La chimie des carbonates dans le cœlentéron d'un corail constructeur de récif : variations spatiales, effet des paramètres environnementaux et lien avec la calcification

Coral reefs, the largest bioconstruction on Earth, are extraordinary biological structures based on the ability of living organisms to produce biominerals in a process known as calcification. The establishment and maintenance of coral reefs depend primarily on the ability of reef-building corals to secrete a calcium carbonate (CaCO3) skeleton, creating a 3D framework essential for the survival of many other marine species. Given the ecological and socio-economic importance of reef-building corals, the calcification process that enables them to efficiently colonize the substrate has been studied in detail at the molecular and physiological level. To build their CaCO3 skeleton, corals require a specific chemical environment found in the extracellular calcifying medium (ECM), a semi-enclosed compartment located between the calicoblastic epithelium and the skeleton. The calicoblastic epithelium is characterized by considerable ion transport capacities, resulting in a supersaturated microenvironment favorable for calcification. Among the key parameters that control chemistry in the ECM, pH and carbonate ion concentration are the most studied, as they control the saturation state of aragonite and thus drive calcification. Whether using indirect or direct methods, all studies show that pH and carbonate ion concentration are higher in ECM than in seawater, suggesting that calcification is a biologically controlled process. Studies dealing with the calcification process of corals often assume a direct link between the external seawater and the ECM. However, the ECM is separated from the seawater by several tissue layers and the cœlenteron. This latter compartment is the only “circulating" fluid body of the coral and is found in both the polyps and the cœnosarc (tissue connecting the polyps). As symbiotic dinoflagellate-containing cells line the cœlenteron, their photosynthetic activity contributes to changes in the chemistry of this compartment, particularly with respect to pH and the concentration of carbonate ions. In the quest to understand the calcification process in corals, numerous data have been obtained independently in the cœlenteron and in the ECM. However, there are still significant gaps in our understanding, particularly regarding the relationship between the carbonate chemistry in the cœlenteron and the ECM. To contribute to this lack of knowledge, the overall aim of my Ph.D. research was to explore how light/dark conditions and environmental stresses influence chemistry in the cœlenteron. To this end, I use liquid ion exchange (LIX) microsensors in combination with the lateral skeleton preparative assay (LSP) to compare zones of different dinoflagellate density in coral tissue: high dinoflagellate density (HDD) in the center of microcolonies growing on glass slides (brown-stained tissue) and low dinoflagellate density (LDD) at the edge of microcolonies (transparent tissue), a zone referred to as the growing edge. These data, collected over time and influenced by variable environmental factors, are essential for the development of dynamic models of calcification. The results of my research provide profound insights into the complex process of calcification, with a focus on deciphering the critical interplay between the cœlenteron and ECM carbonate chemistry.