Due to the increasing production and use of nanomaterials (NMs), their potential toxic impacts on the environment should be considered for a safe application of NMs. In this regard, the potential hazards of MnO 2 nanorods (NRs) on the green microalgae Chlorella sorokiniana during long-term exposure were investigated. Exposure to the high concentration of MnO 2 NRs (100 and 200 mg L−1) significantly reduced the cell number of C. sorokiniana over 20 days of the experiment. The different concentrations of MnO 2 NRs (25–200 mg L−1) induced the remarkable increase in the chlorophyll (a+b) content of algal cells due to the shading effect of NRs. For more than 72 h, the chlorophyll content of microalgae decreased due to the aggregation of NRs and the possible effects of oxidative stress. Long-term exposure to high concentrations of NRs caused a significant decrease in the primary and secondary metabolites of microalgae, including carotenoids, phenolic compounds, proteins, lipids, and carbohydrates. Oxidative stress was one of the possible toxic mechanisms of MnO 2 NRs to microalgae validated by an increase in lipid peroxidation induced by exposure to NRs. The algal cells increased the catalase activity and the amount of extracellular polymeric substances in response to NRs toxicity. The low level of Mn ions in the culture media indicated that MnO 2 NRs dissolution was not the cause of the observed reduction in the microalgae growth. Moreover, the bulk form of MnO 2 was not involved in the toxic impact of MnO 2 , which was documented by an insignificant decrease in the growth, pigment, and lipid peroxidation of C. sorokiniana. These results may provide an additional insight into the potential hazards of MnO 2 NRs on the aquatic ecosystem. • Long-term exposure of MnO 2 nanorods (NRs) to microalgae Chlorella sorokiniana. • Reduction in growth of C. sorokiniana exposure to high concentrations of MnO 2 NRs. • Increase in chlorophyll content due to shading effect during short-term exposure. • Oxidative stress as one of the possible toxic mechanism of MnO 2 NRs. • No significant toxic effects on C. sorokiniana exposure to bulk form of MnO 2. [ABSTRACT FROM AUTHOR]