A reevaluation of the role of glycerol in carbon translocation in zooxanthellae-coelenterate symbiosis

Abstract

Glycerol has been traditionally viewed as the main form of carbon translocated from zooxanthellae to the coelenterate host. Most of this glycerol was postulated to be used by the coelenterate host for lipid synthesis. Recent work suggests that large amounts of photosynthetically fixed carbon is synthesized into lipid in the algae, and then translocated as lipid droplets to the host. These two hypotheses of carbon translocation are not mutually exclusive, but to reconcile them the role of glycerol must be reevaluated. In this study the short term metabolic fate of uniformly labelled 14C-glycerol, 14C-bicarbonate, and 14C-acetate was examined in zooxanthellae and coelenterate host tissue isolated from Condylactis gigantea tentacles. When host and algal triglycerides, synthesized during 90-min light and dark incubations in 14C-bicarbonate and 14C-acetate, were deacylated, more than 80% of the activity was found in the fatty acid moiety. In contrast, triglycerides isolated from zooxanthellae and coelenterate host tissue incubated in 14C-glycerol in the dark for 90 min were found to have more than 95% of their radioactivity in the glycerol moiety. During the 90-min 14C-glycerol incubations in the light, the percentage of radioactivity in the fatty acid moiety of zooxanthellae triglycerides increased to 37%. The percentage of radioactivity in the host tissue triglycerides fatty acid moiety stayed below 5% during the 90-min 14C-glycerol incubations in the light. These results show that neither the zooxanthellae nor the host can rapidly convert glycerol to fatty acid. Radioactivity from 14C-glycerol, which does eventually appear in host lipid, may have been respired to 14CO2, then photosynthetically fixed by the zooxanthellae and synthesized into lipid fatty acid.