Transcriptome analysis reveals vanadium reduction mechanisms in a bacterium of Pseudomonas balearica

The continuously elevated concentration of vanadium (V) in the environment is of great concern due to its potential risks to organisms. Microbial reduction is considered a green way to remediate the V-contaminated sites. This study presents the first demonstration of Pseudomonas balearica, a bacterium flourishing in vanadium-titanium magnetite tailings, exhibiting the capacity to reduce pentavalent V [V(V)]. We found that P. balearica exhibited strong tolerance to high V(V) levels, despite lacking known V(V)-reducing genes. After 48 h of incubation, the reduction rates of V(V) by P. balearica under 0.5 mM and 10 mM V(V) stress reached 81.1% and 60.6%, respectively. The phosphorus group and carboxyl group on the bacterial cell surface contributed to V(V) binding and reduction. Both extracellular and intracellular V(V) reduction was significantly facilitated by many critical V-induced differentially expressed genes (DEGs) encoding nitrate/nitrite reductases, ABC transporters, NADH, and c-type cytochromes. In addition, up-regulation of DEGs associated with reactive oxygen species (ROS)-scavenging enzymes, such as glutathione peroxidase, superoxide dismutase, and catalase, was observed as a protective response to oxidative stress triggered by V(V). Moreover, some up-regulated DEGs (ureC, ureE, ureA, and ureG) encoding urease subunit or urease accessory proteins significantly enriched in the nickel cation binding pathway, indicating that P. balearica may be resistant to Ni stress by regulating urease activity. This study will help to more thoroughly elucidate the characteristics and mechanisms of V(V) reduction by microbes and provide a native bacterium flourishing in tailings for potential bioremediation of V pollution and bioleaching of V-sources.