The critical importance of defined media conditions in Daphnia magna nanotoxicity studies

Due to the widespread use of silver nanoparticles (AgNPs), the likelihood of them entering the environment has increased and they are known to be potentially toxic. Currently, there is little information on the dynamic changes of AgNPs in ecotoxicity exposure media and how this may affect toxicity. Here, the colloidal stability of three different sizes of citrate-stabilized AgNPs was assessed in standard strength OECD ISO exposure media, and in 2-fold (media2) and 10-fold (media10) dilutions by transmission electron microscopy (TEM) and atomic force microscopy (AFM) and these characteristics were related to their toxicity towards Daphnia magna. Aggregation in undiluted media (media1) was rapid, and after diluting the medium by a factor of 2 or 10, aggregation was reduced, with minimal aggregation over 24 h occurring in media10. Acute toxicity measurements were performed using 7 nm diameter particles in media1 and media10. In media10 the EC50 of the 7 nm particles for D. magna neonates was calculated to be 7.46 μg L−1 with upper and lower 95% confidence intervals of 6.84 μg L−1 and 8.13 μg L−1 respectively. For media1, an EC50 could not be calculated, the lowest observed adverse effect concentration (LOAEC) of 11.25 μg L−1 indicating a significant reduction in toxicity compared to that in media10. The data suggest the increased dispersion of nanoparticles leads to enhanced toxicity, emphasising the importance of appropriate media composition to fully assess nanoparticle toxicity in aquatic ecotoxicity tests.