Effects of polystyrene in the brackish water flea Diaphanosoma celebensis: Size-dependent acute toxicity, ingestion, egestion, and antioxidant response

•Nano-sized PS beads have longer retention time than micro-sized ones.•PS beads distribute in various tissues in D. celebensis depending on size.•PS beads cause changes in the antioxidant capacity.•PS beads induce size-dependent oxidative stress response, causing cellular damage. Owing to the increasing usage of plastics, their debris is continuously deposited in marine environments, resulting in deleterious effects on aquatic organisms. Although it is known that microplastics disturb the cellular redox status, knowledge of molecular in marine cladocerans is still lacking. In the present study, we investigated the acute toxicity of different-sized polystyrene (PS) beads (0.05, 0.5, and 6-μm diameter), ingestion and egestion patterns, their distribution in the tissues, and their effects on the antioxidant systems in the brackish water flea Diaphanosoma celebensis. All different-sized PS beads showed no mortality at the concentrations used in this study. After 48 h of exposure to PS beads of different sizes, all microbeads were retained in the digestive tract, but the retention time varied according to the bead size. In particular, the group that was exposed to 0.05-μm beads showed widely distributed fluorescence (e.g., in the embryo, and probably in lipid droplets as well as the digestive tract). The transcriptional level and enzyme activities of antioxidants were modulated depending on the size of the PS beads, and lipid peroxidation was induced in groups exposed to 0.05 and 0.5-μm beads. These findings suggest that the size of PS beads is an important factor for cellular toxicity, and can induce size-dependent oxidative stress in this species. This study provides a better understanding of the molecular modes of action of microplastics in marine zooplankton.