CaCO3 nucleation by cyanobacteria: laboratory evidence for a passive, surface-induced mechanism

Calcite nucleation on the surface of cyanobacteria of the Synechococcus leopoliensis strain PCC 7942 was investigated to assess the influence of photosynthetic uptake of inorganic carbon and active ion exchange processes across the cell membrane on the nucleation and precipitation mechanisms. We performed longterm precipitation experiments at a constant CO2 level in ambient air by adding suspensions of previously washed cyanobacteria to solutions of NaHCO3/CaCl2 which were supersaturated with respect to calcite. Induction times between 4 and 110 h were measured over a range of saturation states, W, between 8 and 4. The kinetics of CaCO3 nucleation was compared between experiments: (i) with ongoing photosynthesis, (ii) with cells metabolizing but not undergoing photosynthetic uptake of inorganic carbon and (iii) in darkness without photosynthesis. No significant differences were observed between the three treatments. The results reveal that under low nutrient concentrations and permanent CO2 supply, photosynthetic uptake of inorganic carbon predominantly uses CO2 and consequently does not directly influence the nucleation process of CaCO3 at the surface of S. leopoliensis. Furthermore, ion exchange processes did not affect the kinetics, indicating a passive nucleation process wherein the cell surface or extracellular polymers provided preferential sites for mineral nucleation. The catalyzing effect of the cyanobacteria on calcite nucleation was equivalent to a similar to 18% reduction in the specific interfacial free energy of the calcite nuclei. This result and the ubiquitous abundance of cyanobacteria suggest that this process may have an impact on local and global carbon cycling.