Quantifying Bioavailability of Pyrene Associated with Dissolved Organic Matter of Various Molecular Weights to Daphnia magna

Dissolved organic matter (DOM) is a key environmental factor for the bioavailability of hydrophobic organic compounds (HOCs) in natural waters. However, the bioavailability of DOM-associated HOCs is not clear. In this research, pyrene was selected as a model HOC, and its freely dissolved concentration (C_free) was maintained by passive dosing systems. The immobilization and pyrene content in the tissues excluding gut of Daphnia magna were examined to quantify the bioavailability of DOM-associated pyrene. The results indicated that DOM promoted the bioavailability of pyrene when the C_free of pyrene was kept constant, and the bioavailability of pyrene associated with DOM of various molecular weights was ordered as middle molecular weight (5 000–10 000 Da) DOM > lower molecular weight (<1 000, 1 000–3 000, and 3 000–5 000 Da) DOM > higher molecular weight (>10 000 Da) DOM. The influencing mechanisms of DOM molecular weight were related with the partition of pyrene between DOM and water, the uptake routes of DOM by D. magna, and the desorption or release of pyrene from DOM in the gut of D. magna. The findings obtained in this research suggest that the bioavailability of DOM-associated HOCs should be taken into account for the eco-environmental risk assessment of HOCs in water systems.