Microplastic (MP) contamination and trace metal pollution are pervasive and often co-occurring stressors in marine ecosystems, particularly in the Mediterranean Sea, one of the world’s most polluted semi-enclosed basins, posing increasing risks to marine biota. This study investigated the individual and combined effects of microplastics and trace metals—cadmium (Cd), lead (Pb), and arsenic (As) on antioxidant and stress-response pathways in the Mediterranean mussel Mytilus galloprovincialis. Mussels (27.04 ± 6.45 g; 4.02 ± 0.21 cm) were exposed for 14 days to four treatments: (1) control (synthetic seawater, 18 °C), (2) microplastics (=1 mg L⁻¹), (3) metals (Cd, Pb, As; 10 µg L⁻¹ each), and (4) combined microplastics + metals. At the end of exposure, gill and digestive gland tissues were analyzed by qPCR to assess the expression of antioxidant genes (cat, sod1, sod2) and the stress-inducible chaperone hsp70.

Both microplastics and metals individually induced moderate up-regulation of antioxidant genes, consistent with enhanced reactive oxygen species (ROS) generation and activation of cellular defense mechanisms. The combined treatment elicited the strongest transcriptional responses, particularly for sod2 and hsp70, indicating pronounced mitochondrial oxidative stress.

Overall, the findings demonstrate that co-exposure to microplastics and trace metals imposes a greater oxidative burden than exposure to either stressor alone, potentially compromising the physiological resilience of M. galloprovincialis. Understanding such multi-stressor interactions is essential for evaluating the ecological risks of emerging pollutants in the Mediterranean coastal environment.