Microbial Consortia from Chilean Salt Flats Drive Coupled Gypsum Dissolution and Mineralization in the Absence of Evaporation: Experimental Evidence

Microbial consortia actively modulate mineralogical processes in polyextreme environments, yet biological mechanisms are often excluded from evaporite evolution models. Here, we investigated gypsum surface dissolution and secondary mineral nucleation driven by microbial mats and natural brines from three Andean salt flats (Salar de Llamara, Salar de Pajonales, and Salar de Gorbea) across 90-day incubation experiments. Synthesized gypsum crystals were exposed to inoculated media and sterile brines (abiotic controls) under controlled conditions, and analyzed using SEM-EDS and epifluorescence microscopy. Results demonstrate that microbial colonization dual-modulates gypsum dynamics: while apparent microbially-mediated boring formed rounded cavities, extracellular polymeric substance (EPS) matrices acted as diffusion barriers that buffered abiotic dissolution. Furthermore, EPS promoted localized supersaturation and authigenic mineral nucleation, yielding fibrous eugsterite and globular Mg-silicates across all sites, alongside site-specific phases such as dumbbell-shaped calcite, allotriomorphic halite (Pajonales), and lenticular alunite (Gorbea). Abiotic controls exhibited simpler parageneses and angular dissolution along cleavage planes without filling. Specific bio-induced textures, including EPS-infilled pores, dumbbell calcite, Mg-Si globules, and alveolar eugsterite, represent potential morphological biosignatures relevant to astrobiology. Our findings demonstrate that purely abiotic geochemical models are insufficient to describe brine evolution, underscoring the critical necessity of integrating geomicrobiological interactions into evaporite formation frameworks.