The short-term effects of polystyrene microplastics on the reproductive function of Mytilus galloprovincialis: morphological and metabolomic evidence.

Microplastic (MP) pollution is one of the most significant environ- mental challenges worldwide. Plastic’s persistence and fragmentation capacity generate microscopic particles that can accumulate in aquatic ecosystems, potentially causing toxic effects on aquatic organisms and, indirectly, humans. In this study, we evaluated the cytotoxic and metabolic effects associated with a short-term (48 h) exposure to polystyrene MP (5 μm) in the gonads of the bivalve Mytilus galloprovincialis, a model species widely used for envi- ronmental biomonitoring due to its wide distribution and relevance, both ecological and commercial. Morphological analyses, performed using specific histological techniques, revealed significant alterations in the structure of sperm cysts in a dose-dependent manner, leading to impaired germ cell interactions and disorgani- sation of gonadal tissue. These changes are caused by both physi- cal damage from direct contact with the particles and the initiation of oxidative stress. Furthermore, metabolomic analysis, based on proton nuclear magnetic resonance (1H NMR)-based metabolomics, combined with chemometrics, allowed for the identification of time-dependent changes in the metabolic profile of the exposed gonads, revealing an imbalance in energy and osmoregulatory pathways. Finally, we observed altered chromatin folding in spermatozoa, suggesting a potential impact on fertility and reproductive success. Our findings suggest that even brief exposures to MP can significantly impair the reproductive health of mussels, negatively affecting the fertilisation capacity of spermatozoa.