Size-dependent oxidative stress effect of nano/micro-scaled polystyrene on Karenia mikimotoi

The effects of polystyrene (PS) of different sizes of diameter (65 nm, 100 nm and 1 μm) with different treat concentrations (control, 1 mg L^-1 and 10 mg L^-1) on growth and oxidative stress for K. mikimotoi were assessed across PS short-term exposures (3 d) and long-term exposures (13 d). The endpoints of physiological parameters such as cell density, antioxidant enzyme activity of SOD and CAT, the content of MDA and ROS level were monitored. The results showed that the CAT activity, SOD activity, MDA content and the relative ROS level reached to 377 U mgprot^-1, 164 U mgprot^-1, 157 nmol mgprot^-1 and 10.8% when treated with 10 mg L^-1 PS of 65 nm diameter; the CAT activity, SOD activity and MDA content in single K. mikimotoi reached to 0.46 U mgprot^-1, 0.36 U mgprot^-1 and 0.16 nmol mgprot^-1 under 10 mg L^-1 PS of 65 nm diameter on the third day. The relative ROS level in single K. mikimotoi was 71% under 10 mg L^-1 PS of 100 nm diameter on the 13th day. The works found that the size of nano/micro-PS was a key factor that cannot be ignored. Smaller size had more serious negative effects on the growth, oxidative stress and cell microstructure. The potential cytotoxicity mechanisms were that monodisperse nanoscaled PS crossed the biological barriers and the agglomerate nanoparticles caused physical blockage, while microscaled PS may not have such an equally strong negative effects. Visualized SEM images also proved that exposing to nano/micro-PS of varies diameters led to apparent size-dependent effects. The arms race of systematic oxidative defensive and offensive between K. mikimotoi and nano/micro-PS would have considerable value in deliberating the relationship between nano/microplastics and marine phytoplankton.