Oxidative stress responses of Daphnia magna exposed to TiO(2) nanoparticles according to size fraction.

Size is one of important factors determining titanium dioxide nanoparticle (TiO(2) NP) toxicity since penetration is eased with decreasing particle size and bioavailability is increased. The effect of particle size on oxidative stress against titanium dioxide nanoparticle (TiO(2) NP) exposure to Daphnia magna was investigated with both acute and chronic toxicity tests. Experiments on biochemical responses, repeatedly performed after size fractionation of the NPs using filtration, focused on the activities of four antioxidant enzymes: catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPX), and glutathione-S-transferase (GST). In the chronic bioassay, the mortality was significantly increased at TiO(2) NP concentrations of 5 and 10mg/L; however, no reduction of the reproduction ability was observed. Biochemical measurements showed that TiO(2) NP exposure significantly increased the antioxidant enzyme activities in D. magna. CAT, GPX and GST, but not SOD, showed a concentration-dependent increase. In terms of size fraction, particles ranging from 400 to 800nm exhibited an increase of antioxidant enzyme activities in GST and GPX. These biochemical level observations suggested that TiO(2) NP toxicity was mediated by reactive oxygen species (ROS) generation via oxidative stress in D. magna. The increased mortality at the concentration of 5mg/L in the chronic bioassay was attributed to accumulated TiO(2) NPs in the intestine of D. magna, which might induce effects such as oxidative stress relating to the induction of antioxidant enzymes.