Microbial-Mediated Iron Sulfide Mineralization in Aragonite Spherules from the Salar de Atacama, Chile

Microbialites are sedimentary deposits formed by microbial activity. This study analyzed the compositions of microbialites from Salar de Atacama to model the process of iron sulfide mineralization within aragonite spherules. We found that microbialite formation could be a multistage process. Initially, <i>Cyanobacterota</i> and <i>Deinococcota</i> phyla produce extracellular polymeric substances (EPS), creating EPS-rich layers that adsorb metal ions dissolved in the lagoon’s water. The degradation of these horizons by <i>Proteobacteria</i> results in the precipitation of calcium carbonate minerals. EPS and aragonite fabric formation restrict access to oxygen-rich, alkaline lake water, transforming the uppermost layer into a geochemical barrier linked with goethite (FeOOH) precipitation. Simultaneously, the inner microenvironment becomes suitable for <i>Desulfobacterota</i>, which reduce sulfate ions and emit HS^-. Meanwhile, microbialite aggregate grows, the goethite-rich layer migrates to the sulfide-ion dominated region, causing pyrite (FeS₂) to replace goethite. Eventually, a three-layer aragonite structure forms: (i) an uppermost layer with <i>Cyanobacterota</i> and <i>Deinococcota</i>, (ii) an anoxic zone with high metal-ion content, and (iii) an inner region with pyrite mineralization. As the lagoon’s salinity increases, the precipitation rate of orthorhombic aragonite decreases, and Mg-rich calcite begins to precipitate. This carbonate mineral’s rhombohedral crystalline structure does not support spherule formation needed for microbial symbiosis. Aragonite spherules form as a result of complex symbiosis between <i>Cyanobacteria</i>, <i>Deinococcota</i>, <i>Proteobacteria</i>, and <i>Desulfobacterota</i>;Aragonite-composed microbialites can grow only in lagoon waters with relatively low mineralization;Aragonite spherules share a great diversity of redox conditions;Micropores of microbialites concentrate many metals and metalloids at orders of magnitude. Aragonite spherules form as a result of complex symbiosis between <i>Cyanobacteria</i>, <i>Deinococcota</i>, <i>Proteobacteria</i>, and <i>Desulfobacterota</i>; Aragonite-composed microbialites can grow only in lagoon waters with relatively low mineralization; Aragonite spherules share a great diversity of redox conditions; Micropores of microbialites concentrate many metals and metalloids at orders of magnitude.