Lipid droplets, medium of energy exchange in the symbiotic anemone Condylactis gigantea: a model coral polyp

Abstract

The symbiotic sea anemone Condylactis gigantea does not make a CaCO3 skeleton, yet its lipid metabolism appears to be very similar to that of reef-building corals. Like hermatypic corals, it is roughly 1/3 lipid on a dry weight basis, its lipid is composed primarily of saturated wax ester and triglyceride, and in the light, its symbiotic algae (zooxanthellae) are the major sites of lipid synthesis in the intact association. Lipid droplets isolated from host tentacle tissue, where the majority of zooxanthellae also occur, contained primarily wax ester and triglyceride in an apparently supercooled liquid state. The diameter of the extra-algal droplets, 3.4±0.7 μm, was similar to that of droplets observed to protrude from isolated zooxanthellae. When anemones were allowed to assimilate acetate-1-14C in the light, only the zooxanthellae and tentacle lipid droplets incorporated significant radioactivity. In the corresponding dark incubations the lipid droplets showed the highest specific activity (5.0 pmol acetate-1-14C/mg lipid). With photic exposure, extra-algal droplet specific activity increased 60% and the zooxanthellae specific activity increased from a dark value of 1.0 to 20.0 pmol acetate-1-14C/mg lipid in the light. A scheme is presented to suggest how carbon may be fixed into lipids by the algae. In C. gigantea, 97% of the total lipid is stored in the polyp column, which is connected to the tentacles by the gastrovascular cavity. In the tentacle tissue, lipid droplets were often found to be inside vesicles; however, upon tentacle contraction free droplets accumulated in the tentacle lumen. Thus, lipid synthesized in the tentacle may be transported to coelenterate tissues via the circulating internal fluid in the form of lipid droplets.