Metabolic responses of the marine mussel Mytilus galloprovincialis after exposure to microplastics of different shapes and sizes

Microplastics (MP) are ubiquitous pollutants with diverse shapes, sizes, and characteristics that pose critical risks to marine organisms and the environment. In this study, we used the Mediterranean mussel Mytilus galloprovincialis as a marine benthic organism model to investigate the metabolic consequences of exposure to different polyethylene terephthalate MP sizes and shapes: round (27-32 μm), small fibers (200-400 μm), large fibers (3000 μm), small fragments (20 μm), medium fragments (45-75 μm), and large fragments (>150 μm). After exposure to high concentrations (100 mg L) of MP for 14 days, round and small fiber-type MP were highly accumulated in mussels. Metabolomic analysis revealed that exposure to round and small fiber-type MP induced significant changes in 150 metabolites. Partial least squares-discriminate analysis (PLS-DA) showed that the round and small fiber MP treatment groups displayed similar cluster patterns that differed from those of the control group. In addition, only 22 annotated metabolites related to histidine, valine, leucine, and isoleucine degradation/biosynthesis and vitamin B6 and aminoacyl-tRNA biosynthesis were significantly affected by round or small fiber-type MP. Among the histidine metabolites, round and small fiber-type MP upregulated the levels of L-histidine, L-glutamate, carnosine, imidazole-4-acetaldehyde, 4-imidazolone-5-propanoate, and methylimidazole acetaldehyde and downregulated methylimidazole acetic acid and N-formimino-L-glutamate. These results suggest novel insights into the potential pathways through which MP of specific sizes and shapes affect metabolic processes in mussels.