Microplastics enhance cadmium bioaccumulation and chronic toxicity in Daphnia magna: Insights from a tropical river case study and controlled exposure experiments

Cadmium (Cd) and microplastics (MPs) frequently co-occur in freshwater environments. However, their combined ecological effects remain insufficiently characterized under controlled experimental conditions. In this study, we integrated field surveys with controlled chronic laboratory bioassays to evaluate the co-occurrence patterns of Cd and MPs in the Perai River (Malaysia) and their co-toxicity. Spatial analysis revealed a clear downstream accumulation of both contaminants, with Cd concentrations reaching 3.68 ± 0.57 μg/L and MP abundances up to 143.67 ± 10.34 items/L. A subsequent 21-day chronic exposure experiment conducted at environmentally relevant Cd concentration, demonstrated that the presence of PS-MPs significantly enhanced Cd bioaccumulation in Daphnia magna by 74.35% and further suppressed reproductive output by 27.62%. Co-exposure was further associated with elevated lipid peroxidation, altered antioxidant responses, mitochondrial-related transcriptional responses, dysregulation of DNA repair-related processes, and apoptosis-like cellular responses. By combining field-based contamination data with controlled chronic exposure experiments, this study provided evidence suggesting that MPs could influence Cd toxicity and contributed to a more cautious and mechanistically informed understanding of contaminant mixture risks in tropical freshwater systems.