Cellular and tissue-level responses of mussels (Mytilus edulis) to aged polyethylene terephthalate (PET) micro- and nanoplastic particles

Micro- and nanoplastic particles (MNPs) are pollutants of global concern due to their persistence, ubiquity, and associated risks. Laboratory studies, however, have predominantly focused on pristine MNPs, which do not adequately reflect the characteristics of environmental plastic debris. To address this gap, this study investigated the cellular and tissue-level responses of mussels (Mytilus edulis) to aged polyethylene terephthalate (PET) MNPs (diameter 600 nm to 3.1 µm) at three environmentally relevant concentrations: 10, 10³, and 10⁵ particles/L. The particles' physicochemical characteristics and stability in exposure media were analyzed using a combination of advanced analytical techniques. The biological responses were analyzed across multiple effect endpoints during both the exposure (days 1, 3, 7, and 14) and the subsequent recovery periods (3 and 10 days post-exposure), via flow cytometry and histopathology. The results revealed the sensitivity of hemocyte subpopulations, including granulocytes and hyalinocytes, to aged PET MNPs. Concentration- and time-dependent changes in lysosomal stability, oxidative activity, and hemocyte mortality were observed, demonstrating both immediate cellular perturbations and recovery potential to alleviate particle-induced effects. Histopathological analysis of key tissues exhibited significant alterations, particularly in the gill, suggesting potential impairment of essential physiological functions. No mussel mortality or significant changes in growth metrics were observed under the tested experimental conditions. These findings underscore the systemic impacts across multiple tissues of aged MNP exposure and highlight the importance of adopting integrative, environmentally realistic approaches to assess the biological consequences in future research.