Mechanistic insights into antimony immobilization by sulfur-metabolizing microorganisms under alternating oxic-anoxic...

Antimony (Sb), as a toxic heavy metal(loid) element, poses potential risks to the environment and human health. The migration and transformation of Sb in the environment are closely linked to the macronutrient sulfur. However, under alternating oxic-anoxic conditions, the roles of sulfur and sulfur-metabolizing microorganisms in Sb mobility remain unclear. This study systematically investigated the role of exogenous thiosulfate (S₂O₃^2-) in Sb(V) immobilization and the underlying mechanisms by integrating microcosm incubations, speciation analyses, and genome-resolved metagenomic analyses. We found that during the oxic-anoxic-oxic transition, Sb(V) was immobilized under anoxic condition and was released under oxic condition. However, thiosulfate amendment significantly inhibited Sb release through facilitating the sulfur-dependent reduction of Sb(V) to Sb(III) and formation of Sb₂O₃ precipitates. Complete Sb(V) immobilization was achieved within 14 days, concurrently with the oxidation of S₂O₃^2- to SO₄^2-, indicating thiosulfate is the primary electron donor for microbial reduction of Sb(V). Key microorganisms, including Ramlibacter (Bin.5), Thiomonas (Bin.105), and Unclassified (Bin.112 and Bin.123), which harbor antimonate/arsenate respiratory reductase gene (arrA) and sulfur oxidation pathway (Sox system), drive the sulfur-dependent Sb immobilization, as evidenced by metagenome-assembled genomes (MAGs) results. These findings enhance our understanding of the coupled cycling of Sb and S in alternating oxic-anoxic environments and provide a potential approach for Sb remediation.