Polyethylene-induced gill alterations in Mytilus galloprovincialis and the mitigation potential of Chlorella vulgaris

Microplastic (MP) pollution has emerged as a major environmental challenge, with polyethylene (PE) being among the most frequently detected microplastics currently found in marine ecosystems. Due to their filtering behaviour and constant exposure to contaminants, bivalve molluscs are particularly susceptible to MPs uptake, also representing a potential vector for the transfer of MPs to humans through the food chain. As the primary site of respiration and feeding, the gills represent a key organ for evaluating MP toxicity. This study provides evidence of histopathological and molecular alterations induced by environmentally relevant concentrations of polyethylene (PE, 20 and 100 μg/L) in the gills of Mytilus galloprovincialis after 14 and 21 days of exposure. Here, we also investigated the potential protective role of co-exposure with Chlorella vulgaris, a nutrient-rich microalga that has recently attracted attention for its bioremediation capacity. Exposure to PE resulted in severe gill alterations, including epithelial detachment, hyperplasia, granulocyte infiltration and haemolymphatic vessel dilation, with severity increasing in a dose- and time-dependent manner. At the molecular level, PE induced the upregulation of sod, cat, and gst, and the downregulation of hsp70. Co-exposure with C. vulgaris mitigated the onset of numerous alterations, although it did not fully prevent gill damage. C. vulgaris also reduced PE-induced oxidative stress, as evidenced by lower expression levels of stress-related genes. Overall, our findings demonstrate the protective role of C. vulgaris against PE-induced toxicity, supporting its potential use as a dietary supplement and bioremediation tool in aquaculture.