Toxicity of zinc oxide and iron oxide engineered nanoparticles to Bacillus subtilis in river water systems

Abstract

Zinc oxide (nZnO) and iron oxide (nFeO_x) engineered nanoparticles (ENPs) are widely used in consumer products and industrial applications, and consequently, are continuously being released into the environment. Numerous studies have reported on the toxicity of ENPs to bacteria, especially in synthetic aqueous exposure media. However, investigations in natural aqueous exposure media such as river water are limited. Herein, the toxicity of nZnO and maghemite iron oxide (γ-nFe₂O₃) to Bacillus subtilis was investigated in two natural river water samples: the Elands River (ER) and the Bloubank River (BR). Four endpoints, namely, cell viability, cell membrane integrity, adenosine triphosphate levels (ATP), and reactive oxygen species (ROS) production, were evaluated to determine the effects of the ENPs on the bacteria. nZnO induced significant reduction in cell viability and membrane integrity at higher tested concentrations of 100 and 1000 μg L⁻¹ in ER; but none were observed in BR. In addition, a higher decrease in ATP levels was observed in ER than in BR, and the ROS production was negligible irrespective of the ENP type and exposure media. γ-nFe₂O₃ induced no effects on B. subtilis on all tested endpoints. These results demonstrated that the observed differences in the effects of nZnO towards B. subtilis were influenced by the physicochemical properties of each river's water. Therefore, the unique physicochemical properties of natural aqueous media were established to be the key determinant attributes in enhancing or inhibiting the effects of ENPs on bacteria.