Dolomite precipitation mediated by Lysinibacillus sphaericus and Sporosarcina psychrophila

Abstract:

[Objective] Dolomite [CaMg(CO3)2], a carbonate mineral composed of calcium and magnesium carbonate is widely distributed both in terrestrial as well as in marine environments including petroleum reservoirs. It has been more than three centuries since dolomite was discovered for the first time. However the origin of dolomite remains unclear, which was referred to as “dolomite problem”. In 1990’s Vasconcelos C. from Swiss Institute of Technology proposed a model for “microbial dolomite formation”, which provided a new perspective on the origin of dolomite. However, this model is not yet optimized to fully clarify the relationship between dolomite formation, bacterial physiology and environmental parameters. The available published data on the dolomite formation mediated by microorganisms were performed at ambient temperature and pressure, which is different from the natural niche of dolomization. In this study, we introduced hydrostatic pressure as an additional environmental parameter in combination with the physiological status of bacteria in order to investigate the dolomite formation under multiple conditions. [Methods] Two strains, Lysinibacillus sphaericus and Sporosarcina psychrophila, which express urea hydrolysis activity, were used as biomass to mediate dolomite precipitation under different environmental conditions like temperature (15 °C and 30 °C), pressures (ambient and 20 MPa) and oxygen concentrations (aerobic and micro-aerobic). To determinate the morphology and component of carbonate precipitation, SEM (scanning electron microscope) combined with EDS (Energy Dispersive X-ray Spectrometry) analysis was performed. To determinate the mineralogy of carbonate precipitation, XRD (X-ray diffraction) analysis was performed. [Results] Both L. sphaericus and S. psychrophila were able to induce carbonate precipitation under all of the given experimental conditions. Both SEM and XRD results confirmed the irregular rhombohedral and spherical dolomite formation mediated by L. sphaericus at 30 °C under 20 MPa pressure and micro-aerobic condition. In addition to dolomite, other minerals (e.g. calcite, nesquehonite, huntite) were also detected to be present in precipitation. [Conclusion] This study has demonstrated that both L. sphaericus and S. psychrophila are able to mediate carbonate precipitation. Especially L. sphaericus is proven to induce dolomite formation under certain conditions. Dolomite formation is significantly influenced by urea hydrolysis activity, temperature and pressure. Our results provide evidence to explain the origin of dolomite from deep sphere and help to optimize the model of “microbial dolomite formation”.