ZnO nanoparticles and Cu2+ enhanced toxicity in acute rather than chronic exposure of the freshwater rotifer Lecane papuana

Zinc oxide nanoparticles (ZnO NPs) are currently used in several fields, including removing dissolved metals and organic contaminants from wastewater by adsorption and/or photocatalytic mechanisms. Thereafter, ZnO NPs can be released into the environment and reach aquatic ecosystems, where their interaction with dissolved trace metals can alter their solubility and toxicity. For these reasons, the present study aimed to assess the enhanced toxicity of ZnO NPs and dissolved copper (Cu²⁺), using the littoral rotifer, Lecane papuana as a test organism. The ZnO NPs synthesized in this research are colloidally stable at high concentrations in either distilled water or test media according to their Z-potential and hydrodynamic diameter. Thus, they remain as colloids along the duration of the exposure experiments. Acute toxicity tests showed median lethal concentrations (LC50) of 28.24×10^-3 mg Cu²⁺/L, 21.34 mg Zn²⁺/L, and 78.74 mg ZnO NP/L. Enhanced acute toxicity was elicited at low concentrations of Cu²⁺ (1.42 to 5.68×10^-3 mg/L) and ZnO NPs (0.841 mg/L). The interaction of dissolved ions, Cu²⁺ and Zn²⁺, was discarded as the main source of toxicity towards L. papuana as the mixtures tested followed a concentration-addition pattern that produced ~50% mortality (1 toxicity unit [TU] = LC50). Thus, we considered that the enhanced toxicity was mainly caused by the synergistic interaction of ZnO NPs and Cu²⁺. In contrast, chronic exposure to Cu²⁺ and ZnO NPs significantly inhibited the rotifers rate of population growth only in the groups exposed to the higher concentrations tested, representing about 10-20% of their respective LC50 values. Therefore, reports based only on acute tests might bias the ecological significance of the results as long-term and more complex matrices are required for a better understanding of the potential risks that nanoparticles and co-contaminants represent to the aquatic biota.