UV-B radiation enhances the toxicity of TiO2 nanoparticles to the marine microalga Chlorella pyrenoidosa by disrupting the protection function of extracellular polymeric substances

Abstract

Enhanced UV-B radiation (UVBR) has been a global environmental problem for decades due to stratospheric ozone depletion. Simultaneously, the potential environmental risks of TiO₂ nanoparticles (NPs) have increasingly attracted attention. In the natural environment, marine microalgae could be simultaneously exposed to enhanced UVBR and TiO₂ NPs. Owing to their photocatalytic properties, the toxicity of TiO₂ NPs to marine microalgae might be exacerbated by UVBR. However, the combined effects and toxicity mechanisms of TiO₂ NPs and UVBR on marine microalgae remain poorly understood. In this study, we investigated the bioavailability and toxicity of TiO₂ NPs to the marine microalga Chlorella pyrenoidosa under UVBR. The results demonstrated that growth inhibition derived from TiO₂ NPs was further enhanced by UVBR. Although UVBR accelerated the sedimentation of TiO₂ NPs, the remaining suspended particles had smaller hydrodynamic diameters and caused stronger growth inhibition, indicating elevated effects of TiO₂ NPs on algal cells after UVBR exposure because of the size-effect. Importantly, TiO₂ NPs and UVBR showed a significant antagonistic interaction on the extracellular polymeric substances (EPS) secreted by C. pyrenoidosa with increasing concentration/dose, thus allowing more TiO₂ NPs to enter the algal cells. The production of intracellular reactive oxygen species was triggered by internalized TiO₂ NPs under UVBR exposure, resulting in lipid peroxidation and serious cellular damage in C. pyrenoidosa. Photosynthesis of C. pyrenoidosa was inhibited after exposure to TiO₂ NPs combined with UVBR due to a combination of oxidative damage and NP shading. Overall, weakened EPS protection of algal cells, which contributed to the increased internalization of TiO₂ NPs, was identified as the key mechanism for the enhanced toxicity of TiO₂ NPs under UVBR conditions. This work provides new insights into the marine environmental risks associated with TiO₂ NPs under increasing UVBR conditions.