Exploring coral biomineralization in gelling environments by means of a counter diffusion system

The crystallization of skeletal aragonite by corals takes place in sites whose physical characteristics resemble those of a highly viscous sol or a gel. In these sites, biomolecules are secreted by calicoblast cells of the coral and some of them become entrapped in the skeleton. To explore the biomineralization process, a series of calcium carbonate crystallization experiments were carried out in a counter-diffusion system (CDS) containing a viscous agarose sol with two dissolved intra-skeletal soluble organic matrices (SOM) that were extracted from Balanophyllia europaea, a zooxanthellate coral, and Leptopsammia pruvoti, an azooxanthellate species. The influence of the viscosity of the media and the presence of Mg2+ were investigated in two additional sets of experiments, one using an agarose gel of variable viscosity, and another allowing Mg2+ to diffuse from the cationic reservoir. The main findings are the following: (i) the species-specific molecular composition of the two SOMs has a different impact on the crystallization parameters and morphology of calcium carbonate; (ii) the viscosity of the gelling media, and thus its porosity, is important in regulating the SOM action; (iii) Mg 2+ is important in defining specific and sharp limits of supersaturation under which crystallization occurs; (iv) the polymorph distribution is determined by SOM concentration. Thus, through the use of the CDS, it was possible to first study in vitro the biomineralization of zooxanthellate and azooxanthellate corals