Adverse Effects of Polystyrene Microplastic Exposure on Daphnia magna: a Comprehensive Assessment of Acute Toxicity, Behavioral Changes, and Oxidative Stress

Microplastics (MPs) pollution has emerged as a pressing environmental concern. In this study, we comprehensively evaluated the adverse effects of polystyrene microplastic (PSMP) on Daphnia magna, focusing on acute toxicity, behavioral changes, and oxidative stress responses. Exposure experiments demonstrated that the acute toxicity of PSMP (1, 6.5, and 50 µm in size) to D. magna increased with concentration, with 1-µm particles being the most toxic (48-h LC50 value: 23.57 mg/L). Behavioral analyses revealed that PSMP exposure reduced swimming speed from 5.8 ± 0.2 mm/s (control) to 3.7 ± 0.4 mm/s (1/2 LC50) after 24 h, further decreasing to 3.0 ± 0.5 mm/s at 48 h. Similarly, thoracic limb movement frequency, jumping frequency, and mandibular activity were suppressed, with transient hyperactivity observed at low doses (1/16 LC50) followed by inhibition. Furthermore, exposure to smaller-sized PSMP caused significant changes in the levels of superoxide dismutase, catalase, malondialdehyde, and glutathione in D. magna, indicating that PSMP induces oxidative stress by increasing reactive oxygen species levels, ultimately leading to oxidative damage. While particle size is a well-established critical parameter, behavioral endpoints (e.g., reduced swimming speed and feeding efficiency) and oxidative damage at sublethal concentrations serve as more sensitive early-warning biomarkers for environmental risk assessment. Future research should validate these biomarkers in natural ecosystems and investigate the effects of chronic exposure on population dynamics.