Organic matter reduces acute toxicity for Daphnia magna exposed to polystyrene nanoparticles

The global production and use of plastic is increasing and a substantial part will end up in aquatic environments where it will degrade into micro- and nanoplastic particles (NPPs). NPPs will interact with naturally occurring biomolecules present in aquatic environments which in turn will affect the properties of the particles. The purpose of this study was thus to investigate how organic matter affects the toxicity of 53 nm polystyrene nanoparticles (PS-NPs). The mortality and reproduction of Daphnia magna, exposed to PS-NPs with and without organic matter were observed during short- and long term experiments. Furthermore, the size and aggregation of the PS-NPs, were determined using Dynamic Light Scattering (DLS) in a variety of conditions, including organic matter, to evaluate how the organic matter affects the physical properties of the particles. The organic matter reduced the acute toxicity observed at a PS-NP concentration of 3.2 mg/L. The PS-NPs aggregated in the exposure media but were stabilized by the organic matter which limited the diameter of the aggregates to 200 nm – 300 nm. Enhanced mortality was observed at a PS-NP concentration of 0.32 mg/L in the first long term toxicity test but not in the second. The protective effect of the organic matter was not seen in the long term experiments and the reproduction was not affected in any of the treatments. We provide pristine knowledge about how organic matter might reduce the toxicity and affect the properties of NPPs in natural environments and highlight the importance of assessing NPP toxicity under more environmentally realistic conditions. (Less)