Effects of light on the Ba/Ca ratios in coral skeletons

Abstract The ratios of barium to calcium (Ba/Ca) in reef coral skeletons have been used as a proxy to reconstruct temporal change in barium concentration in surface ocean. To reduce the uncertainty of the coral Ba/Ca proxy, the Ba distribution coefficients between seawater and coral skeletons were determined under different light conditions. Coral planulae (Acropora tenuis) were metamorphosed and settled in dishes with commercial peptide Hym-248 in the laboratory. Juvenile corals were infected with symbiotic algae (Durusdinium sp.) and cultured in 5 Ba concentrations (5, 10, 20, 35, and 150 μmol/mol) for 2 months. Half of the dishes were placed under low light conditions (10–33 μmol m-2 s−1), and another half of the dishes were placed under high light conditions (67–84 μmol m-2 s−1). The Ba/Ca ratios in the coral skeletons were highly correlated with those in ambient seawater under both high and low conditions (R2 > 0.8). The Ba distribution coefficients between seawater with 5–35 μmol/mol of Ba concentration and coral skeletons were 0.72 and 2.83 under high and low light conditions, respectively. These results suggest that biological processes related to light availability could influence to Ba/Cacoral.