ABSTRACT

Cyanobacteria induced biomineralization of atmospheric CO₂ is a natural process leading to the formation of carbonates by spontaneous precipitation or through the presence of nucleation sites, under supersaturated conditions. As importance of basaltic rocks in the carbon cycle has already been highlighted, basaltic glass was chosen to test its ability to release cations needed for carbonate formation in presence of Synechococcus sp. cyanobacteria. Active cyanobacteria were expected to generate a local alkaline environment through photosynthetic metabolism. This process produces oxygen and hydroxide ions as waste products, raising the pH of the immediate cell surface vicinity and indirectly enhancing the carbonate CO₃^2- concentration and providing the a degree of saturation that can lead to the formation of calcite CaCO₃ or magnesite MgCO₃. In the presence of active cells, the saturation index (SI) increased from -10.56 to -9.48 for calcite and from -13.6 to -12.5 for magnesite, however they remained negative due to the low Ca²⁺ and Mg²⁺ activities. Dead cells were expected to act as nucleation sites by the stepwise binding of carbonate with Ca²⁺ and Mg²⁺ on their surface. In the presence of inactive cells, SI values were closer to 0 but still negative due to the low pH and cation concentrations. Our results highlight that our current understanding of the carbon cycle suggests that Earth’s climate is stabilized by a negative feedback involving CO₂ consumption and especially during chemical weathering of silicate minerals.