Microbial Consortia from Chilean Salt Flats Drive Coupled Gypsum Dissolution and Mineralization in the Absence of Evaporation: Experimental Evidence
Microbial consortia are partially mediating mineralogical processes within polyextreme environments. However, it remains necessary to better evidence how the interplay between microbial activity and mineral nucleation occurs. The present study investigates dissolution and mineral precipitation on synthesized gypsum crystals under abiotic and biotic conditions using microbial mats and natural brines sourced from three Andean salt flats: Salar de Llamara, Salar de Pajonales, and Salar de Gorbea. Initially, gypsum crystals were synthesized via evaporation of field-collected brines under sterile conditions. Subsequently, these crystals were incubated for 90 days at 25°C with 12-hour light cycles, using both BG-11-supplemented inoculated medium and sterile brines for abiotic control. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), epifluorescence microscopy and biovolume estimations were performed at progressive stages of microbial colonization of gypsum to assess crystal surface alteration, culture growth and microbe-mineral interactions. Results revealed that microbial activity influenced gypsum dissolution in two ways: in some cases, microbial colonization was strongly associated with crystal dissolution, whereas in others, the formation of extracellular polymeric substances (EPS) on top of the crystal surface appeared to act as a diffusion barrier that limits abiotic dissolution. Moreover, EPS matrices facilitated the nucleation of fibrous eugsterite and globular Mg-silicates in samples from all three salt flats in different proportions. Additionally, dumbbell-shaped calcite and allotriomorphic halite were observed in Pajonales samples, as well as lenticular alunite in Gorbea samples. On the other hand, abiotic controls showed less complex mineral paragenesis and precipitate content compared to microbially colonized samples. Additionally, microbes were often observed accumulating within the pores in the gypsum crystals, with EPS and newly formed minerals frequently infilling pores. Furthermore, microbes, EPS and precipitates were also observed on the surface of gypsum crystals forming alveolar and framboid structures. The pore rich gypsum, dumbbell-shaped calcite and Mg-Si globule assemblages alongside the biologically mediated alveolar eugsterite found in this experiment can represent potential biosignatures relevant to astrobiology. Finally, the reported evidence emphasizes the intricate geomicrobiological complexity generated by microbe-mineral interactions in polyextreme environments. Consequently, these findings indicate that purely abiotic models of brine evolution are insufficient, strongly supporting the integration of biological processes to fully understand the dynamics of evaporite formation.