Polystyrene nanoplastic exposure induces immobilization, reproduction, and stress defense in the freshwater cladoceran Daphnia pulex

The widespread occurrence and accumulation of plastic waste have been globally recognized as a critical issue. However, there is limited information on the adverse effects of nanoplastics on freshwater invertebrates. In this study, the effects of a polystyrene nanoplastic on physiological changes (e.g., survival, growth, and reproduction) and expression levels of stress defense genes (oxidative stress-mediated and heat shock proteins) in the freshwater flea Daphnia pulex were measured. The results showed that the digestive organs of D. pulex were strongly fluorescent after exposure to the nanoplastic particles, and the 48-h median lethal concentration (LC 50) of the nanoplastic was determined to be 76.69 mg/L. In the 21-day chronic toxicity test, dose- and time-dependent relationships were observed for body length, and the time to first eggs was significantly prolonged in the 0.5 and 1 mg/L groups. The time to clutch was delayed, and total offspring per female and number of clutches were decreased in all the treatment groups. In addition, the offspring per clutch were significantly decreased in the 0.1 mg/L group. As the nanoplastic concentration increased, expression of stress defense genes (SOD, GST, GPx, and CAT) was first induced and then inhibited. The gene expressions of heat shock proteins (HSP70 and HSP90) were induced in all the treatment groups. Our results suggest that nanoplastics can be ingested by the freshwater cladoceran D. pulex and affect its growth and reproduction as well as induce stress defense.