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Abstract

Abstract Microplastics (MPs) are pervasive pollutants in marine ecosystems and pose risks to aquatic organisms. The blue mussel ( Mytilus edulis ) is widely used as a sentinel species for microplastic studies. This study investigated the accumulation of fragment-shaped microplastics of different polymers—polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC)—and sizes (100–200 µm, 200–300 µm, 300–500 µm) in mussels, and assessed physiological and gut microbial responses to 300–500 µm PS MPs. Short-term exposure revealed rapid ingestion of all microplastic types, with peak abundances at 2 hours for PET (0.7–1.87 items/g) and PVC (0.55–1.5 items/g), and at days 4–5 for PS (2.92–6.63 items/g). Smaller fragments (100–200 µm) were ingested more readily but excreted efficiently, whereas larger fragments (300–500 µm) were retained longer. PS microplastics induced oxidative stress: superoxide dismutase (SOD) and catalase (CAT) peaked at 87.3 U/mg and 62.5 U/mg on day 7, while reduced glutathione (GSH) decreased to 3.2 µmol/g. Digestive enzymes were affected: α-amylase increased to 48.6 U/mg, while trypsin and pepsin decreased to 12.5 U/mg and 15.2 U/mg before partial recovery. Gut microbiota shifts were observed at phylum (Proteobacteria, Verrucomicrobia, Bacteroidetes) and genus levels ( Rubritalea , Mycoplasma , Arcobacter ), with reduced Chao1 and Shannon indices (p < 0.05). Principal Coordinate Analysis confirmed substantial compositional changes across exposure stages. These results indicated that microplastic size and polymer type jointly regulate accumulation in mussels, and that PS MPs cause multi-dimensional physiological stress and gut microbiota disruption, highlighting potential ecological risks for bivalves.