Mechanism of Cr(VI) removal by efficient Cr(VI)-resistant Bacillus mobilis CR3

Abstract Cr(VI) is a hazardous environmental pollutant that poses significant risks to both ecosystems and human health. We successfully isolated a novel strain of Bacillus mobilis , strain CR3, from Cr(VI)-contaminated soil. The strain CR3 could show 86.70% removal capacity at 200 mg/L Cr(VI) and also good Cr(VI) removal capacity at different pH, temperature, coexisting ions and electron donor conditions. Different concentrations of Cr(VI) affected the activity of Bacillus mobilis CR3 cells and the removal rate of Cr(VI), and about 3.46% of total Cr was immobilized at the end of the reaction. The combination of SEM-EDS and TEM-EDS analysis showed that Cr accumulated both on the cell surface and inside the cells after treatment with Cr(VI). XPS analysis showed that both Cr(III) and Cr(VI) were present on the cell surface, and FTIR results indicated that the presence of Cr on the cell surface was mainly related to functional groups such as O-H, phosphate groups and -COOH. The removal of Cr(VI) was mainly bioreduction, which primarily occurred outside the cell. Metabolomics analysis revealed upregulation of five metabolites, including phenol and L-carnosine, closely associated with Cr(VI) reduction, heavy metal chelation, and detoxification mechanisms. Additionally, numerous other metabolites linked to cellular homeostasis exhibited differential expression. The presence of Cr(VI) exerted inhibitory effects on division rate and influenced critical pathways like energy metabolism, nucleotide metabolism, and amino acid synthesis and catabolism. These findings reveal the molecular mechanism of Cr(VI) removal by strain CR3 and provide valuable insights for guiding the remediation of Cr(VI)-contaminated sites.