The Role of Ca2+/CO32- Ratio in Calcite Growth: An Underestimated Factor in Calcium Carbonate Biomineralization

Laboratory investigations of calcium carbonate growth often employ stoichiometric solution compositions, with equivalent activities of calcium and carbonate species, or simply consider precipitation kinetics in the context of saturation state alone. However, nonstoichiometric solutions, with nonequivalent Ca2+ and CO32- activities, are the rule in natural biomineralizing systems where metabolic reactions produce microenvironments exhibiting disparate Ca2+/CO32- ratios proximal to biological membranes. Additionally, recent evidence suggests that the calcium/carbonate ratio of the oceans has varied over geologic time, making the examination of this solution parameter important for understanding paleoenvironmental signatures in biominerals, as well as for predicting the response of biomineralizing systems to anthropogenic forcing of seawater chemistry under elevated atmospheric carbon dioxide levels. Here we investigate the role of Ca2+/CO32- ratio, at constant saturation state, in determining calcite growth using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). These coupled techniques provide kinetic measurements across multiple length-scales. Our results indicate that changes in Ca2+/CO32- ratio significantly affect the overall growth rate as well as the anisotropy of growth features on the crystal surface. We further demonstrate that carbonate biomineralization cannot be understood in terms of bulk solution chemistry alone, but requires specific knowledge of both the structure of the biomineral surface and the interaction of solution species with elementary steps on the surface. These findings suggest that differences in Ca2+/CO32- ratio may account for some of the complexity associated with carbonate biomineral proxies and may be an underestimated aspect of biological control over CaCO3 mineralization.