COMBINED EFFECTS OF POLYSTYRENE MICROPLASTICS, NANOPLASTICS, AND MERCURY ON THE GILL STRUCTURE AND FUNCTION OF <i>MYTILUS GALLOPROVINCIALIS</i>

Marine pollution from plastics and heavy metals is an escalating global concern. Microplastics (MPs) and nanoplastics (NPs), originating from industrial waste and synthetic textile degradation, have become widespread contaminants in the aquatic environment. Additionally, heavy metals such as mercury (Hg), largely released through anthropogenic activities, persist in the environment and pose significant ecotoxicological risks to marine ecosystems. Co-exposure to these contaminants may result in synergistic interactions, amplifying their individual toxic effects, and potentially disrupting key structures and functions in marine organisms. Filter-feeding organisms like the mussel Mytilus galloprovincialis, a key species in coastal ecosystems, are particularly vulnerable due to their capacity to accumulate contaminants in their tissues. Therefore, this study aimed to investigate the effects of polystyrene MPs, polystyrene NPs, and Hg, both separately and in combination (MPs + Hg and NPs + Hg) in the gills of M. galloprovincialis after 11 days of exposure under controlled laboratory conditions. Gills, the primary site of interaction with waterborne xenobiotics, were analysed using histological techniques and oxidative stress and neurotoxicity biomarkers as endpoints. Histological analyses showed significant structural damage, disorganization of the respiratory epithelium, cytoskeletal disruption, and altered mucus cell distribution, especially in the gills of mussels simultaneously exposed to NPs and Hg. Exposure to Hg alone and in combination with MPs and NPs led to a decline in catalase activity, along with increased glutathione-S- transferase activity and depletion of total glutathione, suggesting a compensatory shift in the antioxidant defence strategies. Despite these biochemical alterations, no oxidative damage or neurotoxicity was observed, indicating effective antioxidant responses. Results support the "Trojan Horse" hypothesis, in which plastic particles help metals enter cells, increasing their uptake and toxicity, and leading to pronounced biochemical and structural alterations, compared to single exposures. These findings highlight the urgent need for stricter environmental regulations to reduce and monitor plastic and heavy metal pollution in marine environments.