Combined effect of microplastics and tire particles on Daphnia magna: Insights from physiological and transcriptomic responses

Microplastics (MPs) and tire particles (TP) are ubiquitous in freshwater ecosystems, raising growing concerns about their ecological impacts. However, limited studies have explored their combined effects. This study investigates the physiological and transcriptomic responses of Daphnia magna to environmentally relevant concentrations (1 and 5 mg/L) of MPs, TP, and their mixture (Mix-MPsTP). Microscopic analyses confirmed particle ingestion and accumulation in the gut, with greater retention observed at the lower concentration, suggesting a non-linear dose-response possibly driven by particle aggregation or avoidance behavior. Oxidative stress biomarkers revealed significant elevation in reactive oxygen species (ROS) and superoxide dismutase (SOD) activity, particularly in the Mix-MPsTP 1 mg/L group, while catalase (CAT) activity was suppressed at higher doses. Despite ROS induction, malondialdehyde (MDA) levels remained unchanged, indicating successful mitigation of lipid peroxidation. Transcriptomic data aligned with physiological findings, showing upregulation of antioxidant-related genes such as peroxidases, cytochrome b5, and endochitinase A, along with metabolic genes like vitellogenin-2 and C-reactive protein. While glycogen levels were significantly affected, protein content remained unchanged, implying preserved metabolic stability under short-term stress. Importantly, this study fills a critical knowledge gap, as no prior research has comprehensively examined the mixture effects of different MPs, TP, and Mix-MPsTP on aquatic organisms. The results highlight the complexity of pollutant interactions and underscore the need for mixture-based assessments in ecotoxicological research and risk management strategies.