Effects of acute exposure to polystyrene nanoparticles and water-soluble diesel oil fraction alone and combined in the ascidian Styela plicata: Synergistic gonadal tissue damage.

Plastic and oil pollution are nowadays among the two major threats to marine ecosystems. They harm marine organisms through a series of toxicological effects from oxidative stress to DNA and tissue damage, up to mortality. Ascidians, as active filter-feeders, are vulnerable to suspended contaminant exposure and subject also to bioaccumulation. In the present study the single and combined short-term effects (48h) of nanoplastics as polystyrene nanoparticles (PS NPs) and the water-soluble fraction of diesel oil (WFDO) were investigated for the first time on adults of the pleated sea squirt Styela plicata (Lesueur, 1823) at environmentally realistic concentrations of PS NPs (PS NP 20 nm, 50 μg/L; WFDO 5% in FSW). Oxidative stress biomarkers as well as histological alterations were evaluated. No significant changes in the acetylcholinesterase activity (AChE) was observed, while the activities of the antioxidant enzyme, Catalase (CAT) and Glutathione S-Transferase (GST) were significantly reduced showing also a strong positive correlation. The Integrated Biomarker Response index (IBRv2i) confirms the antioxidant system perturbation. No changes in reactive oxygen species (ROS) was observed across all treatments either in single and combined exposures, suggesting that the reduced CAT and GST activities likely result from direct contaminant-enzyme interactions rather than oxidative stress. Histological analysis revealed no tissue alterations upon single exposure while significant structural changes were observed upon their combination. This inconsistency - no synergy for biochemical endpoints, yet synergy for gonadal damage exclusively in the combined group - suggests they operate through distinct mechanisms. These findings highlight the increased ecological risk from contaminant mixtures.