Heteroaggregation between polystyrene micro(nano)plastics and dibutyl phthalate mediates toxicity reduction in macroalgae: Physiological and transcriptomic evidence from Gracilariopsis lemaneiformis.

Although micro(nano)plastics (MNPs) and phthalate esters (PAEs) ubiquitously co-occur in marine environments, their interactions and combined toxicity on macroalgae are poorly characterized. This study investigated the toxic effects and underlying mechanisms of individual and combined exposure to polystyrene (PS) particles (SPS, 80 nm; LPS, 5 µm) and dibutyl phthalate (DBP) on Gracilariopsis lemaneiformis. Results showed that DBP significantly inhibited algal relative growth rate (RGR) and photochemical efficiency while disrupting redox homeostasis. Both SPS and LPS adsorbed onto the surface of G. lemaneiformis, but they did not affect RGR. Combined experimental characterization and molecular dynamics simulations unveiled that DBP adsorption onto PS accelerated heteroaggregation, leading to decreased DBP and PS bioavailability. Consequently, DBP + PS co-exposure reduced PS particles adsorption on algal surface and alleviated the adverse effects of DBP on G. lemaneiformis growth. Transcriptomic analysis indicated that DBP+PS co-exposure and DBP alone similarly downregulated the key metabolic pathway of G. lemaneiformis, including photosynthesis, carbohydrate metabolism, oxidative phosphorylation, and nitrogen metabolism. Single exposure to SPS or LPS suppressed photosynthetic and carbon fixation gene expression while upregulating energy metabolic pathways. This study can provide a molecular-scale perspectives for assessing the combined ecological risks of MNPs and PAEs in the marine environment.