| dc.description.abstract | Recent advances in nanotechnology have created numerous and promising applications in all sectors of society; as a result, large scale developments of engineered nanomaterials (ENMs) have increased the likelihood of the release of these novel materials to the environment. Yet, the behavior of these nanomaterials in the environment is still poorly understood, thus raising the concerns of their potential ecological health risks. Given that microbes are the foundation of many ecosystems, a better understanding of the factors that control the microbial toxicity of ENMs is crucial for their sustainable use. To date, available data have indicated that toxicity of quantum dots (QDs) to bacteria is predominantly attributed to the release of toxic inorganic ions, in particular metal species, from weathered QD-cores. Hence, extracellular metal speciation is considered to play an important role in determining the ultimate toxicity of QDs towards microbial cells. In this study, we conducted exposure experiments to investigate the importance of cadmium (Cd) speciation in toxicity to pure cultures of Escherichia coli K-12 (ATCC 25404), a known Gram-negative strain lacking metal resistance czc genes in its genome and thus can be used as a model organism to represent generic non-resistant bacterial cells. Variable chloride chemistry experiments were carried out to modify Cd(II) speciation in assay media, and cell death or growth inhibition was used as an indicator of toxicity. Results show that the toxicity of Cd decreased along the chloride gradient where the severest growth inhibition occurred at the lowest salinity. Under the experimental conditions, inhibition was not a function of the total Cd(II) concentration but strongly correlated with free Cd2+ concentration. In addition, when strong chelating agents such as citrate and EDTA were supplemented to the assay medium, the Cd2+ concentrations were also modulated, resulting in reduced deleterious effects. As such, Cd toxicity to non-czc-mediated Cd resistant bacteria seemed to be explained by the free ion activity model (FIAM). We further examined if energy dependence of Cd uptake was required by strain K-12. Interestingly, differential viability was not observed when starved cells were exposed to media containing Cd(II) and variable chloride concentrations, suggesting that active transport may be the underlying uptake mechanism of Cd in this strain. Together, these results suggest that Cd toxicity to non-resistant bacterial cells is directly proportional to the uncomplexed, free Cd activity in solution, and uptake of Cd may be mediated via an energy-dependent transport system. Accordingly, on the basis of the predictions from the FIAM, these results also suggest that except for acidic and oligotrophic environments, under circumneutral and alkaline conditions cadmium-based QDs may not pose a significant threat to the ecosystem. | en_US |