The role of light in mediating the effects of ocean acidification on coral calcification.

We tested the effect of light and Pco₂ on the calcification and survival of Pocillopora damicornis recruits settled from larvae released in southern Taiwan. In March 2011, recruits were incubated at 31, 41, 70,122 and 226μmol photons m^-2s^-1 under ambient (493patm) and high Pco₂ (878μatm). After 5days, calcification was measured gravimetrically and survivorship estimated as the number of living recruits. Calcification was affected by the interaction of Pco₂ with light, and at 493patm Pco₂ the response to light intensity resembled a positive parabola. At 878μatm Pco_2- the effect of light on calcification differed from that observed at 493 μatm Pco₂, with the result that there were large differences in calcification between 493 patm and 878 patm Pco₂ at intermediate light intensities (ca. 70 μmol photons m^-2 s^-1), but similar rates of calcification at the highest and lowest light intensities. Survivorship was affected by light and Pco₂, and was highest at 122(μmol photons m^-2S^-1 in both PcO₂ treatments, but was unrelated to calcification. In June 2012 the experiment was repeated, and again the results suggested that exposure to high Pco₂ decreased calcification of P. damicornis recruits at intermediate light intensities, but not at lower or higher intensities. Together, our findings demonstrate that the effect of PCo₂ on coral recruits can be light dependent, with inhibitory effects of high Pco₂ on calcification at intermediate light intensities that disappear at both higher and lower light intensities. [ABSTRACT FROM AUTHOR]