Whereas prospects of bioremediation for a vanadium­(V) [V­(V)]-contaminated environment are widely recognized, reported functional species are extremely limited, with the vast majority of Gram-negative bacteria in Proteobacteria. Herein, the effectiveness of V­(V) reduction is proved for the first time by Lactococcus raffinolactis, a Gram-positive bacterium in Firmicutes. The V­(V) removal efficiency was 86.5 ± 2.17% during 10-d operation, with an average removal rate of 4.32 ± 0.28 mg/L·d in a citrate-fed system correspondingly. V­(V) was bio-reduced to insoluble vanadium­(IV) and distributed both inside and outside the cells. Nitrite reductase encoded by gene nirS mainly catalyzed intracellular V­(V) reduction, revealing a previously unrecognized pathway. Oxidative stress induced by reactive oxygen species from dissimilatory V­(V) reduction was alleviated through strengthened superoxide dismutase and catalase activities. Extracellular polymeric substances with chemically reactive hydroxyl (−OH) and carboxyl (−COO–) groups also contributed to V­(V) binding and reduction as well as ROS scavenging. This study can improve the understanding of Gram-positive bacteria for V­(V) bio-detoxification and offer microbial resources for bioremediation of a V­(V)-polluted environment.

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