Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: A review on interactions and control by carbonate chemistry

Photosynthesis and calcification in zooxanthellate scleractinian corals and coral reefs are reviewed at several scales: cellular (pathways and transpo rt mechanisms of inorganic carbon and calcium), organismal (interaction bet ween photosynthesis and calcification, effect of light) and ecosystemic (co mmunity primary production and calcification, and air-sea CO2 exchanges). The coral host plays a major role in supplying carbon for the photosynthesi s by the algal symbionts through a system similar to the carbon-concentrati ng mechanism described in free living algal cells;The details of carbon sup ply to the calcification process are almost unknown, but metabolic CO2 seem s to be a significant source; Calcium supply for calcification is diffusion al through oral layers, and active membrane transport only occurs between t he calicoblastic cells and the site of calcification. Photosynthesis and ca lcification are tightly coupled in zooxanthellate scleractinian corals and coral reef communities. Calcification Is, on average, three times higher in light than in darkness. The recent suggestion that calcification is dark-r epressed rather than light-enhanced is not supported by the literature. The re is a very strong correlation between photosynthesis and calcification at both the organism and community levels, but the ratios of calcification to gross photosynthesis (0.6 in corals and 0.2 in reef communities) differ fr om unity, and from each other as a function of level. The potential effect of global climatic changes (pCO(2) and temperature) on the rate of calcification is also reviewed. In various calcifying photosyn thetic organisms and communities, the rate of calcification decreases as a function of increasing pCO(2) and decreasing calcium carbonate saturation s tate, The calculated decrease in CaCO3 production, estimated using the scen arios considered by the International Panel on Climate Change (IPCC), is 10 % between 1880 and 1990, and 9-30% (mid estimate: 22%) from 1990 to 2100. I nadequate understanding of the mechanism of calcification and its interacti on with photosynthesis severely limits the ability to provide an accurate p rediction of future changes in the rate of calcification.