This paper provides strong evidence for the contribution of the phylum Firmicutes in mediating the primary precipitation of Mg-rich carbonates (hydromagnesite, dolomite, magnesite, and nesquehonite) in recent microbialites from a highly alkaline and ephemeral inland lake (Las Eras, Central Spain). The carbonate mineral precipitation occurs sequentially as the microbial mats decay. Scanning electron microscopy (SEM) provided solid proof that hydromagnesite nucleation is initiated on the exopolymeric substances (EPS) and the microbial cells associated to the microbial mat degradation areas. The progressive mineralization of the EPS and bacterial cells by hydromagnesite plate-like crystals on their surface, results in the entombment of the bacteria and formation of radiating aggregates of hydromagnesite crystals. The hydrous phases, mostly hydromagnesite, were produced at a high percentage in the first stages of the microbial degradation of organic matter. When the availability of organic substrates declines, the heterotrophs tend to reduce their number and metabolic activity, remain dormant. At this stage, the anhydrous phases, dolomite and magnesite, nucleate on bacterial nanoglobules and/or collapsed cells. Evidence for the sequential formation of the Mg-rich carbonates trough the decay of organic matter by a fermentative EPS-forming bacterium isolated from the microbialites, Desemzia incerta, is drawn through a comparative analysis of carbonate formation in both natural and experimental settings. This study will help to constrain potential mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars.

Based on petrographic, mineralogical, isotope, and facies assemblage analysis, a microbial origin is established for the formation of dolomite associated with gypsum in Miocene evaporite lake deposits of the Madrid Basin, central Spain. In these deposits, dolomite is present as both intercalated carbonate beds, locally showing domal stromatolite structures between packages of selenite Christmas tree-like gypsum, and patches replacing macrocrystalline gypsum. Texture of the dolomite is characterized by crystal aggregates showing a variety of crystal sizes and morphologies, e.g., platelets, rhombs, micro-rods, and rings, whilst larger crystals are commonly spherical and/or wheat-grain shaped. Organic remains, in the form of filaments, shrubs, micro-fibrils, and strands, are also common and contain significant amounts of carbon. These textural features are also recognized in dolomite replacing gypsum, where Fe oxide and sulfide as well as celestite are ubiquitous mixed with the dolomite groundmass. The dolomite, whether primary or replacing gypsum, is poorly ordered and slightly Ca-rich, thus non stoichiometric. Stable-isotope compositions are characterized by negative values for both oxygen and carbon. Dolomite beds featuring domal stromatolites have δ18O values ranging from −2.99‰ and −3.79‰ and δ13C values ranging from −4.67‰ and −7.35‰, whilst δ13C values determined in the dolomite replacive of gypsum shows a small range of variation between −5.70‰ and −6.96‰. By contrast, δ18O values of replacive dolomite oscillate in a wider range (from −3.04‰ to −7.99‰). Formation of the dolomite was associated mainly with microbial mats, having taken place in relatively diluted lake water. Further evaporative concentration resulted in precipitation of gypsum crystals sealing the mats and creating endoevaporitic microenvironments in which endolithic cyanobaterial activity produced extensive boring and corrosion of the gypsum crystals. Hiatuses in gypsum growth caused an intensification of the corrosion process and favored the precipitation of dolomite mediated by microbes, resulting in pervasive replacement of the sulfate.