Imbalance between oxidative and antioxidative systems: toward an understanding of visible light-induced titanium...

The broad application of titanium dioxide nanoparticles (TiO(2) NPs) has resulted in the release of substantial quantities of untreated TiO(2) NPs into aquatic systems, which serve as the terminal sink for nanomaterials. These TiO(2) NPs may induce some unexpected toxic effects to aquatic systems. The objectives of this research were to evaluate toxic effects of low dosage TiO(2) NPs on Tetrahymena pyriformis under visible-light illumination conditions and further discuss possible mechanisms. The results showed that TiO(2) NPs accumulated as aggregates in cytosolic vesicles of T. pyriformis. Cell numbers and viability of TiO(2) NPs-exposed groups decreased to (20.2 ± 1.8)% and (17.3 ± 1.9)% after 24h continuous 12000 Lx illumination. Compared with the dark control, the intracellular reactive oxygen species (ROS) levels increased (1.9-fold) by the combination of 5 mg L(-1) TiO(2) NPs and 12000 Lx illumination. Even incubation in the dark for 6h after 12h illumination, superoxide dismutase (SOD) activity of TiO(2) NPs exposed groups still maintained at the high level (about 3.9-fold of dark control). Meanwhile, illumination-induced TiO(2) NPs led to a reduction of scavenging hydroxyl radicals (OH) activity with an average of (14.3 ± 6.5)%. Under illumination conditions, overproduction of reactive oxygen species (ROS) by TiO(2) NPs exceeded the scavenging capability of T. pyriformis and then led to the breakdown of oxidative/anti-oxidative systems, further resulting in cell growth delay and mitochondria dysfunction. All the results revealed that via visible light excitation, oxidative injury may be responsible for the toxic effects of TiO(2) NPs on T. pyriformis.