Exploring synergistic interactions in Cr(VI) removal by highly Cr(VI)-reductive strain Alcaligenes faecalis S1 and Cr(VI)-tolerant strain Lysinibacillus macrolides S2: genomic and mechanistic insights

Public health and ecological security are seriously threatened by hexavalent chromium [Cr(Ⅵ)], whereas bioreduction of Cr(Ⅵ) to trivalent chromium [Cr(Ⅲ)] presents a far lesser risk to the environment. The study examined the performance and removal mechanisms of Cr(VI) by Alcaligenes faecalis S1, known for its strong reducing capacity, and Lysinibacillus macrolides S2, which shows tolerance to Cr(VI). The results indicated that the extracellular secretions of strains S1 and S2 were the main sites of Cr(VI) reduction, primarily in the form of extracellular reduction of hexavalent chromium to trivalent chromium. The genomic findings of bacteria substantiated this idea, as both strains S1 and S2 possessed chromate reduction genes, concurrently demonstrating chromate transporter proteins and DNA repair proteases, among others, that function in reducing Cr(VI) toxicity to themselves. The construction of bacterial consortium S3 was further pursued in order to investigate the synergistic resistance mechanism of strains S1 and S2 against Cr(VI). It was found that S1 and S2 synergistically enhance the Cr(VI) resistance of the bacterial consortium S3 through the expression of reduction and resistance genes, endowing it with broader pH adaptability and excellent Cr(VI) reduction capabilities. This complementary effect allowed S3 to have multiple biological reduction mechanisms: enzyme-mediated reduction of Cr(VI), DNA-repaired enzymes, detoxification enzymes, and Cr(VI) efflux, significantly reducing Cr(VI) toxicity damage. In conclusion, this study has deepened the understanding of the fundamental characteristics of strains S1 and S2, as well as the remediation synergistic mechanisms of Cr(VI), providing a molecular basis and scientific guidance for future bioremediation.