BIOMINERALIZATION OF CARBONATES IN MODERN MICROBIAL SEDIMENTS

Microbial mats are lithified or unlithified sedimentary structures often associated with precipitating carbonate minerals. Sulfate reducing bacteria (SRB), one of the groups of microorganisms constituting the microbial mats, play an active role in cycling elements such as carbon and sulfur and induce precipitation of carbonates by modifying pH and alkalinity as a result of their metabolic activity. The ability of SRB to induce biomineralization of carbonates was investigated by conducting experiments using sulfate-rich sediments collected from two hypersaline lakes: Lake Estancia, New Mexico and Great Salt Plains Lake, Oklahoma. Lake Estancia sediments contain dolomite (CaMg(CO₃)₂) and high Mg-calcite (CaCO₃ containing >4 mole% MgCO₃) whereas both lakes had gypsum (CaSO₄.2H₂O) precipitating in their natural environments. The sediments from both locations were tested in small batch reactors under atmospheric conditions and with the presence or absence of light. Light availability was included as an important factor to investigate the activity of SRB under different light conditions and to determine the effect of any photosynthesizing bacteria. Carbonate mudrock was used as another test variable in the experiments to determine if its presence would have any influence on the precipitation process. Sediments were further inoculated in an enrichment media to enhance the growth of SRB, which were then used in another series of similar batch experiments. Preliminary analysis of pH, Eh and calcium concentration revealed conditions favorable for carbonate formation. An increase in pH is especially a potential indicator of microbial activity and carbonate formation. Lake Estancia reactors that were exposed to light showed a more prominent pH increase while the presence of carbonate mudrock did not affect the pH conditions much. The reverse was observed with the Great Salt Plain reactors in which a drastic pH increase was observed when there was no carbonate source added while light had little to no effect on pH. Reactors set with the lake sediments and those with SRB enrichment cultures showed a similar trend in pH changes, indicating that the microbial community in both lakes might be dominated by SRB. It can further be proposed that this microbial activity can be used as a potential CO₂ and SO₂ sequestration mechanism.