A nonlinear calcification response to CO2-induced ocean acidification by the coral Oculina arbuscula

Anthropogenic elevation of atmospheric pCO(2) is predicted to cause the pH of surface seawater to decline by 0.3-0.4 units by 2100 AD, causing a 50% reduction in seawater [CO3 (2-)] and undersaturation with respect to aragonite in high-latitude surface waters. We investigated the impact of CO2-induced ocean acidification on the temperate scleractinian coral Oculina arbuscula by rearing colonies for 60 days in experimental seawaters bubbled with air-CO2 gas mixtures of 409, 606, 903, and 2,856 ppm pCO(2), yielding average aragonite saturation states (Omega(A)) of 2.6, 2.3, 1.6, and 0.8. Measurement of calcification (via buoyant weighing) and linear extension (relative to a Ba-137/Ba-138 spike) revealed that skeletal accretion was only minimally impaired by reductions in Omega(A) from 2.6 to 1.6, although major reductions were observed at 0.8 (undersaturation). Notably, the corals continued accreting new skeletal material even in undersaturated conditions, although at reduced rates. Correlation between rates of linear extension and calcification suggests that reduced calcification under Omega(A) = 0.8 resulted from reduced aragonite accretion, rather than from localized dissolution. Accretion of pure aragonite under each Omega(A) discounts the possibility that these corals will begin producing calcite, a less soluble form of CaCO3, as the oceans acidify. The corals' nonlinear response to reduced Omega(A) and their ability to accrete new skeletal material in undersaturated conditions suggest that they strongly control the biomineralization process. However, our data suggest that a threshold seawater [CO3 (2-)] exists, below which calcification within this species (and possibly others) becomes impaired. Indeed, the strong negative response of O. arbuscula to Omega(A) = 0.8 indicates that their response to future pCO(2)-induced ocean acidification could be both abrupt and severe once the critical Omega(A) is reached.