Titanium dioxide nanoparticle exposure reduces algal biomass and alters algal assemblage composition in wastewater effluent-dominated stream mesocosms

Highlights: • TiO₂NP are under consideration for treating pharmaceuticals in wastewater. • Outdoor stream mesocosms using effluent as sourcewater were dosed with TiO₂NP. • There was significant periphyton Ti accumulation in the high concentration group. • Periphyton biomass declined and algal assemblage structure was altered. • Significant ecological impacts could result from using TiO₂NP to treat wastewater. A 5-week mesocosm experiment was conducted to investigate the toxicity of titanium dioxide nanoparticles (TiO₂NPs) to periphytic algae in an environmentally-realistic scenario. We used outdoor experimental streams to simulate the characteristics of central Texas streams receiving large discharges of wastewater treatment plant effluent during prolonged periods of drought. The streams were continually dosed and maintained at two concentrations. The first represents an environmentally relevant concentration of 0.05 mg L⁻¹ (low concentration). The second treatment of 5 mg L⁻¹ (high concentration) was selected to represent a scenario where TiO₂NPs are used for photocatalytic degradation of pharmaceuticals in wastewater. Algal cell density, chlorophyll-a, ash-free dry mass, algal assemblage composition, and Ti accumulation were determined for the periphyton in the riffle sections of each stream. The high concentration treatment of TiO₂NPs significantly decreased algal cell density, ash-free dry mass, and chlorophyll-a, and altered algal assemblage composition. Decreased abundance of three typically pollution-sensitive taxa and increased abundance of two genera associated with heavy metal sorption and organic pollution significantly contributed to algal assemblage composition changes in response to TiO₂NPs. Benefits of the use of TiO₂NPs in wastewater treatment plants will need to be carefully weighed against the demonstrated ability of these NPs to cause large changes in periphyton that would likely propagate significant effects throughout the stream ecosystem, even in the absence of direct toxicity to higher trophic level organisms.