Marginal Reefs Under Stress: Physiological Limits Render Galapagos Corals Susceptible to Ocean Acidification and Thermal Stress

Ocean acidification (OA) and thermal stress may undermine corals' ability to calcify and support diverse reef communities, particularly in marginal environments. Coral calcification depends on aragonite supersaturation (omega >> 1) of the calcifying fluid (cf) from which the skeleton precipitates. Corals actively upregulate pH(cf) relative to seawater to buffer against changes in temperature and dissolved inorganic carbon, which together control omega(cf). Here we assess the buffering capacity of modern and fossil corals from the Galapagos Islands that have been exposed to sub-optimal conditions, extreme thermal stress, and OA. We demonstrate a significant decline in pH(cf) and omega(cf) since the pre-industrial era, trends which are exacerbated during extreme warm years. These results suggest that there are likely physiological limits to corals' pH buffering capacity, and that these constraints render marginal reefs particularly susceptible to OA.