| Summary: | The effects of the photocatalytic toxicity of titanium dioxide nanoparticle (nano-TiO<sub>2</sub>) on phytoplankton are well understood. However, as UV light intensity decreases sharply with the depth of the water column, the effects of nano-TiO<sub>2</sub> itself on deeper water phytoplankton, such as green algae, need further research. In this research, we investigated the effects of three sizes of TiO<sub>2</sub> (10, 50 and 200 nm) on the photosynthetic and antioxidative processes of <i>Scenedesmus obliquus</i> in the absence of UV light. We found that 50 nm and 10 nm TiO<sub>2</sub> (10 mg/L) inhibited growth rates and the maximal photosystem II quantum yield compared to the control in <i>Scenedesmus obliquus</i>. The minimal and maximal fluorescence yields, and the contents of reactive oxygen species and lipid peroxidation, increased, indicating that photosynthetic energy/electrons transferred to oxygen and induced oxidative stress in nano-TiO<sub>2</sub>-treated samples. In addition, we found that aggregations of algae and 10 nm TiO<sub>2</sub> were present, which could induce cell membrane disruption, and vacuoles were induced to cope with nano-TiO<sub>2</sub> stress in <i>Scenedesmus obliquus</i>. These results enhance our understanding of the effects of nano-TiO<sub>2</sub> on the photosynthetic and antioxidative processes of green algae, and provide basic information for evaluating the ecotoxicity of nano-TiO<sub>2</sub> in freshwater ecosystems.
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