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Front. Mar. Sci. | doi: 10.3389/fmars.2019.00699

Linking internal carbonate chemistry regulation and calcification in corals growing at a Mediterranean CO2 vent

  • 1GEOMAR Helmholtz Center for Ocean Research Kiel, Germany
  • 2Department of Biological Sciences, Geological and Environmental, University of Bologna, Italy
  • 3Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Israel
  • 4Department of Physics and Astronomy, University of Bologna, Italy
  • 5University of Haifa, Israel
  • 6Institute of Marine Science, National Research Council, Italy
  • 7Department of Chemistry, University of Bologna, Italy

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledge on their capability of long-term acclimatization to ocean acidification. We used a dual geochemical proxy approach to estimate the calcifying fluid pH (pHcf) and carbonate chemistry of a Mediterranean coral (Balanophyllia europaea) naturally growing along a pH gradient (range: pHTS 8.07-7.74). The pHcf derived from skeletal boron isotopic composition (11B) was 0.3-0.6 units above seawater values and homogeneous along the gradient (mean  sem: Site 1 = 8.39  0.03, Site 2 = 8.34  0.03, Site 3 = 8.34  0.02). Also carbonate ion concentration derived from B/Ca was homogeneous (mean  sem (µmol kg-1): Site 1 = 579  34, Site 2 = 541  27, Site 3 = 568  30) regardless of seawater pH. Furthermore, gross calcification rate (GCR, mass of CaCO3 deposited on the skeletal unit area per unit of time), estimated by a “bio-inorganic model” (IpHRAC), was homogeneous with decreasing pH. The homogeneous GCR, internal pH and carbonate chemistry confirm that the features of the ‘building blocks’ – the fundamental structural components - produced by the biomineralization process were substantially unaffected by increased acidification. Furthermore, the pH up-regulation observed in this study could potentially explain the previous hypothesis that less ‘building blocks’ are produced with increasing acidification ultimately leading to increased skeletal porosity and to reduced net calcification rate obtained by including the total volume of the pore space. In fact, assuming that the available energy at the three sites is the same, this energy at the low pH sites could be partitioned among fewer calicoblastic cells that consume more energy given the larger difference between external and internal pH compared to the control, leading to the production of less building blocks (i.e., formation of pores inside the skeleton structure, determining increased porosity). However, we cannot exclude that also dissolution may play a role in increasing porosity. Thus, the ability of scleractinian corals to maintain elevated pHcf relative to ambient seawater might not always be sufficient to counteract declines in net calcification under ocean acidification scenarios.

Keywords: pH up-regulation, ocean acidfication, Balanophyllia europaea, Mediterranean Sea, Boron, Calcifying fluid, Carbonate chemistry

Received: 03 Jun 2019; Accepted: 31 Oct 2019.

Copyright: © 2019 Wall, Prada, Fietzke, Caroselli, Dubinsky, Brizi, Fantazzini, Franzellitti, Mass, Montagna, Falini and Goffredo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Stefano Goffredo, Department of Biological Sciences, Geological and Environmental, University of Bologna, Ravenna, Emilia-Romagna, Italy, s.goffredo@unibo.it