Ocean Acidification and Control of Reef Coral Calcification by Boundary Layer Limitation of Proton Flux

Since the beginning of the industrial revolution, the concentration of atmospheric CO₂ has been rising due to the burning of fossil fuels. increased absorption of this CO₂ by the oceans is lowering the seawater pH and aragonite saturation state (Ω_ar). This process is known as ocean acidification (OA). numerous studies have shown a direct correlation between declining ocean pH, declining Ω_ar, and declining coral growth, but the mechanism is not understood. Various experiments designed to evaluate the relative importance of pH, CO₃ ^2–, Ω_ar, HCO₃ ^–, aqueous CO₂, total alkalinity, and total inorganic carbon (C_t) to coral calcification have led to opposing conclusions. A reanalysis of existing data suggests that the mechanism is diffusion limitation of net H⁺ transport through the boundary layer caused by increasing [H⁺] in the water column. The resulting “proton flux hypothesis” offers an explanation for the reduction in calcification caused by OA and other phenomena associated with increasing acidification. The hypothesis states that the lowered calcification rate observed in corals under increasing conditions of OA can be attributed to higher [H⁺] in the seawater with consequent decrease in the efflux of H⁺ through the boundary layer.