Toxicity Mechanisms of ZnO Nanoparticles in Nanochloropsis Oculata with a Comparative Approach with CuO and Ag Nanoparticles

The purpose of present work was the investigation of different concentrations of zinc oxide nanoparticles on the marine microalga Nannochloropsis oculata and compare the results of this study with previous studies. Dissolution of ZnO NPs in nanopure water was 0.378-3.12 mg/L and the rate solubility decreased with increasing the concentrations of ZnO NPs. ZnO NPs were toxic to this microalga with EC50 of 153/72 mg/L. The toxicity of 200 mg/L ZnO NPs was 59.36% for the cell number, 61.27% for MTT test, and 57.34% for the chlorophyll content. Increase the content of malondialdehyde and hydrogen peroxide in response to increasing the concentration of ZnO NPs was indicated the induction of oxidative stress in N. oculata. The activity of catalase and lactate dehydrogenase increased in the treated cells, while the activity of ascorbate peroxidase was decreased. Concurrently, an increase in the content of carotenoids and phenolic compounds was observed in the treated cells. SEM and TEM analyses confirmed the aggregation of algal cells, damages in cell membrane and atypical changes in morphology of cell wall after NPs treatments. The FTIR results cofirmed the interaction of ZnO NPs with C-H, C-O and C=O groups on the cell surface. All of these changes were indicated the significant toxic impacts of ZnO NPs on the N. oculata cells. Comparison between the results obtained in previous studies with our results showed that the defensive mechanisms of N . oculata probably was not effective against the oxidative stress by >10 mg/L of ZnO NPs, > 5 mg/L of CuO NPs and > 1 mg/L of Ag NPs. Therefore, N. oculata is sensitive to such concentrations of these NPs.