative respiration rates were 1: 1.47 :3.4 for green hydra, aposymbionts, and FISA, respectively. The photosynthetic rate of the algae of a single green hydra was 46% greater than that of the intact hydra. In green hydra, different nutrients stimulated both respiration and photosynthesis, photosynthesis alone, or had no effect on either process. No nutrient tested stimulated respiration by intact aposymbionts; however, several stimulated O₂ consumption in homogenates of these hydra. No substrate tested stimulated photosynthesis in FISA, although many resulted in increased respiratory rates. We conclude that the establishment of the algae-hydra symbiosis involves regulatory changes in metabolism and cellular permeability of the cosymbionts.