Rayon microfibers exacerbate the toxic effects of polychlorinated biphenyls on foot integrity and byssal performance in the mussel Mytilus coruscus.

Rayon microfibers (RY) and polychlorinated biphenyls (PCBs) are common emerging contaminants in marine environments, and their ecological impacts have attracted increasing research attention. Although these contaminants frequently coexist and pose potential risks to aquatic organisms, their combined toxicity remains poorly understood. This research examined the effects of PCBs, RY, and their combined exposure on foot structure, byssus thread functionality, and the expression of byssus-related genes in the mussel Mytilus coruscus. Mussels were exposed for 14 days to PCBs at low (100 ng/L) and high (1000 ng/L) concentrations, rayon microfibers alone (1000 pieces/L), and combined treatments of PCBs with RY at both concentration levels. The results unveiled that RY and PCBs caused significant structural alterations in foot tissue (P < 0.05), including ciliary deterioration and loss, as well as changes in muscular fiber content. These effects were more pronounced under combined exposure. The mechanical properties of byssus threads, including thread number, length, diameter, and breaking force, were significantly reduced by combined exposure. At the molecular level, exposure to PCBs and RY altered the expression of key byssus-related genes, including collagen precursor genes (Mcfp), with pronounced downregulation observed under combined exposure (P < 0.05). These molecular responses suggest that combined exposure may disrupt the regulatory pathways involved in byssus synthesis and attachment performance. Such impairments may reduce the mussel's ability to maintain stable attachment in dynamic coastal environments. Together, these findings suggest that combined exposure to PCBs and RY can result in greater impairment of key attachment-related traits than either stressor alone. These outcomes emphasize the ecological risks posed by the interaction between microfibers and persistent pollutants in the coastal ecosystem.