Non-monotonic concentration–response relationship of TiO2 nanoparticles in freshwater cladocerans under environmentally relevant UV-A light

• Toxicity of TiO2 NPs under environmentally relevant level of UV-A was studied.
• Under UV-A light, TiO2 NPs showed non-monotonic response curves in waterflea.
• ROS in Daphnia filter was associated with the observed toxicity.
• TiO2 NPs greater than filter size but smaller than precipitating appeared to be toxic.

The effects of UV-A on the toxicity of TiO2 nano-particles (NPs) were evaluated using Moina macrocopa and Daphnia magna under environmentally relevant level of UV-A. The waterfleas were exposed to TiO2 NPs with different sizes of ~298 nm, ~132 nm, or ~72 nm for up to 48 h, with or without UV-A light. Whole body reactive oxygen species and transcription of antioxidant enzyme genes were measured, as well as the survival of the waterflea. In the presence of UV-A, the survival rates of M. macrocopa significantly decreased in concentration dependent way until ~1 mg/L TiO2 NPs, but the survivals were reversed at greater concentrations. This peculiar non-monotonic trend of concentration–response relationship might be explained by changes of particle size under different light conditions. TiO2 NPs within a certain size range could be trapped in the filter apparatus and exert toxicity, and the NPs of greater size were subject to either precipitation or ingestion leading to no or little toxicity. Observed TiO2 toxicity was associated with oxidative stress in the filter apparatus. The results of this study showed that the size change due to UV-A irradiation should be considered in evaluation of ecological risks of TiO2 NP.