Lithifying microbial mats in Lagoa Vermelha, Brazil: Modern Precambrian relics?

Bacterial populations, microbial and mineral-forming processes, and their products were analyzed in Lagoa Vermelha, Brazil. The microbial mat and underlying sediment were studied as a unique system to define the boundary conditions responsible for high Mg-calcite and dolomite formation. In the uppermost layers of the microbial mat, oxygenic photosynthesis and aerobic respiration resulted in calcite precipitation, whereas, in the underlying anoxic layers of the mat, sulfide oxidation and sulfate reduction induced formation of a range of carbonate minerals with increasing Mg concentrations. The chemical, mineralogical, and biological conditions presently found in Lagoa Vermelha may have been more common in the Precambrian. The microorganisms performing the metabolic processes related to carbonate mineral formation within Lagoa Vermelha's hypersaline microbial mat may have already been present in the Precambrian. Thus, microbial carbonate as a biomineral could be a record of metabolism throughout geological time.