Abstract

Reverse osmosis (RO) brine is a concentrated hypersaline waste stream that poses an environmental burden while offering potential for mineral recovery. In this study, we show that the halotolerant ureolytic strain Bacillus pumilus Soc1 functions as a robust biological chassis for microbially induced calcium carbonate precipitation (MICP) in real hypersaline RO brine from a full-scale desalination facility in northern Chile. Soc1 sustained rapid ureolysis under brine stress, driving alkalinization from approximately pH 7.0 to 9.0 within 3 days and producing ammonium concentrations exceeding 13 mM. FE-SEM, EDS and XRD analyses showed a complex mineral assemblage dominated by amorphous calcium carbonate, vaterite, monohydrocalcite and Mg-rich phases, together with evaporitic salts. Minor struvite was detected only in batch assays with sus pended cells; phosphate and inorganic nitrogen in the raw RO brine were below analytical detection limits, indicating that struvite formation in those assays was associated with phosphate carry-over from the growth medium rather than with the brine matrix itself. Additional immobilized-cell experiments conducted without growth medium addressed this ambiguity: under phosphate-limited conditions, carbonate valorization and brine polishing were maintained, and no struvite was detected, indicating that struvite formation is not intrinsic to the raw RO brine. Under these conditions, Soc1 achieved pronounced Ca 2+ removal and partial Mg 2+ depletion, with mineral recovery dominated by carbonate phases. Whole-genome and transcriptomic profiling further supported this phenotype by revealing a complete urease cluster, multiple compatible-solute transport and biosynthesis pathways, Na + /H + antiporters, and genes involved in exopolysaccharide production and cell-envelope remodeling. Collectively, these findings support B. pumilus Soc1 as a mechanistically informed candidate for carbonate- based valorization of hypersaline desalination brines.