The effect of cyanobacteria on silica precipitation at neutral pH: implications for bacterial silicification in geothermal hot springs

In this study, we performed silica precipitation experiments with the cyanobacteria Calothrix sp. to investigate the mechanisms of silica biomineralization. Batch silica precipitation experiments were conducted at neutral pH as a function of time, Si saturation states, temperature and ferrihydrite concentrations. The experimental results show that in solutions undersaturated with respect to amorphous silica, the interaction between Si and cell surface functional groups is weak and minimal Si sorption onto cyanobacterial surfaces occurs. In solutions at high Si supersaturation states, abiotic Si polymerization is spontaneous, and at the time scales of our experiments (1-50 h) the presence of cyanobacteria had a negligible effect on silica precipitation kinetics. At lower supersaturation states, Si polymerization is slow and the presence of cyanobacteria do not promote Si-solid phase nucleation. In contrast, experiments conducted with ferrihydrite-coated cyanobacteria significantly increase the rate of Si removal, and the extent to which Si is removed increases as a function of ferrihydrite concentration. Experiments conducted with inorganic ferrihydrite colloids (without cyanobacteria) removes similar amounts of Si, suggesting that microbial surfaces play a limited role in the silica precipitation process. Therefore, in supersaturated hydrothermal waters, silica precipitation is largely nonbiogenic and cyanobacterial surfaces have a negligible effect on silica nucleation.