Exploring Coral Calcification by Calcium Carbonate Overgrowth Experiments

The Scleractinia coral biomineralization process is a representative example of a heterogeneous process of nucleation and growth of biogenic CaCO3 over a mineral phase. Indeed, even if the biomineralization process starts before settlement, the bulk formation of the skeleton takes place only when the larvae attach to a solid substrate, which can be Mg-calcite from coralline algae, and the following growth proceeds on the Mg-calcite surface of the formed baseplate of the planula. Despite this peculiarity and central role of the Mg-calcite substrate, the in vitro overgrowth of CaCO3 on single crystals of Mg-calcite, or calcite, in the presence of magnesium ions and the soluble organic matrix (SOM) extracted from coral skeletons has not been performed until now. In this study, the SOMs from Stylophora pistillata and Oculina patagonica skeletons were used in a set of overgrowth experiments. The overgrown CaCO3 was characterized by microscopic, diffractometric, and spectroscopic techniques. Our results showed that CaCO3 overgrowth in the presence of S. pistillata or O. patagonica SOM produces different effects. However, there appears to be a minor distinction between samples when magnesium ions are present in solution. Moreover, the Mg-calcite substrate appears to be a favorable substrate for the overgrowth of aragonite, differently from calcite. These observations fit with the observed settling of coral larvae on Mg-calcite-based substrates and with the in vivo observation that in the planula aragonite forms on first-formed Mg-calcite crystals. The overall results of this study highlight the importance of magnesium ions, either in the solution or in the substrate, in defining the shape, morphology, and polymorphism of biodeposited CaCO3. They also suggest a magnesium-dependent biological control on the deposition of coral skeletons.