Beyond carrier effects: Polyamide microplastics and TCPP jointly drive physiological toxicity in mussels at environmental concentrations

Marine microplastic pollution is a global problem that has received widespread attention, making the risk assessment of marine microplastics a crucial topic of concern. However, unscientific assessment methods have been widely used in the past, highlighting the necessity to recalibrate the judgment of the ecological risks posed by marine microplastics. In this study, we conducted exposure experiments on the filter-feeding shellfish, thick-shelled mussels (Mytilus coruscus), using ambient concentrations of marine microplastic PA debris compounded with the common organophosphorus flame retardants pollutant, Tris (1-chloro-2-propyl) phosphate (TCPP). The results of our experiments showed that the combined stress of TCPP and PA exhibited an adsorption effect on the physiological toxicity on thick-shell mussel at environmental concentrations rather than a carrier effect, and there was no significant difference in the toxic effects between the current environmental concentration and a predicted environmental concentration in 2050. However, microplastics and TCPP appeared to exert irreversible effects on the digestive glands of mussels, compared to their effects on the gill tissues. The diversity of bacterial populations in the mussels' internal environment was affected, indicating potential ecological consequences.