Assembly of the Intraskeletal Coral Organic Matrix during Calcium Carbonate Formation

Scleractinia coral skeleton formation occurs by a heterogeneousprocess of nucleation and growth of aragonite in which intraskeletalsoluble organic matrix molecules, usually referred to as SOM, playa key role. Several studies have demonstrated that they influencethe shape and polymorphic precipitation of calcium carbonate. However,the structural aspects that occur during the growth of aragonite havereceived less attention. In this research, we study the depositionof calcium carbonate on a model substrate, silicon, in the presenceof SOM extracted from the skeleton of two coral species representativeof different living habitats and colonization strategies, which wepreviously characterized. The study is performed mainly by grazingincidence X-ray diffraction with the support of Raman spectroscopyand electron and optical microscopies. The results show that SOM macromoleculesonce adsorbed on the substrate self-assembled in a layered structureand induced the oriented growth of calcite, inhibiting the formationof vaterite. Differently, when SOM macromolecules were dispersed insolution, they induced the deposition of amorphous calcium carbonate(ACC), still preserving a layered structure. The entity of these effectswas species-dependent, in agreement with previous studies. In conclusion,we observed that in the setup required by the experimental procedure,the SOM from corals appears to present a 2D lamellar structure. Thisstructure is preserved when the SOM interacts with ACC but is lostwhen the interaction occurs with calcite. This knowledge not onlyis completely new for coral biomineralization but also has strongrelevance in the study of biomineralization on other organisms.