Contributions of Zn Ions to ZnO Nanoparticle Toxicity on Microcystis aeruginosa During Chronic Exposure

In this work, we assessed the toxic effects of ZnO nanoparticles (NPs; 1, 10, and 50 mg L-1) and the corresponding dissoluble Zn ions (0.71, 8.66, and 35.59 mg L-1) on Microcystis aeruginosa. After chronic exposure (28 days), significantly higher growth inhibition was observed under ZnO NPs at 1 mg L-1 (47%) than under Zn ions at 0.71 mg L-1 (-15%). The opposite effect pattern was observed for ZnO NPs at 10 (71% vs. 80%) and 50 mg L-1 (73% vs. 95%) compared to Zn ions at the corresponding concentrations. After 7 days of exposure, ZnO NPs at 10 and 50 mg L-1 led to an increase of 83 and 53% in malondialdehyde content, as well as an increase of 106 and 61% in superoxide dismutase activity, respectively. However, Zn ions at the corresponding concentrations showed negligible impacts on the two parameters. The different results indicate that the insoluble NPs during the initial exposure mostly account for lipid peroxidation, which further lead to microalgal antioxidant response. During the subsequent exposure, the contributors of ZnO NP toxicity shift with the concentration and exposure time of ZnO NPs. In conclusion, the study presents new insights into the different contributions of insoluble NPs and dissoluble metallic ions to metallic NP toxicity.