Integrated physiological and transcriptomic evidence for enhanced phenanthrene toxicity under microplastics...

Microplastics (MPs) are increasingly recognized as vectors for hydrophobic organic contaminants, yet whether they enhance the toxicity and intracellular bioavailability of Phenanthrene (Phe) in marine microalgae remains unclear. Here, Isochrysis galbana was exposed to Phe (5 and 30 µg L^-1), polystyrene MPs (2000 µg L^-1), and their combination for 12 days. Co-exposure significantly intensified toxicity compared with single treatments. Cell density decreased by more than 50%, and chlorophyll a, chlorophyll c, and carotenoids declined by 17.0%, 24.7%, and 31.2%, respectively. Photosynthetic efficiency was markedly impaired, with F_v/F_m and qN declining by 24.7% and 75.0%, respectively. Early-stage membrane damage was evident from a 126% increase in PI signal, accompanied by elevated esterase activity (up to 2.07-fold). Transmission electron microscopy revealed severe chloroplast deformation and thylakoid disorganization. Chemical analysis showed significantly higher intracellular Phe concentrations under co-exposure, confirming enhanced bioavailability and uptake. Transcriptomic analysis further indicated coordinated suppression of photosynthesis, oxidative phosphorylation, and protein synthesis, alongside activation of lipid remodeling, DNA repair, and detoxification pathways. These results demonstrate that MPs enhance Phe accumulation and toxicity via vector-mediated processes, driving metabolic reprogramming from growth to stress adaptation. Such interactions may impair primary productivity and amplify ecological risks in marine ecosystems.