Zinc oxide and iron oxide engineered nanoparticles toxicity on Bacillus subtilis in river water systems
Zinc oxide (nZnO) and iron oxide (nFeOx) engineered nanoparticles (ENPs) are widely used in consumer products and industrial applications, and consequently, are continuously being emitted 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) on Bacillus subtilis was investigated in two natural river water samples; the Elands river (ER) and Bloubank river (BR). Four end points, 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 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, higher decrease in ATP levels were observed in ER than in BR, and ROS production was negligible irrespective of ENPs type and exposure media. The γ-nFe2O3 induced no effects on B. subtilis on all tested endpoints. These results demonstrated that the observed differences in effects of nZnO towards B. subtilis were influenced by the physicochemical properties of each river water. Therefore, the unique physicochemical properties of natural aqueous media were established to be key determinant attributes in enhancing or inhibiting the effects of ENPs to bacteria.