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Ocean acidification is predicted to impact ecosystems reliant on calcifying organisms, potentially reducing the socioeconomic benefits these habitats provide. Here we investigate the acclimation potential of stony corals living along a pH gradient caused by a Mediterranean CO2 vent that serves as a natural long-term experimental setting. We show that in response to reduced skeletal mineralization at lower pH, corals increase their skeletal macroporosity (features >10 μm) in order to maintain constant linear extension rate, an important criterion for reproductive output. At the nanoscale, the coral skeleton's structural features are not altered. However, higher skeletal porosity, and reduced bulk density and stiffness may contribute to reduce population density and increase damage susceptibility under low pH conditions. Based on these observations, the almost universally employed measure of coral biomineralization, the rate of linear extension, might not be a reliable metric for assessing coral health and resilience in a warming and acidifying ocean.

The demographic characteristics of the solitary zooxanthellate scleractinian Balanophyllia europaea, endemic to the Mediterranean, were determined in six populations, on a latitudinal gradient along the Italian coast, and compared with the mean annual sea surface temperature (SST). Growth rate correlated negatively, and asymptotic length of the individuals positively with SST. With increasing SST, the distributions of age frequencies moved away from a typical steady state structure (i.e., exponential decrease in the frequency of individuals with age), indicating less stable populations and showed a deficiency of individuals in the younger-age classes. These observations suggest that high temperatures are an adverse factor to the B. europaea symbiosis. Using projected increases in seawater temperature, most of the B. europaea populations in the Mediterranean are expected to be close to their thermal limits by 2100 and the populations at that time may support few young individuals.

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The correlation between solar radiation and sea surface temperature (SST) and growth was assessed along a latitudinal gradient. Extension rate and skeletal density were both correlated with calcification rate, indicating that calcium carbonate deposition was allocated evenly between skeletal density and linear extension. Unlike most studies on other tropical and temperate corals, in which calcification was positively correlated with solar radiation and SST, in the present study calcification was not correlated with solar radiation, whereas it was negatively correlated with SST. We hypothesize that photosynthesis of the symbiotic algae of Balanophyllia europaea is inhibited at high temperatures, consequently causing an inhibition of calcification. The regressions between calcification and SST predicted that the calcification of B. europaea would be depressed at 20.5-21.0uC mean annual SST. The scenarios of the Intergovernmental Panel on Climate Change conclude that by 2100, SST will exceed this physiological threshold for most of the populations considered in this study. This study comprises the first field investigation of the relationships between environmental parameters and calcification of a Mediterranean coral and highlights the risks of losing Mediterranean marine biodiversity over the course of future decades.