The symbiotic algae of the coral Plesiastrea versipora have previously been shown to release photosynthate to the host, much of it in the form of glycerol. Glycerol uptake was investigated as part of studies of the mechanisms of translocation of organic carbon between zooxanthellae and their coral hosts. Experiments were done on isolated zooxanthellae suspended in coral host homogenate or sea water. Uptake of [14C]glycer°l was linear with time in the light and dark. The Photosystem II

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... DCMU, had no effect on glycerol uptake rates in the light or dark, but glycerol uptake was strongly inhibited by KCN. [14C]glycer°l taken up by the zooxanthellae was rapidly metabolized and the intracellular glycerol concentration was near zero. Uptake curves were curvilinear and resolvable into a saturable Michaelis-Menten component and a linear component. Glycerol is apparently taken up both by a transport mechanism and by simple diffusion. Glycerol-3-phosphate, propan-l-ol, propan-2-ol, ethanediol, 1,2 propanediol and erythritol do not inhibit glycerol uptake, but DL-glyceraldehyde is a non-competitive inhibitor. The permeability of the zooxanthellae to glycerol is similar to that of other eukaryotic cells (p ~ 0.65 nm s_1). Host homogenate decreases the net rate of uptake of glycerol into zooxanthellae by diffusion in both the light and the dark. The apparent permeability of glycerol is slightly higher in the dark than in the light. Host homogenate does not make the plasma membrane 'leaky' to glycerol. Models of the mechanism of action of host homogenate are discussed in the light of these results.