Influence of temperature, salinity and Mg:Ca ratio on microbially-mediated formation of Mg-rich carbonates by Virgibacillus strains isolated from a sabkha environment

Studies have demonstrated that microbes facilitate the incorporation of Mg into carbonate minerals, leading to the formation of potential dolomite precursors. Most microbes that are capable of mediating Mg-rich carbonates have been isolated from evaporitic environments in which temperature and salinity are higher than those of average marine environments. However, how such physicochemical factors affect and concur with microbial activity influencing mineral precipitation remains poorly constrained. Here, we report the results of laboratory precipitation experiments using two mineral-forming Virgibacillus strains and one non-mineral-forming strain of Bacillus licheniformis, all isolated from the Dohat Faishakh sabkha in Qatar. They were grown under different combinations of temperature (20°, 30°, 40 °C), salinity (3.5, 7.5, 10 NaCl %w/v), and Mg:Ca ratios (1:1, 6:1 and 12:1). Our results show that the incorporation of Mg into the carbonate minerals is significantly affected by all of the three tested factors. With a Mg:Ca ratio of 1, no Mg-rich carbonates formed during the experiments. With a Mg:Ca ratios of 6 and 12, multivariate analysis indicates that temperature has the highest impact followed by salinity and Mg:Ca ratio. The outcome of this study suggests that warm and saline environments are particularly favourable for microbially mediated formation of Mg-rich carbonates and provides new insight for interpreting ancient dolomite formations.