Mixtures effects of nanoscale and microscale contaminants to Bacillus subtilis in natural water systems

Abstract

Engineered nanoparticles (ENPs) (e.g. zinc oxide (nZnO) and iron oxide (nFeOx) and organic pollutants (e.g. triclosan (TCS)) are among emerging contaminants (ECs) of environmental concern. However, to date there is limited knowledge on their fate and potential deleterious effects in the ecological systems. Following simultaneous and/or sequential release of ECs in the ecological systems; they co-exist as mixtures defined by complex permutations. As such, toxicological outcomes of individual ECs may be altered as those of mixtures formed may exhibit synergistic, antagonistic or additive effects. Influenced by water physicochemical parameters, chemical interactions between multiple contaminants and the unique properties of ENPs like photoactivity, adsorption capacity and dissolution may alter the toxic outcomes to bacteria. Yet, currently, the environmental fate and toxicity ENPs as mixtures, particularly in natural water are limited. In this work, Bacillus subtilis was used as a model organism to assess the toxicity of nZnO and maghemite iron oxide (γ-nFe2O3) as individual ENPs, binary mixtures, and ternary mixtures with TCS. Natural water from two river sources, the Elands River (ER) and the Bloubank River (BR) were used to generate environmentally relevant data; and four endpoints were used to evaluate toxicological outcomes (cell viability, cell membrane integrity, ATP production, oxidative stress from reactive oxygen species (ROS). Aggregation of the two ENPs were significantly different between the two river water matrixes with higher aggregates observed in BR water. nZnO induced significant reduction in cell viability and membrane integrity at higher tested concentrations in ER; but none in BR under visible light. A higher decrease in ATP levels was observed in ER than in BR, and ROS production was negligible irrespective of the ENP type and exposure media under visible light. Conversely, γ-nFe2O3 induced no significant effects on B. subtilis on all tested endpoints. nZnO induced concentration-dependent effects on the cell membrane integrity of B. subtilis in both river water samples under solar irradiation. For binary mixtures of ENPs under solar irradiation, nZnO toxicity was found to be concentration-dependent, with more pronounced effects in ER than BR water due to water chemistry. However, toxic effects were mitigated by γ-nFe2O3 in the binary mixtures, linked to heteroaggregation between the ENPs. Solar irradiation induced ROS had minimal effect on the toxicity of ENPs. For ternary mixtures, toxicity of TCS was more pronounced at the highest concentration. However, the effects were not water chemistry dependent, compared to the observed effects from nZnO. In addition, more distinctive mitigating effects were observed in ternary mixtures, where nZnO dissolution was significantly lower in the presence of TCS. These findings demonstrated that observed differences in the effects of nZnO towards B. subtilis, either in binary or ternary systems were influenced by the nature of interactions (TCS and γ-nFe2O3) as well as water chemistry of natural water in focus. 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, and, the co-existence of ECs of different types and classes may lead to reduction of toxic effects of individual contaminants.