Prioritising nano- and microparticles: identification of physicochemical properties relevant for toxicity to Raphidocelis subcapitata and Daphnia magna

Background: Advanced/innovative materials are an undefined group of nano- and micro-particles encompassing diverse material compositions, structures and combinations. Due to their unique properties that enable specific functions during applications, there are concerns about unexpected hazards to humans and the environment. In this study, we provide ecotoxicity data for 36 nano- and microparticles of various inorganic species (single constituents and complex compositions; materials releasing toxic ions and others), morphologies (spheroidal, cubic, flaky, elongated/fibrous) and sizes (10 nm–38 µm). By applying Raphidocelis subcapitata algae growth inhibition and Daphnia magna immobilisation tests according to OECD test guidelines 201 and 202, and extensive material characterisation, we aimed to identify indicators of concern. This would allow better predictions of the hazardous properties of these materials in the future.Results: The chemical identity (toxic ion-releasing materials vs. other materials) and agglomeration behaviour, which is affected by size (nm vs. µm) and morphology (fibres vs. others), were obvious drivers of ecotoxicity on R. subcapitata. Differences in morphology had an impact on agglomeration behaviour. Fibres formed agglomerates of varying sizes with entrapped and attached algae. Small compact (e.g. spheroidal) particles attached to algae. A high coverage resulted in high ecotoxicity, while less toxic materials attached to a much lesser extent. No agglomeration of algae and particles was observed for particles with a µm size. Small toxic components of large hybrid materials did not affect ecotoxicity. For D. magna, despite uptake of all materials studied into the gut, the sole indication of toxicity was the release of toxic ions. This is in line with previous observations on nanomaterials. Based on the identified criteria, charts were developed to indicate the expected toxicity of advanced/innovative materials toward algae and daphnia. Conclusion: Indicators for the toxicities of advanced materials differ for algae and daphnia. Thus, different materials give rise to concerns for the two aquatic organisms. For D. magna, only the toxic ion-releasing materials are relevant, but for R. subcapitata, more complex interactions between particular materials and cells must be considered.