Evidence that nano-TiO2 induces acute cytotoxicity to the agronomically beneficial nitrogen-fixing bacteria Sinorhizobium meliloti

Abstract

When nano-sized titanium dioxide (nano-TiO2) absorbs ultra-violet (UV-A) radiation, it produces reactive oxygen species that can be toxic to bacteria. We used the agronomically beneficial nitrogen-fixing bacterium Sinorhizobium meliloti strain 1021 as a model microorganism to detect nano-TiO2 toxicity. Sinorhizobium meliloti was exposed to aqueous dispersions of micrometer-sized TiO2 (micron-TiO2, 44 μm) or nanometer-sized TiO2 (nano-TiO2, 21 nm) at nominal concentrations of 0, 100, 300, 600, 900, and 1800 mg TiO2/L. There were fewer viable S. meliloti cells after exposure to nano-TiO2 under dark and UV-A light conditions. Nano-TiO2 was more toxic to S. meliloti with UV-A irradiation (100% mortality at 100 mg TiO2/L) than under dark conditions (100% mortality at 900 mg TiO2/L). Micron-TiO2 concentrations less than 300 mg TiO2/L had no effect on S. meliloti viability under dark or UV-A light conditions. Exposure to 600 mg/L or more of micron-TiO2 under UV-A light could also photo-kill S. meliloti cells (100% mortality). Further studies are needed to ascertain whether nano-TiO2 interferes with the growth of N2-fixing microorganisms in realistic agricultural environments.