Manifestations and environmental implications of microbially-induced calcium carbonate precipitation (MICP) by the cyanobacterium Dolichospermum flosaquae

Abstract. The aim of this work is to explore the ability and magnitude of the temperate cyanobacterium Dolichospermum flosaquae in MICP (microbially-induced calcium carbonate precipitation). Environmentally, MICP controls the availability of calcium, carbon and phosphorus in freshwater lakes and simultaneously controls carbon exchange with the atmosphere. Cultures of flosaquae were grown in BG11 medium containing 0, 1, 1.5, 2 and 4 mg Ca²⁺ L⁻¹, as cardinal concentrations previously reported in freshwater lakes, in addition to a control culture (BG11 containing 13 mg Ca²⁺ L⁻¹). Growth (cell number, chlorophyll a, and protein content) of D. flosaquae was generally reduced by elevating calcium concentrations of the different salts used (chloride, acetate, or citrate). D. flosaquae seems able to perform MICP as carbonate alkalinity was sharply induced up to its highest level (six times that of the control) at a citrate concentration of 4 mg Ca²⁺ L⁻¹. Calcium carbonate was formed at a pre-precipitation stage as the minimum pH necessary for precipitation (8.7) has been scarcely approached under such conditions. MICP took place mostly relying on photosynthesis and respiration, but not on urease activity, as urea was not supplemented in the growth media. However, D. flosaquae exhibited strong urease specific activity in in vitro assays (four times that of the control at 4 mg Ca²⁺ citrate L⁻¹). Residual calcium exhibited its lowest value at 4 mg Ca²⁺ citrate L⁻¹, coinciding with the highest alkalinity level. Consumed calcium was increasing with chlorophyll a content, but not with increasing cell numbers. The experiments should be repeated in a future study, but in the presence of urea, to evaluate the full potential of calcium carbonate precipitation of D. flosaquae, its dynamics and impact on biogeochemical cycles of calcium, carbon and phosphorus in freshwater lakes.