Seawater carbonate chemistry and calcification, survival, concentrations of Symbiodiniaceae, chlorophyll a and protein of Caribbean corals

Coral calcification is expected to decline as atmospheric carbon dioxide concentration increases. We assessed the potential of Porites astreoides, Siderastrea siderea and Porites porites to survive and calcify under acidified conditions in a 2-year field transplant experiment around low pH, low aragonite saturation (Omega arag) submarine springs. Slow-growing S. siderea had the highest post-transplantation survival and showed increases in concentrations of Symbiodiniaceae, chlorophyll a and protein at the low Omega arag site. Nubbins of P. astreoides had 20% lower survival and higher chlorophyll a concentration at the low Omega arag site. Only 33% of P. porites nubbins survived at low Omega arag and their linear extension and calcification rates were reduced. The density of skeletons deposited after transplantation at the low Omega arag spring was 15–30% lower for all species. These results suggest that corals with slow calcification rates and high Symbiodiniaceae, chlorophyll a and protein concentrations may be less susceptible to ocean acidification, albeit with reduced skeletal density. We postulate that corals in the springs are responding to greater energy demands for overcoming larger differences in carbonate chemistry between the calcifying medium and the external environment. The differential mortality, growth rates and physiological changes may impact future coral species assemblages and the reef framework robustness.