Molecular biomineralization of octocoral skeletons: calcite versus aragonite (MINORCA)

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Biomineralization refers to the process by which organisms produce minerals. This process that has shaped the spectacular diversity of shells and skeletons throughout large periods of the Earth's history, and forms the three dimensional framework of coral reefs, one of the most diverse ecosystem on the planet. Anthozoan corals, main reef builders today, produce different polymorphs of calcium carbonate (CaCO3). Scleractinians deposit aragonite and octocorals calcite, with a notable exception, the aragonitic Heliopora. It is however unknown why and how corals produce different CaCO3 polymorphs. Changes in the molar ratio of seawater Mg:Ca during certain periods of Earth history have been proposed as drivers of preferential selection of specific CaCO3 polymorphs at clade origin. While the process of biomineralization is comparatively well studied at the molecular level in Scleractinia, knowledge is still deficient in Octocorallia but necessary for a complete understanding of the ability of corals to control and counter the effects of environmental changes.Here we will combine transcriptomic and proteomic approaches to identify and characterize the gene repertoire involved in the octocoral biomineralization toolkit. We will use three species with a rigid skeleton, which we culture in our research aquaria and for which we recently generated reference transcriptomes, as models: the aragonitic octocoral Heliopora coerulea and the calcitic Tubipora musica, and, to allow comparative analyses, the aragonitic scleractinian Montipora digitata. Three different strategies will be employed to achieve the project's objectives:First, the existing target species transcriptomes will be data mined and compared with published data on corals and other taxa (e.g., calcareous sponges, molluscs, brachiopods, echinoderms) to identify biomineralization-related proteins.Second, for each of our target species, the skeletal proteomes (i.e. organic matrix proteins occluded in the skeleton) will be characterized to further characterize proteins putatively involved in biomineralization.Third, we will expose our three target species to artificial Calcite Sea-like seawater (with a mMg:Ca = 1), because apparently aragonitic corals start to produce calcite under those conditions, but the molecular processes responsible for such CaCO3 polymorph shift are unknown yet. We will monitor the differential expression of biomineralization-related genes in short- and longer-term experiments aiming to fill that knowledge gap.By focusing on corals that produce different CaCO3 polymorphs and employing different experimental approaches, we will provide novel data on coral biomineralization in general, and octocoral biomineralization in particular, which might help to further our understanding of the biological processes that control biomineralization in changing oceans.