Quantifying impacts of titanium dioxide nanoparticles on natural assemblages of riverine phytobenthos and phytoplankton in an outdoor setting.

Impacts of widespread release of engineered titanium dioxide nanoparticles (nTiO) on freshwater phytoplankton and phytobenthic assemblages in the field, represents a significant knowledge gap. Using outdoor experiments, we quantified impacts of nTiO on phytoplankton and periphyton from UK rivers, applied at levels representative of environmentally realistic concentrations (0.05 mg/L) and hot spots of accumulation (5.0 mg/L). Addition of nTiO to river water led to rapid temporal size changes in homoagglomerates and many heteroaggregates of nTiO with cells in the phytoplankton, including green algae, pennate and centric diatoms, increasing settlement of some cells. Changes in phytoplankton composition were evident after 72-h resulting from a significant decline in the relative abundance of very small phytoplankton cells (1-3 μm), often accompanied by increases in centric diatoms at both concentrations. Significant changes detected in the composition of the phytobenthos after 12 days, following nTiO treatments, were not evident when using benthic diatoms alone after 56 days. A lack of inhibition in the maximum quantum yield (Fv/Fm) in phytobenthos after 72-h exposures contrasted with a significant inhibition in Fv/Fm in 75% of phytoplankton samples, the highest recorded in Rutile nTiO exposures at both concentrations of nTiO. After 12 days, strong positive stimulatory responses were recorded in the maximum relative electron transport rate (rETR) and the maximum non-photochemical coefficient (NPQ), in phytoplankton and phytobenthos samples exposed to the higher Anatase nTiO concentration, were not measured in Rutile exposed biota. Collectively, these results indicate that the Rutile phase of nTiO has more negative impacts on freshwater algae than the Anatase form, at specific time scales, and phytoplankton may be more impacted by nTiO than phytobenthos. We caution that repeated release of nTiO, could lead to significant changes in riverine algal biomass and species composition, dependent on the phase and concentration of nTiO.