GLYCEROL UPTAKE BY ZOOXANTHELLAE OF THE TEMPERATE HARD CORAL, PLESIASTREA-VERSIPORA (LAMARCK)

The symbiotic algae of the coral Plesiastrea versipora have previously been shown to release photosynthate to the host, much of it in the fo rm of glycerol. Glycerol uptake was investigated as part of studies of the mechanisms of translocation of organic carbon between zooxanthell ae and their coral hosts. Experiments were done on isolated zooxanthel lae suspended in coral host homogenate or sea water. Uptake of [C-14]g lycerol was linear with time in the light and dark. The Photosystem II inhibitor, DCMU, had no effect on glycerol uptake rates in the light or dark, but glycerol uptake was strongly inhibited by KCN. [C-14]glyc erol taken up by the zooxanthellae was rapidly metabolized and the int racellular glycerol concentration was near zero. Uptake curves were cu rvilinear and resolvable into a saturable Michaelis-Menten component a nd a linear component. Glycerol is apparently taken up both by a trans port mechanism and by simple diffusion. Glycerol-3-phosphate, propan-1 -ol, propan-2-ol, ethanediol, 1,2 propanediol and erythritol do not in hibit glycerol uptake, but DL-glyceraldehyde is a non-competitive inhi bitor. The permeability of the zooxanthellae to glycerol is similar to that of other eukaryotic cells (p almost-equal-to 0.65 nm s-1). Host homogenate decreases the net rate of uptake of glycerol into zooxanthe llae by diffusion in both the light and the dark. The apparent permeab ility of glycerol is slightly higher in the dark than in the light. Ho st homogenate does not make the plasma membrane 'leaky' to glycerol. M odels of the mechanism of action of host homogenate are discussed in t he light of these results.