Abstract
Uptake of anthropogenic CO2 by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (Ωarag). The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Porites sp. and Stylophora pistillata, exposed to high pCO2 (or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Porites sp. and S. pistillata were kept for 6–14 months under controlled aquarium conditions characterized by normal and elevated pCO2 conditions, corresponding to pHT values of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental timescale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater Ωarag < 1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C), and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density, and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO2 (low pH) conditions. Both species showed similar trends of δ11B depletion and δ18O enrichment under reduced pH, whereas the δ13C results imply species-specific metabolic response to high pCO2 conditions. The skeletal δ11B values plot above seawater δ11B vs. pH borate fractionation curves calculated using either the theoretically derived αB value of 1.0194 (Kakihana et al. (1977) Bull. Chem. Soc. Jpn. 50, 158) or the empirical αB value of 1.0272 (Klochko et al. (2006) EPSL 248, 261). However, the effective αB must be greater than 1.0200 in order to yield calculated coral skeletal δ11B values for pH conditions where Ωarag ⩾ 1. The δ11B vs. pH offset from the seawater δ11B vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal δ13C and δ18O. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeletons.
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