Linking Internal Carbonate Chemistry Regulation and Calcification in Corals Growing at a Mediterranean CO2 Vent

Marlene Wall, Fiorella Prada, Jan Fietzke, Erik Caroselli, Zvy Dubinsky, Leonardo Brizi, Paola Fantazzini, Silvia Franzellitti, Tali Mass, Paolo Montagna, Giuseppe Falini, Stefano Goffredo

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10 Scopus citations


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 (OA). 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 (δ11B) 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 computed 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 OA scenarios.

Original languageEnglish
Article number699
JournalFrontiers in Marine Science
StatePublished - 19 Nov 2019

Bibliographical note

Publisher Copyright:
© Copyright © 2019 Wall, Prada, Fietzke, Caroselli, Dubinsky, Brizi, Fantazzini, Franzellitti, Mass, Montagna, Falini and Goffredo.


The research leading to these results was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number (249930-CoralWarm: Corals and global warming: the Mediterranean versus the Red Sea). EC was supported by the ALMA IDEA grant of the University of Bologna for the project “STRAMICRO.” MW was supported by the Austrian Science fund (FWF), Schrödinger Fellowship J-3667: pH up-regulation in tropical corals: a key mechanism? Implications for the future and the past.

FundersFunder number
Seventh Framework Programme249930
FP7 Ideas: European Research Council
European Research Council
Austrian Science Fund
Università di Bologna


    • Balanophyllia europaea
    • Mediterranean Sea
    • boron
    • calcifying fluid
    • carbonate chemistry
    • ocean acidification
    • pH up-regulation


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