Toxicity of VO₂ micro/nanoparticles to nitrogen-fixing bacterium Azotobacter vinelandii

Vanadium dioxide (VO₂) has been used in a variety of products due to its outstanding phase transition properties. However, as potential heavy metal contaminants, the environmental hazards and risks of VO₂ should be systematically investigated. Biological nitrogen fixation is one of the most dominant processes in biogeochemical cycle, which is associated with nitrogen-fixing bacteria. In this study, we reported the environmental bio-effects of VO₂ micro/nanoparticles on the nitrogen-fixing bacterium Azotobacter vinelandii. VO₂ at 10 and 30 mg/L caused severe hazards to A. vinelandii, such as cell apoptosis, oxidative damage, physical damage, genotoxicity, and the loss of nitrogen fixation activity. The up-regulated differentially expressed genes of A. vinelandii were related to stress response, and the down-regulated genes were mainly related to energy metabolism. Surprisingly, VO₂ of 10 mg/L decreased the nif gene expression but elevated the vnf gene expression, which enhanced the ability of A. vinelandii to reduce acetylene in anaerobic environment. In addition, under tested conditions, VO₂ nanoparticles exhibited insignificantly higher toxicity than VO₂ microparticles.