Polylactic acid bioplastics (PLA BPs): A threat to the reproductive health of the Mediterranean mussel Mytilus galloprovincialis

Polylactic acid-based bioplastics (PLA-BPs) are considered the green innovation of the 21^st century because they are considered more environmentally friendly than conventional plastics. However, their growing production has led to their increased presence in environmental matrices at concentrations of mg/L (in liquid samples) and mg/g (in solid samples), a sign of the limited capacity of treatment plants to mitigate their release into the environment. Their slow degradation makes them a potential threat to biota health in marine environments. This study aims to evaluate the impact of environmental concentrations of PLA-BPs (0.5 µg/mL) on the reproductive health of the Mediterranean mussel Mytilus galloprovincialis during a 7-day exposure. Histomorphological analysis by Hematoxylin/Eosin (H/E) staining revealed that gonads of both sexes underwent precocious spawning, intense haemocyte recruitment and adipogranular cell hypertrophy. Positive results for the diastase Periodic Acid-Schiff/PAS (dPAS/PAS) reaction indicated an increase in the glycogen level, a key energy reserve during gametogenesis, which was more evident in females than in males. Metabolomic data based on Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy revealed augmentation of energy demands from both the aerobic and anaerobic pathways, as well as the occurrence of redox imbalance by alterations in the concentration of glutathione constituents. This finding was also confirmed by biochemical analysis showing, primarily in males, an increase in the enzymatic activity of catalase and glutathione S-transferase. These results were validated by Partial Least Squares Discriminant Analysis (PLS-DA), which highlighted a greater variance in females between the treated and control groups. Overall, the study findings, besides highlighting some similarities with conventional plastics, suggest that PLA-BPs represent an emerging threat to the reproductive health of mussels. However, these ecological engineers of the intertidal zone are able to implement, at least for a period of 7 days, sex-dependent adaptive mechanisms to preserve their reproductive capacity and survival.