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Studies have demonstrated that microbes facilitate the incorporation of Mg2+ 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 Mg2+:Ca2+ ratios (1:1, 6:1 and 12:1). Our results show that the incorporation of Mg2+ into the carbonate minerals is significantly affected by all of the three tested factors. With a Mg2+:Ca2+ ratio of 1, no Mg-rich carbonates formed during the experiments. With a Mg2+:Ca2+ ratios of 6 and 12, multivariate analysis indicates that temperature has the highest impact followed by salinity and Mg2+:Ca2+ 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. [ABSTRACT FROM AUTHOR]

The origin of dolomite – a common mineral in the geological record – is the subject of an ongoing debate. Among different hypotheses, it has been proposed that extracellular polymeric substances (EPS) excreted by microbes include organic molecules that catalyze the incorporation of Mg in the carbonate mineral. However, limited information exists on the composition of the EPS produced by Ca Mg carbonate-forming microbes, which in turn hampers a precise understanding of their role in the mineralization mechanism. Here, we present the results of laboratory experiments in which we cultured different strains of microbes, characterized their EPS, and identified components associated with carbonate minerals with high mol% Mg. Two Virgibacillus strains known to mediate the formation of Mg-rich carbonates, as well as a strain of Bacillus licheniformis – a negative control that does not mediate mineral formation but produces EPS, were grown under different salinities and temperatures, which caused them to produce EPS with different compositions. The EPS were subsequently characterized by measuring total carbohydrate (TCHO) and total protein (TP) contents, as well as by Fourier-transform infrared spectroscopy (FTIR). At the tested conditions, we found that Mg-carbonates with a mol% Mg higher than 40% (i.e., potential dolomite precursor phases) formed exclusively in association with EPS rich in carbohydrates (TCHO > than 75% of the total mass). FTIR spectra of Ca Mg carbonate-forming strains were distinct from those of the non-mineral-forming strain in areas associated with the protein structures responsible for the formation of hydrogels, which contribute to hydration or dehydration of ionic clusters; further differences have been observed in the regions of phosphoryl functional groups. These results provide insight on which fraction of organic molecules and specific functional groups are, among the many constituents of EPS, important for mineral nucleation and incorporation of Mg into carbonate minerals, a crucial step for the formation of dolomite in natural environments. • EPS associated with protodolomite are characterized by a high proportion of total carbohydrate (> 75% of the total mass). • FTIR spectra of the EPS produced by mineral-forming strains differ from those of the non-mineral-forming strain. • Negatively charged functional groups favor —likely by dehydration of Mg— the incorporation of Mg into carbonate minerals. [ABSTRACT FROM AUTHOR]