Microbial Dolomite Precipitation under Aerobic Conditions: Results from Brejo do Espinho Lagoon (Brazil) and Culture Experiments

Abstract

Microbially mediated high-Mg calcite and dolomite precipitation occurs under oxic conditions in Brejo do Espinho lagoon, Brazil, within the upper 5 cm below the sediment–water interface. With burial to < 25 cm in the sediment sequence, early diagenesis associated with sulphate-reducing bacterial activity transforms the mixed carbonate mineralogy to 100% dolomite, as the pore-water becomes undersaturated with respect to calcite, while remaining supersaturated with respect to dolomite. Laboratory culture experiments using moderately halophilic aerobic bacteria (Virgibacillus marismortui and Marinobacter sp.) isolated from the uppermost part of the microbial mat in Brejo do Espinho demonstrate that microbially mediated dolomite precipitation can occur under ambient Earth’s surface conditions in the presence of oxygen. These results add an additional metabolic process, aerobic respiration, to bacterial sulphate reduction and methanogenesis, which have previously been identifi ed with dolomite formation. Furthermore, the formation of carbonate minerals with spherulitic structures in both the natural environment and laboratory culture experiments points to microbial involvement, as recognized in numerous other modern environments and ancient systems. This study suggests that previously recognized modern dolomite-forming environments, such as the supratidal areas of Andros Island, Bahamas, with recent dolomite crusts should be revisited to evaluate the importance of aerobic respiration in dolomite precipitation.