Halite crystallization at Sfax solar salterns, Tunisia, driven by brine geochemistry and biogeochemical interactions

Solar salterns are hypersaline systems characterized by evaporation-driven salinity gradients that promote the precipitation of evaporite minerals. Here, we investigate the influence of halophilic microbial communities on halite crystallization along the salinity gradient of the Sfax solar saltern (Tunisia). Geochemical analyses of 26 brine samples (mean salinity range ~28-570 g L^-1) demonstrated marine-fed evaporative sequences, allowing the selection of three representative basins along a gradient of 150 g/L to 280 g/L (M22, B22, and TS5 sites). Enrichment cultures inoculated with surface (SB) and bottom (BB) brines were established under conditions reproducing in situ brine chemistry. Three halophilic archaeal species, Halorubrum xinjiangense, H. africanae, and H. ezzemoulense, were isolated from cultures able to tolerate salinities of up to 280 g L^-1. Subsequently, controlled crystallization experiments using high-salinity biomass as a progressively increasing inoculum revealed that biomass-associated microbial populations influenced halite precipitation kinetics, yield, and crystal morphology relative to abiotic controls. SB-derived cultures reached maximum halite-dominated precipitate mas at a lower biomass level (25% inoculum), yielding 5.550 ± 0.127 g after 10 days at 0.555 ± 0.013 g d^-1, whereas BB-derived cultures peaked at 75% inoculum (64.20 ± 2.632 mg mL^-1), producing 3.570 ± 0.085 g after 12 days at 0.299 ± 0.028 g d^-1. This contrast shows that biomass affects crystallization differently in surface- and bottom-derived cultures, with differences in crystal size, shape, aggregation, and uniformity. These results demonstrate that halophilic microorganisms could modulate halite crystallization under extreme salinity, highlighting the importance of microbial processes in evaporite mineralization within hypersaline environments.