Bioavailability quantification and uptake mechanisms of pyrene associated with different-sized microplastics to Daphnia magna.

Microplastics (MPs) are the significant environmental factor for bioavailability of hydrophobic organic contaminants (HOCs) in aquatic environments. Nevertheless, the bioavailability of microplastic-associated HOCs remains unclear. In this research, the freely dissolved pyrene concentrations were kept stable with passive dosing devices, and the pyrene content in D. magna tissues as well as D. magna immobilization were analyzed to quantify bioavailability of pyrene (a representative HOC) associated with naturally-aged polystyrene (PS) MPs. Furthermore, the uptake mechanisms of pyrene associated with MPs of different sizes were explored by investigating the distribution of MPs in D. magna tissues with scanning electron microscopy. Especially, a new schematic model of bioavailability process was established. The results demonstrated that a part of pyrene associated with 0-1.5 μm MPs could directly cross cell membrane through endocytosis from intestine and exposure solutions to D. magna tissues except the 10-60 and 60-230 μm MPs. The bioavailability of microplastic-associated pyrene was ordered as 0-1.5 μm (20.0-21.6%) > 10-60 μm (10.7-13.8%) > 60-230 μm MPs (6.0-9.8%), which were essentially resulted from the difference in uptake mechanisms of pyrene associated with MPs of different sizes. This work suggests that the bioavailability of microplastic-associated HOCs should be considered when assessing water quality and environmental risk of HOCs in natural waters.