Single strain mediated biogeochemical cycling of vanadium during redox fluctuations

The biogeochemical cycling of vanadium, particularly the redox transformation between tetravalent [V(IV)] and pentavalent [V(V)] species, is environmentally significant. Whereas V(IV) oxidation and V(V) reduction have previously been attributed to different microorganisms, this study demonstrated that a single bacterium, Bacillus subtilis, can mediate bidirectional V(IV)/V(V) transformations under fluctuating redox conditions. Under aerobic incubation, solid VO(OH)₂ was biooxidized to soluble V(V), achieving a concentration of 35.8 ± 1.79 mg/L. Upon switching to anaerobic conditions, the released V(V) precipitated again as VO(OH)₂ through biological reduction. Microbially improved reactive oxygen species production contributed to V(IV) oxidation in an aerobic environment. These findings indicated that microbial metabolic activity tightly couples vanadium redox transformations to environmental oxygen availability. Cyclic voltammetry revealed accelerated electron transfer under anaerobic conditions, facilitating efficient V(V) bioreduction mediated by elevated levels of electron carriers such as cytochrome c, nicotinamide adenine dinucleotide, and glutathione. In contrast, increased catalase activity under aerobic conditions likely enhanced V(IV) oxidation and vanadium mobilization, whereas elevated lactate dehydrogenase activity under anaerobic conditions promoted V(V) reduction. High extracellular polymeric substance production under both conditions further supported vanadium transformation, enhancing vanadium leaching in the presence of oxygen and V(V) reduction in anaerobic environments. Collectively, these results demonstrated that oxygen availability governs enzyme activity and electron-transfer pathways, thereby controlling the direction and efficiency of microbially mediated vanadium redox cycling. This study established for the first time, that single bacterium mediated vanadium redox cycling through oxygen-dependent metabolic switching, revealing the novel biogeochemical phenomenon of vanadium in the environment.