Interactions between titanium dioxide nanoparticle exposure and grazing organisms on periphyton biomass and community composition.

Titanium dioxide nanoparticles (TiO₂NPs) are important contaminants of emerging concern, due to their ubiquity and use in such a broad range of consumer products. Freshwater ecosystems represent the most important aquatic reservoir for TiO₂NPs. Outdoor stream mesocosm and indoor stream microcosm experiments were conducted to determine if titanium dioxide nanoparticles (TiO₂NPs) were toxic to periphyton under environmentally realistic conditions and in the absence/presence of fish or snail grazing. In the riffle section of the streams, the high concentration treatment of TiO₂NPs (5 mg L⁻¹) significantly decreased periphyton biomass and altered algal assemblage composition. In the glide section where fish grazing was present, there were significant effects of high concentrations of TiO₂NP on algal and bacterial community composition regardless of grazing pressure. There was a nonadditive effect of treatment and grazing in the streams exposed to both the low (0.05 mg L⁻¹) and high concentrations of TiO₂NPs, demonstrating that the effects of treatment and grazing combined were larger than the sum of their effects. Unexpectedly, biodiversity was greatest in the grazed high concentration streams. In the indoor stream microcosm experiment, low doses of TiO₂NP (0.05 and 0.5 mg L⁻¹) caused increases in the change in periphyton biomass at study termination. Significant changes in community structure were only detected at the study mid-point. There was significant mortality for the Elimia snails used in the experiment, even though the analytical data showed little uptake of titanium. The research presented here suggests that using TiO₂NPs to treat pharmaceuticals in wastewater may be very costly, based on the non-additive effects in the presence of grazing on periphyton biomass for the environmentally relevant and wastewater treatment concentrations (0.05, 0.5, and 5 mg TiO₂NPs L⁻¹). The differential alterations in community composition in the presence and absence of grazing highlight the need for the documentation of the presence/absence of grazers in order to inform toxicological assessments of stream ecosystems. This research demonstrates the complicated nature of using engineered systems approximating natural habitats and conditions and the complexities associated with studying toxicity in the face of biological and biochemical stochasticity.