Microbial Mediation in Gypsum Dissolution and Its Role in Evaporitic System Transformation in Northern Chilean Salt Flats

The role of microbial consortia in modulating mineralogical processes within polyextreme environments remains underexplored, particularly under controlled laboratory conditions that simulate these environments. This study investigates dissolution and secondary mineral precipitation on synthesized gypsum crystals under abiotic and biotic conditions using microbial mats and natural brines sourced from three Andean salt flats: Llamara, Pajonales, and Gorbea, respectively. Initially, gypsum crystals were synthesized via evaporation of field-collected brines under sterile conditions and then, these crystals were incubated with microbial cultures supplemented by Cyanobacteria medium (BG-11) and sterile brines (controls) under controlled conditions for 90 days. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and epifluorescence microscopy were performed at progressive stages of microbial colonization to monitor gypsum surface alteration, culture growth and microbe-mineral interactions. Results reveal that microbial activity influenced gypsum dissolution in two ways: in some cases, microbial metabolism promoted crystal dissolution, whereas in others, the formation of extracellular polymeric substances (EPS) on top of the crystal surface created diffusion barriers that reduced dissolution. Moreover, EPS matrices facilitated the nucleation of allotriomorphic halite, fibrous Na-Ca sulfates, globular Mg-silicates, dumbbell-shaped calcite (Pajonales-specific), and lenticular Al-K sulfates, while abiotic controls were mineralogically monotonous in comparison. Additionally, microbes showed a tendency to accumulate in the porosities within gypsum crystals and occasionally fill them with EPS and the above-mentioned minerals, although these minerals were also observed on the surface of gypsum crystals. These findings underscore the geomicrobiological complexity of evaporitic systems and support the integration of biotic processes into models of evaporite formation, with implications in astrobiology.