Comparative multi-generation study on long-term effects of pristine and wastewater-borne silver and titanium dioxide nanoparticles on key lifecycle parameters in Daphnia magna

The rising production volume of engineered nanoparticles (NPs) leads to an increasing risk of environmental pollution. After passing sewage treatment plants (STPs), a significant concentration of NPs may end up in the aquatic environment where NPs can accumulate in the aquatic food chain and may cause harmful effects on aquatic organisms. However, when passing STPs some NPs such as silver nanoparticles (AgNPs) are transformed and enter the aquatic environment mostly as sulphide species with lower bioavailability and reduced toxicity compared to pristine NPs. For the environmental risk assessment of NPs, it is thus crucial to consider the transformation processes of nanomaterials during STP processes. For other NPs, such as titanium dioxide nanoparticles (TiO2NPs), knowledge about the acute and chronic toxicity of NPs from STP effluents on aquatic organisms is still missing. Chronic studies, such as the Daphnia reproduction test following OECD TG 211, cover a period of only 21 days and hence allow only to evaluate the reproduction performance of a single generation. Multi-generation studies provide a more realistic exposure scenario and offer the opportunity to identify transgenerational effects which may possess a significant impact on the population dynamic. Hence, the aim of this study was to assess the impact of wastewater-borne AgNPs and TiO2NPs on the aquatic invertebrate Daphnia magna in a multi-generation approach covering six generations. The effects of long-term exposure to pristine AgNPs (NM-300K; 14.9 ± 2.4 nm) and TiO2 NPs (NM-105; 21 ± 9 nm) on the reproductive success (number of offspring), mortality, time to first brood and body size of adult Daphnia were measured and compared to those caused by wastewater-borne AgNP and TiO2NPs. In all six generations, the exposure to environmentally relevant concentrations (determined by inductively coupled plasma mass spectrometry, ICP-MS) of pristine AgNPs caused a significant reduction in the mean number of offspring compared to the control. However, wastewater-borne AgNPs had no effects on reproduction in any generation. STEM analysis shows, that the AgNPs particles were transformed to Ag2S while passing the STP. No effects could be detected following exposure to environmentally relevant concentrations (determined by inductively coupled plasma optical emission spectrometry, ICP-OES) of pristine TiO2NPs and wastewater-borne TiO2NPs. The present study is the first multi-generation study on long-term effects of pristine and wastewater-borne nanoparticles on Daphnia. No transgenerational effects of wastewater-borne AgNPs nor TiO2NPs were observed. The results confirm that realistic exposure conditions are required in order to allow for a reliable environmental risk assessment of NPs.