Acute toxicity of copper and chromium oxide nanoparticles to Daphnia similis

Copper oxide nanoparticles (CuO NPs) are employed in antifouling paints, and chromium oxide nanoparticles (Cr2O3 NPs) have been used as a green pigment. Their extensive use can contaminate aquatic ecosystems, and the toxicological effects of these NPs to the biota are poorly known. In this study, we evaluated the acute toxicity induced by CuO and Cr2O3 NPs, comparing with CuSO4 and Cr(NO3)3 as Cu2+ and Cr3+ ion source, respectively, using the microcrustacean Daphnia similis. The mean EC50-48h for CuO NPs was 0.064 mg L-1 and for CuSO4 was 0.015 mg L-1. CuO NPs tend to agglomerate, which may have reduced the release of Cu2+ in the test medium in relation to CuSO4.The mean EC50-48h for Cr2O3 NPs was 6.74 mg L-1 and for Cr(NO3)3 was 11.98 mg L-1. The reduced size of the Cr2O3 NPs (15-30 nm) and the higher zeta potential may have contributed to the higher stability in suspension and less potential for agglomeration, partially explaining the higher toxicity of NPs in relation to Cr(NO3)3. After the tests, we observed morphological damages such as increase in fat droplets, internal organ exposure and partially disintegration in organisms exposed to all tested substances, NPs or the salts.