Toxicity and environmental impact of multi-walled carbon nanotubes to nitrogen-fixing bacterium Azotobacter chroococcum

With the large-scale production and wide applications of carbon nanotubes (CNTs), they are inevitably entering the environment and bringing unknown threats to environmental organisms. Nitrogen-fixing bacteria dominate 80% of the nitrogen fixation in nitrogen cycle. Herein, we investigated the toxicity and environmental impact of multi-walled CNTs (MWCNTs) to nitrogen-fixing bacterium Azotobacter chroococcum. MWCNTs slightly inhibited the growth of A. chroococcum and induced small increase of death rate at high concentrations. MWCNTs did not penetrate the cell walls of A. chroococcum and no cytoplasm loss was observed. MWCNTs were only observed surrounding the A. chroococcum cells. According to oxidative stress assays, MWCNTs induced meaningful oxidative damage at 0.5 mg/mL after 6 d exposure, but the oxidative damage diminished at 14 d. MWCNTs did not affect the nitrogen fixation activity of A. chroococcum and the soil nitrogen contents. Our findings provided the fundamental data for the environmental safety of CNTs to nitrogen-fixing bacteria.