Distribution of environmental microplastics in different tissues of indigenous freshwater bivalves (Unio spp.): a case study from the Sejenane River in Northern Tunisia

Abstract Microplastic (MP) pollution is a worldwide concern and represents an ecological threat to the aquatic environment, particularly freshwater ecosystems. It can pose risks to the health of organisms and potentially lead to bioaccumulation of these tiny particles in the food chain. This study focused in MP determination on three species of freshwater mussels (Unio gibbus, Unio ravoisieri, and Unio dureui) as potential models for ecological assessment in the Sejenane stream in Northern Tunisia. To achieve this, we assessed ingested microplastics in the gills and digestive gland tissues of these mussels. Raman microspectroscopy was used to examine and identify microparticles with size ranges under 5,000 μm. Our results indicated that the microparticles are categorized into three sequential size ranges: S1 (< .45–1.2 μm), S2 (< 1.2–3 μm), and S3 (≥ 3 μm). Over 50% of the S1 class was found in Unio gibbus. Our findings showed a higher occurrence of the S3 size class of microplastics (≥ 3 µm) in the gills of all studied mussels. More than 60% of the S3 class was identified in Unio durieui, followed by S2 (< 3–1.2 µm) and S1 (< 1.2–.45 µm). Polyethylene-vinyl acetate, polypropylene, low-density polyethylene, polyethylene terephthalate, high-density polyethylene, and polyethylene are the six different types of polymers that were found. Polyethylene terephthalate emerged as the dominant polymer type in Unio dureui, accounting for up to 59% of the gills and 55% of the digestive gland. Overall, it seems that freshwater mussels are capable of accumulating microplastics from environmental contamination. However, further studies in diverse freshwater ecosystems are necessary to validate the findings of this study.