Light driven lipid peroxidation of coral membranes and a suggested role in calcification

Low concentrations of calcium ions (Ca2+) are maintained in living cells by the Ca-ATPase calcium pump of the plasma membrane. The presence of a calcium pump in corals has been confirmed and its tempera- ture related breakdown underlies coral bleaching. There is also mounting evidence from stable calcium isotope ratios in the skeleton for involvement of a calcium pump in coral calcification. Hydrogen peroxide (H 2O2) is known to be generated by zooxanthellae during photosynthesis and passes easily through cell membranes. H2O2 is known to cause lipid peroxidation of the plasma membrane and make it leaky to Ca2+. In this study lipid peroxidation was measured in the tissues of Agaricia agaricites. Lipid peroxides in the light doubled from 5.83 to 11.23 μMol.cm-2 (n=17) over the first 3-4 hours then slowly decreased over the next four hours. Glutathione, which in many organisms acts as a lipid peroxidation repair system, was also found at a mean level of 79 μg.cm -2 (n=14) and probably accounts for the observed reduction of lipid peroxide levels. It is suggested that the lipid peroxidation makes the plasma membrane leaky to Ca2+ and thus provides a route by which Ca 2+ enters cells of the calicoblastic layer. A model of calcification, following Adkins et al. (2003), is presented in which calcifica- tion takes place from the extracellular calcifying fluid (ECF) between the calicoblastic layer and the skeleton. The Ca-ATPase calcium pump drives calcification by actively transporting Ca 2+ into the ECF and setting up pH and carbon dioxide (CO2) gradients that enhance the passive diffusion of CO 2 and the formation of calcium carbonate. It is suggested that, in light, H2O2 produced by zooxanthellae makes the membranes leaky to Ca2+. The calcium pump has to work harder to maintain low internal levels of calcium and thus more Ca 2+ would