The puzzling presence of calcite in skeletons of modern solitary corals from the Mediterranean Sea

Stefano Goffredo, Erik Caroselli, Francesco Mezzo, Leonardo Laiolo, Patrizia Vergni, Luca Pasquini, Oren Levy, Francesco Zaccanti, Aline Tribollet, Zvy Dubinsky, Giuseppe Falini

Research output: Contribution to journalArticlepeer-review

23 Scopus citations

Abstract

The skeleton of scleractinian corals is commonly believed to be composed entirely of aragonite due to the current Mg/Ca molar ratio of seawater, which thermodynamically favours the deposition of this polymorph of calcium carbonate (CaCO 3). However, some studies have shown that other forms of CaCO 3 such as calcite can be present in significant amount (1-20%) inside tropical coral skeletons, significantly impacting paleo-reconstructions of SST or other environmental parameters based on geochemical proxies. This study aims at investigating for the first time, (1) the skeletal composition of two Mediterranean solitary corals, the azooxanthellate Leptopsammia pruvoti and the zooxanthellate Balanophyllia europaea, across their life cycle, (2) the distribution of the different CaCO 3 forms inside skeletons, and (3) their implications in paleoclimatology. The origin of the different forms of CaCO 3 observed inside studied coral skeletons and their relationships with the species' habitat and ecological strategies are also discussed. CaCO 3 composition of L. pruvoti and B. europaea was investigated at six sites located along the Italian coasts. Skeleton composition was studied by means of X-ray powder diffraction and Fourier transform infrared spectroscopy. A significant amount of calcite (1-23%) was found in more than 90% of the studied coral skeletons, in addition to aragonite. This calcite was preferentially located in the basal and intermediate areas than at the oral pole of coral skeletons. Calcite was also mainly located in the epitheca that covered the exposed parts of the coral in its aboral region. Interestingly in B. europaea, the calcite content was negatively correlated with skeleton size (age). The presence of calcite in scleractinian corals may result from different mechanisms: (1) corals may biologically precipitate calcite crystals at their early stages in order to insure their settlement on the substrate of fixation, especially in surgy environments; (2) calcite presence may result from skeletons of other calcifying organisms such as crustose coralline algae; and/or (3) calcite may result from the infilling of galleries of boring microorganisms which are known to colonize coral skeletons. We suggest that more than one of the above mentioned processes are involved.

Original languageEnglish
Pages (from-to)187-199
Number of pages13
JournalGeochimica et Cosmochimica Acta
Volume85
DOIs
StatePublished - 15 May 2012

Bibliographical note

Funding Information:
This study is part of the FP7 IDEAS EU-Research Project “CoralWarm” (Grant Agreement no. 249930 ). We thank L. Bortolazzi, A. Comini, M. Cova, G. Gasparini, P. Bonzi, M. Ghelia, G. Neto, and L. Tomesani for their underwater assistance in collecting the samples. The diving centres Centro Immersioni Pantelleria, Il Pesciolino, Polo Sub, and Sub Maldive supplied logistic assistance in the field. The Bologna Scuba Team (BST) and the Scientific Diving School (SDS) collaborated in the underwater activities. The Marine Science Group ( http://www.marinesciencegroup.org ) supplied scientific, technical, and logistical support. This research was financed by the Ministry of Education, University and Research (MIUR), the Ministry of Tourism of the Arab Republic of Egypt, the Associazione dei Tour Operator Italiani (ASTOI), the Project AWARE Foundation, the Scuba Nitrox Safety International (SNSI), the Scuba Schools International (SSI), the Underwater Life Project (ULP), the Marine & Freshwater Science Group Association, and the Consorzio Interuniversitario di Ricerca della Chimica dei Metalli nei Sistemi Biologici. The experiments complied with current Italian law.

Funding

This study is part of the FP7 IDEAS EU-Research Project “CoralWarm” (Grant Agreement no. 249930 ). We thank L. Bortolazzi, A. Comini, M. Cova, G. Gasparini, P. Bonzi, M. Ghelia, G. Neto, and L. Tomesani for their underwater assistance in collecting the samples. The diving centres Centro Immersioni Pantelleria, Il Pesciolino, Polo Sub, and Sub Maldive supplied logistic assistance in the field. The Bologna Scuba Team (BST) and the Scientific Diving School (SDS) collaborated in the underwater activities. The Marine Science Group ( http://www.marinesciencegroup.org ) supplied scientific, technical, and logistical support. This research was financed by the Ministry of Education, University and Research (MIUR), the Ministry of Tourism of the Arab Republic of Egypt, the Associazione dei Tour Operator Italiani (ASTOI), the Project AWARE Foundation, the Scuba Nitrox Safety International (SNSI), the Scuba Schools International (SSI), the Underwater Life Project (ULP), the Marine & Freshwater Science Group Association, and the Consorzio Interuniversitario di Ricerca della Chimica dei Metalli nei Sistemi Biologici. The experiments complied with current Italian law.

FundersFunder number
AWARE Foundation
Associazione dei Tour Operator Italiani
Consorzio Interuniversitario di Ricerca della Chimica dei Metalli nei Sistemi Biologici
Ministry of Tourism of the Arab Republic of Egypt
Scuba Nitrox Safety International
Scuba Schools International
Seventh Framework Programme249930
Ministero dell’Istruzione, dell’Università e della Ricerca
Statens Serum Institut

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