Characterization of the CaCO3 biomineral in coralline red algae (Corallinales) from the Pacific coast of Mexico

Abstract

Coralline red algae assimilate HCO₃^– to precipitate CaCO₃ in their tissues in the form of calcite or aragonite. A characterization of the biomolecular content and the crystalline structure of the biomineral of coralline red algae from the Pacific coast of Mexico was performed by powder X-ray diffraction (XRD), scanning electron (SEM) and tunneling microscopy (STM), and Fourier transform infrared spectroscopy (FTIR). The preliminary conclusion drawn from the results is that this type of calcite-aragonite biomineral has a very low organic content occluded within the crystals. FTIR bands at 2945 and 2889 cm^–1 indicate that the most likely organic molecules are carbohydrates; moreover, peptide bond bands (amide I ~1640 and amide II ~1540 cm^–1) were not detected, suggesting that proteins are not related to mineral synthesis or their stabilization. This could be explained if the biomineral is synthesized by a biologically controlled extracellular mineralization process. The XRD study showed two main mineral phases, calcite and aragonite, with very similar structural parameters to the inorganic mineral counterparts. The crystallite shapes, seen by STM, were found as plates and needles with different sizes, between 20 and 100 nm.