Why does the white tip of stony coral grow so fast without zooxanthellae?

The photosynthesis of zooxanthellae in a coral polyp greatly enchances the calcification rate of a coral. However, the white tip of a coral branch is free of zooxanthellae yet still has a very high calcification rate. Furthermore, the reason for the difference is not clear. In this study, the amount of photopigment, total protein (TP), total organic carbon (TOC), ATP, and lipid in polyps from the white tip and brown stalk of a branch of stony coral were measured. Samples of Acropora hyacinthus and A. formosa were collected from southern Taiwan between 1985 and 1987. The results showed that the ATP concentration in polyps of the white tip was much higher than that in polyps of the brown stalk. Conversely, the amount of TP, TOC and measured lipids in polyps of the brown stalk were all higher than those of the white tip. It was the high concentration of ATP in cells that gave these polyp tips the vitality to sustain the energy requirements of such a rapid calification rate. Facilitated diffusion, due to the high metabolite gradient created by cell activity, could be the major driving force for the transport of photosynthetic product from stalk to tip.