Antimonite oxidation by microbial extracellular superoxide in Pseudomonas sp. SbB1

Antimony (Sb) is a re-emerging contaminant, and its redox transformation is mainly driven by microorganisms. Sb(III) oxidation has been attributed to enzymatic catalyzed reaction, though the physiological reason for this process remains unclear. Herein, we isolated a bacterium named Pseudomonas sp. SbB1 which can oxidize Sb(III) to Sb(V) without known Sb-oxidizing genes in its genome. The Sb(III) oxidation followed a zero-order kinetics with an appreciably lower rate (0.068 μM/h) than known Sb(III) oxidases (0.159–0.210 μM/h). Our incubation experiments show that Sb(III) was oxidized by extracellular superoxide and the superoxide production is NADH-dependent. By in-gel analysis and Sb K-edge XANES, we found proteins at ∼100 kDa position were responsible for Sb(III) oxidation by producing superoxide. Further, our nano LC-MS/MS results suggest that dihydrolipoyl dehydrogenase was the source of extracellular superoxide. In vivo evidence with mutant indicated that strain ΔdldH was incapable of producing superoxide and oxidizing Sb(III), whereas complementation by dldH rescued the mutant’s ability. Beyond strain SbB1, superoxide generation and Sb(III) oxidation were also observed in diverse bacteria with DLDH orthologous across five classes. Our finding shows a previously unknown pathway used by widespread bacteria to mediate the transformation of redox-sensitive pollutants in the environment.