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

Anthropogenic elevation of atmospheric pCO₂ 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 pCO₂, yielding average aragonite saturation states (Ω_A) of 2.6, 2.3, 1.6, and 0.8. Measurement of calcification (via buoyant weighing) and linear extension (relative to a ¹³⁷Ba/¹³⁸Ba spike) revealed that skeletal accretion was only minimally impaired by reductions in Ω_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 Ω_A = 0.8 resulted from reduced aragonite accretion, rather than from localized dissolution. Accretion of pure aragonite under each Ω_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 Ω_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 Ω_A = 0.8 indicates that their response to future pCO₂-induced ocean acidification could be both abrupt and severe once the critical Ω_A is reached.