Combined effects of titanium dioxide nanoparticles and bisphenol-A on freshwater algae Scenedesmus obliquus, and the importance of humic acid in reducing toxicity

The rising usage of titanium dioxide nanoparticles (nTiO₂) in household products has led to their widespread occurrence in freshwater systems. Bisphenol-A (BPA), a secondary contaminant, can modify the toxic potential of nTiO₂. This study aimed to investigate the effects of BPA on the poisonous effects of nTiO₂ on algae. Four concentrations of nTiO₂, 0.4, 0.8, 1.6, and 3.2 mg L⁻¹, were mixed with 1 mg L⁻¹ of BPA to perform the mixture toxicity tests with Scenedesmus obliquus, a freshwater alga. To understand the effects of natural organics on the combination toxicity of nTiO₂ and BPA, humic acid (HA) was used as a model compound. The cell viability in the treatment groups containing both nTiO₂ and BPA was significantly reduced compared to that with the single components. This was accompanied by increased oxidative stresses like ROS generation, LPO production, and resultant loss in the photosynthetic yield in the mixture-treated samples. The combination toxicity data was validated using Abbott’s independent action model. Furthermore, optical microscopic analysis was conducted to examine the morphological alterations in the algae. In another set of tests, 1 and 10 mg L⁻¹ of HA were mixed with 3.2 mg L⁻¹ of nTiO₂ and 1 mg L⁻¹ of BPA. Humic acid ameliorated the effects of the nTiO₂ and BPA in the cells. This was evident from enhanced cell viability and decreased oxidative stress-enhanced photosynthetic activities in the cells. In addition, the fate of BPA in the presence of nTiO₂ and HA was also studied during algal interaction.