Lipid contents and compositions of the bleached Okinawan corals were analyzed for the first time. Bleached corals collected at Sesoko Okinawa, Japan (26 ∞ 38'N, 127 ∞ 52'E), after the 1998 bleaching event showed decreased lipid content with a concomitant decline of wax in the lipid composition. The reduction in the lipid content of corals showed diversity depending on their colony morphology. On the whole, corals of massive morphology were relatively rich in lipid even after the bleaching event, and hence lesser reduction in the lipid content compared with the cases for those of branching morphology. Furthermore, there was a positive correlation between lipid content and zooxanthellae density in the bleached corals, suggesting again the importance of symbiont for the supply of lipid to the host cells, and hence for the survival of the bleaching event.

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>ABSTRACT: The lipid profile was studied along the branch length, from the top, middle to base portion, of coral Montipora digitata to gain more insight into the physiological significance of lipids in the coral energy budget. The lipids of M. digitata consisted of seven major lipid classes: polar lipid, sterol, free fatty acid, unknown lipids 1 and 2, triacylglycerol (TG), and wax ester. The concentration of storage lipids, TG, and wax ester showed a top–base gradient along the length, whereas the levels of free fatty acid and unknown lipids showed a base–top gradient. The proportions of polar lipid and sterol in the top portion of the branch were slightly higher than those in the base portion. This observation appeared to be compatible with the view that the increased energy expenditure for proliferation enhanced the mobilization of the storage fuel lipids of wax ester and TG rather than the structure lipids of polar lipids and sterols at the top portion of the branch. Compositions of fatty acid also showed a length‐wise diversity. The top portion had a lower proportion of palmitic acid (16 : 0) in all lipid classes of fatty acid ester, suggesting that this fatty acid was preferentially mobilized at the top portion, probably for the growth of coral cells.

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Glucose has been implicated in functioning as a form of carbon translocated from symbiont zooxanthellae to the host coral cell. The present paper describes the lipid biosynthesis from [14C]‐glucose in the coral tissue. To study the incorporation of [14C]‐glucose into lipids, the branch tips of the coral Montipora digitata were incubated with [14C]‐glucose or another radiolabeled substrates. The lipid biosynthesis from [14C]‐glucose was dependent on light, and was decreased by dark conditions or by photosystem II inhibitor, 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea. Of the lipid classes, the light dependency was more pronounced with the biosynthesis of triacylglycerols (TG) and wax compared with phospholipids. Examination of [14C]‐label distribution in the glycerolipids suggested that [14C]‐glucose supplied mainly the fatty acid moiety of newly synthesized TG, while [14C]‐glucose provided evenly the fatty acid moiety and the glycerol skeleton of phospholipids. The comparison of [14C]‐labeling of lipid from host coral tissue and symbiont zooxanthellae suggested that [14C]‐glucose entered the coral cell and was processed in parallel in the zooxanthellae and host cells. Furthermore, the coral cells used various [14C]‐labeled sugars for lipid synthesis with similar lipid labeling profile as was the case for glucose. The current study thus supports the view that the low‐molecular‐weight compound, sugars and amino acids, once translocated from zooxanthellae to host cell were metabolized toward lipogenesis as well as glycerol.