Assessing the ecological risks of ZnO and CuO nanoparticles to black sea picophytoplankton

The toxic effect of zinc oxide (ZnO NPs) and copper oxide (CuO NPs) nanoparticles on natural communities of Black Sea picophytoplankton was investigated during a year-long cycle of in-situ experiments. The results revealed pronounced seasonal and taxonomic specificity of toxicity. It was established that CuO NPs exert a complex toxic effect on algae, disrupting most key physiological processes in the cells. Pronounced oxidative stress, suppression of pigment synthesis, changes in morphology, dose-dependent inhibition of photosynthetic activity (EC₅₀ = 18.3 μg L^-1), and a sharp restructuring of the community in favor of picoeukaryotes (up to 90-98% at 20 μg L^-1) were observed. In contrast, ZnO NPs acted selectively on the algal community, primarily suppressing the growth of phycoerythrin-containing (PEC) forms of cyanobacteria, while phycocyanin-containing (PCC) forms exhibited resistance. The peak sensitivity of Synechococcus sp. cyanobacteria to CuO NPs was observed in winter-spring (EC50 6-14 μg L^-1), whereas their resistance to ZnO NPs increased in summer (EC₅₀ up to 110 μg L^-1). Picoeukaryotes, on the contrary, demonstrated the highest vulnerability to ZnO NPs during periods of ecological stress (EC₅₀ 12-15 μg L^-1 in July and December). The nanomaterials also induced a cellular morphological stress response, manifested in a significant increase in Synechococcus sp. cell size (up to 2.5 times at 60 μg L^-1 CuO NPs) and a decrease in pigment indices. The obtained data show that the ecotoxicological assessment of nanomaterials cannot be based on constant values and must necessarily account for seasonal fluctuations in the resilience of aquatic organisms, which are linked to the natural life cycles of planktonic communities and changes in abiotic environmental parameters. The results are of particular relevance due to their direct ecological relevance. The calculated EC₅₀ threshold concentrations correspond to real levels of metal nanoparticle pollution in coastal waters.