Assessing the Effects of Titanium Dioxide Nanoparticles on Microbial Communities in Stream Sediment Using Artificial Streams and High Throughput Screening

Nano-titanium dioxide (nano-TiO2) is an engineered nanomaterial used in a wide array of commercial products. The production and use of large amounts of nano-TiO2 is resulting in the unintended release to the environment. Nano-TiO2 is known to be cytotoxic due primarily to its ability to generate reactive oxygen species, and negative impacts on a variety of organisms have been demonstrated, but the effects of nano-TiO2 on complex microbial communities under ecologically relevant conditions have rarely been tested. We conducted a controlled manipulative experiment using recirculating model streams dosed with a one-time amendment of 1mg L-1 nano-TiO2 (specifically P25). Within one day bacterial cell numbers in the treated streams were 25% lower than in the control streams, but by 30 days post-treatment bacterial numbers in the treated streams had recovered. Treated streams also showed significantly higher per-cell respiration rates than controls on days 8 and 15. Tag pyrosequencing of bacterial 16S rRNA genes indicated that bacterial communities in the nano-TiO2 treated streams were highly similar to each other and distinct from the control streams on days 1 through 23, but by day 30 the community composition in the treated streams had returned to being indistinguishable from the control streams. Our results demonstrate that one-time addition of nano-TiO2, representative of an accidental release, can have a rapid but temporary effect on the size, activity and composition of sediment bacterial communities. The use of high-throughput screening (HTS) enabled us to test the effects of four different types of nanoTiO2 (P25, PW6, pure anatase particles, and pure rutile particles) at various concentrations to bacterial communities collected from sediments from two different streams. To simulate the effects of sunlight, various concentrations of nanoTiO2 and bacteria suspension was exposed under a xenon arc lamp for one hour. In Chicago River communities, incubation of all types of nanoTiO2 of concentrations 1 mg/L and higher significantly decreased bacterial viability compared to 0 mg liter-1 controls. Although Nippersink Creek communities were sensitive to nanoTiO2, incubation of only the highest concentration of P25 (25 mg liter-1) with illumination lowered bacterial viability significantly. Our results confirmed that illumination is an important contributor to short-term nanoTiO2 toxicity and indicate a difference in the bacterial community responses to nanoTiO2 based on anthropogenic pollution in the habitat