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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.

Anthropogenic CO 2 is a major driver of current environmental change in most ecosystems 1 , and the related ocean acidification (OA) is threatening marine biota 2 . With increasing pCO 2 , calcification rates of several species decrease 3 , although cases of up-regulation are observed 4 . Here, we show that biological control over mineralization relates to species abundance along a natural pH gradient. As pCO 2 increased, the mineralogy of a scleractinian coral (Balanophyllia europaea) and a mollusc (Vermetus triqueter) did not change. In contrast, two calcifying algae (Padina pavonica and Acetabularia acetabulum) reduced and changed mineralization with increasing pCO 2 , from aragonite to the less soluble calcium sulphates and whewellite, respectively. As pCO 2 increased, the coral and mollusc abundance was severely reduced, with both species disappearing at pH < 7.8. Conversely, the two calcifying and a non-calcifying algae * Corresponding authors: Stefano Goffredo -s.goffredo@unibo.it, Tel. +39 051 2094244, Fax +39 051 2094286.; Giuseppe Falinigiuseppe.falini@unibo.it, Tel. +39 051 2099484 . Author Contributions S. G., Z. D., and G. F. conceived and designed research. S. G., F. P., E. C., and B. C. collected the samples and performed the diving fieldwork. L. P., S. F., M. R., and G. F. performed the lab experiments. S. G., F. P., E. C., B. C., L. P., P. F., M. R., and G. F. analysed the data. All authors wrote the manuscript and participated in the scientific discussion. Competing Financial InterestsThe authors declare no competing financial interests. Europe PMC Funders GroupAuthor Manuscript Nat Clim Chang. Author manuscript; available in PMC 2015 January 01. Published in final edited form as:Nat Clim Chang. 2014 July 1; 4(7): 593-597. doi:10.1038/nclimate2241. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts (Lobophora variegata) showed less severe or no reductions with increasing pCO 2 , and were all found at the lowest pH site. The mineralization response to decreasing pH suggests a link with the degree of control over the biomineralization process by the organism, as only species with lower control managed to thrive in the lowest pH.Several studies on the influence of pH on crystallography and texture of calcified regions are ex situ, short-term experiments on isolated organisms 5 , providing important information, but unrepresentative of natural ecosystems and failing to assess long-term effects 6 . There is a great need of long-term analyses on OA effects on marine ecosystems acclimated to high pCO 2 , as found around CO 2 vents. Vents are not perfect predictors of future oceans, owing to pH variability, proximity of unaffected populations, and co-varying environmental parameters 7 . However, vents acidify seawater on sufficiently large temporal and spatial scales to integrate ecosystem processes 6 , acting as "natural laboratories". In Papua New Guinea vents, reductions in coral diversity, recruitment, abundance, and shifts in competitive interactions are found 8 . I...