Fragmented polystyrene and polyethylene microplastics modulate brain antioxidant defences and behaviour in adult zebrafish under Copper Co-exposure.

Microplastics (MPs) frequently co-occur with trace metals in aquatic environments, yet their combined effects on fish neurobiology remain insufficiently understood. This study evaluated the effects of fragmented polystyrene (PS) and polyethylene (PE) MPs, alone and combined with copper (Cu), on brain redox homeostasis and behaviour in adult zebrafish (Danio rerio). Prior to exposure, both polymers were characterized by optical microscopy and scanning electron microscopy, confirming irregular fragmented morphologies and comparable particle dimensions. Adult zebrafish were exposed for 21 days to MPs of PS or PE (1 mg/L), Cu (25 μg/L), or their respective mixtures. Behavioural endpoints (shoaling, exploratory activity and anxiety-related responses) were assessed together with oxidative stress biomarkers, antioxidant enzyme activities and oxidative damage in the brain. MPs altered shoaling behaviour independently of polymer type, whereas locomotor and exploratory performance remained largely unaffected. At the biochemical level, exposure modulated antioxidant defences, including glutathione-related enzymes and glutathione S-transferase activity, indicating adjustments in redox homeostasis. Copper was the main factor influencing several redox-related endpoints, while significant interactions between MPs and Cu were restricted to selected biomarkers. Despite these biochemical responses, no evidence of DNA damage was detected under any exposure condition. Overall, the results indicate that chronic exposure to fragmented PS and PE under an experimentally defined high-end microplastics contamination scenario induced subtle behavioural alterations and modulated brain antioxidant defences in zebrafish. In contrast, the influence of Cu on these responses depends on the specific endpoint evaluated.