Response of an aerobic denitrifier to titanium dioxide nanoparticles exposure

The cytotoxicity of titanium dioxide nanoparticles (TiO₂ NPs) to microorganisms has attracted great attention over the past few decades. As an important participator in the nitrogen cycle, aerobic denitrifiers have been proven to be negatively affected by TiO₂ NPs, but the mechanism of this effect remains unclear. In this study, the bacteria-nanoparticle interaction was investigated by exposing an aerobic denitrifier, Pseudomonas stutzeri PCN-1 to different concentrations of TiO₂ NPs at the dark condition, in order to investigate the cytotoxicity mechanism. The results illustrated that aerobic denitrification was inhibited at different TiO₂ NPs concentrations from 1 to 128 mg/L, accompanied by the postponement of nitrate reduction and the accumulations of nitrite and nitrous oxide. But this inhibitory effect was mitigated with increasing TiO₂ NPs concentrations. Further studies revealed that expressions of aerobic denitrification genes were also inhibited with the presence of TiO₂ NPs, and the inhibition effect on napA and nirS genes was more significant than that on nosZ and cnorB, which might directly bring about the delayed nitrate reduction and hindered nitrite transfer. Moreover, the decreased toxicities at higher TiO₂ NPs concentrations could be attributed to the formation of larger aggregates (>1000 nm), which greatly reduced the chance for direct interactions between NPs and bacterial membranes, as well as the interruption of denitrifying genes expressions. These findings were meaningful for the formation of deep insights into the mechanism of TiO₂ NPs cytotoxicity as well as the development of strategies to control the negative effect of nanoparticles in the environment.Aerobic denitrification characteristics of strain PCN-1 under different carbon sources.