In Vitro Coral Biomineralization under Relevant Aragonite Supersaturation Conditions

The biomineralization of corals occurs under conditions of high and low supersaturation with respect to aragonite, which corresponds to day- or night-time periods of their growth, respectively. Here, in vitro precipitation of aragonite in artificial seawater was investigated at a high supersaturation, allowing spontaneous nucleation and growth, as well as at low supersaturation conditions, which allowed only the crystal growth on the deliberately introduced aragonite seeds. In either chemical systems, soluble organic matrix (SOM) extracted from Balanophyllia europaea (light sensitive) or Leptopsammia pruvoti (light insensitive) was added. The analyses of the kinetic and thermodynamic data of aragonite precipitation and microscopic observations showed that, at high supersaturation, the SOMs increased the induction time, did not affect the growth rate and were incorporated within aggregates of nanoparticles. At low supersaturation, the SOMs affected the aggregation of overgrowing crystalline units and did not substantially change the growth rate. On the basis of the obtained results we can infer that at high supersaturation conditions the formation of nanoparticles, which is typically observed in the skeleton's early mineralization zone may occur, whereas at low supersaturation the overgrowth on prismatic seeds observed in the skeleton fiber zone is a predominant process. In conclusion, this research brings insight on coral skeletogenesis bridging physicochemical (supersaturation) and biological (role of SOM) models of coral biomineralization and provides a source of inspiration for the precipitation of composite materials under different conditions of supersaturation

Systematic study of the effects of polyamines on calcium carbonate precipitation

While negatively charged organic additives are widely used as an effective means to control CaCO3 precipitation, positively charged additives are generally considered to be much less active. Nevertheless, the cationic polyelectrolyte poly(allylamine hydrochloride) has recently been shown to exert significant control over CaCO3 precipitation, driving the formation of thin films and fibers, and other examples suggest that many positively charged additives promote vaterite formation. This article aims to bring together these sometimes conflicting views of the activity of positively charged additives. The effect of a series of polyamines on CaCO3 precipitation was studied, where the polyamines were selected such that the amine group type, the pKa value (of the corresponding conjugated acid), the molecular weight, and the side chain length of the polymers could be evaluated. The results unambiguously demonstrate that polyamines carrying primary amine groups are capable of exerting a significant effect and that the activity of this class of polyamines is strongly dependent on the length of the side chain. In contrast, polyamines comprising with quaternary amines have negligible effect, despite carrying a permanent positive charge. The activity of the most active polyamines therefore depends on their ability to complex with carbonate ions present in solution, and electrostatic attraction alone is not sufficient