The overlooked role of denitrifying bacteria in mediating vanadate reduction

Microbial vanadate [V(V)] reduction is the key process of vanadium geochemical cycling, while the identified functional microbes are rather limited. Considering the structural similarity of metavanadate and nitrate, widespread denitrifying bacteria are expected to be able to mediate V(V) reduction, but this process has not been fully understood. This study confirmed the effectiveness of two denitrifying bacteria, i.e., Acidovorax sp. strain BoFeN1 and Pseudogulbenkiania sp. strain 2002, in reducing V(V). During 120-h incubation, the V(V) removal efficiency was 89.0 ± 1.83% and 96.0 ± 1.43% by strain BoFeN1 and strain 2002, respectively. V(V) was reduced to tetravalent vanadium [V(IV)] both intracellularly and extracellularly. Use of inhibitors and metabolomic analysis revealed enzyme catalysis and electron transfer pathways for V(V) reduction. Enzyme catalysis was identified by analyses of gene abundance, transcript abundance, and enzymatic activity. Electron transfer was verified through quantifying electron transporters, monitoring electron transport system activity, and performing cyclic voltammetry analysis. For strain BoFeN1, nitrite and nitrous oxide reductases played vital roles in V(V) reduction, together with cytochrome c (Cyt c) and nicotinamide adenine dinucleotide (NADH) mediated electron transfer. Differentially expressed riboflavin produced by strain BoFeN1 also promoted electron transfer. Strain 2002 mainly utilized periplasmic nitrate reductase to reduce V(V). Aquifer sediment bioaugmented with these strains enhanced V(V) reduction by 10.5%−51.1%. They well colonized and collaborated with indigenous microorganisms to reduce V(V) to V(IV). This study advances understanding of environmental biogeochemistry of vanadium and provides molecular insights into V(V) bio-reduction as a sustainable bioremediation strategy.