Polystyrene micro and nanoplastics: A comparative study of the cytotoxic effects exerted on Mytilus galloprovincialis gills.

Polystyrene microplastics (MP) and nanoplastics (NP) are significant contributors to the pollution of aquatic ecosystems. Their high persistence, small size, and high potential to enter food chains pose risks to humans and all living organisms. Mytilus galloprovincialis is a mussel native to the Mediterranean Sea, widely distributed in coastal environments. As a filter-feeder, it accumulates pollutants, making it a valuable bioindicator and model organism in environmental monitoring and research. In this work, we used adult mussels to evaluate the cytotoxic effects of polystyrene MP (5 µm) and NP (0.1 µm) on gill lamellae cyto-anatomy. The animals were exposed for 1, 3, and 11 days. Results showed increased reactive oxygen species (ROS), oxidative damage to lipids, and in vitro susceptibility to oxidants on day 3, as well as increased total antioxidant capacity on days 3 and 11. Parallel histological investigation demonstrated epithelial alterations from day 1, characterised by increased PCNA expression, particularly in the most exposed frontal epithelium. Marked alterations were also observed in the connective septa, where collagen deposition and disorganisation were detected. The activation of an immune response was evident by the increased presence of haemocytes and mucus cells. In all cases, the effects were more marked after exposure to NP. These results suggest a significant impairment of gill function, particularly in food collection, as the frontal and front lateral cilia were markedly affected. In conclusion, plastic reduction in the oceans is an issue that can no longer be postponed; the first step seems to be a more conscious use of this material.