Unpuzzling pH up-regulation and calcification in corals growing at CO2 Mediterranean vents

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledgeon their capability of long-term acclimatization to ocean acidification (OA). We used a dual geochemicalproxy approach to estimate the calcifying fluid pH (pHcf) and carbonate chemistry of a Mediterranean coral(Balanophyllia europaea) naturally growing along a pH gradient. The pHcf derived from skeletal boronisotopic composition (11B) was homogeneous along the gradient, exhibiting an increased up-regulationcapability with increasing acidification. Also, carbonate ion concentration derived from B/Ca washomogeneous regardless of seawater pH. Furthermore, gross calcification rate (GCR), estimated by a “bio-inorganic model” (IpHRAC), was homogeneous with decreasing pH. This determined an apparentlyunresolved puzzle with the previously observed decrease in net calcification rates on the same specimensalong this gradient. However, on the contrary, the homogeneous GCR was in agreement with the previousresults in which the ‘building blocks’ produced by the biomineralization process were substantiallyunaffected by increased acidification. Furthermore, the pH up-regulation observed in this study provides asubstantial contribution in solving the puzzle. In fact, it confirms the previous hypothesis that less buildingblocks are produced with increasing acidification by showing thatB. europaea likely produces less buildingblocks because part of its energy is used to maintain elevated calcifying fluid pH with increasing acidity.Thus, the ability of scleractinian corals to maintain elevated pHcf relative to ambient seawater might notalways be sufficient to counteract declines in net calcification under ocean acidification scenarios.