Quantifying Bioavailability of Pyrene Associated with Dissolved Organic Matter of Various Molecular Weights to <i>Daphnia magna</i>

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 (<i>C</i><sub>free</sub>) was maintained by passive dosing systems. The immobilization and pyrene content in the tissues excluding gut of <i>Daphnia magna</i> were examined to quantify the bioavailability of DOM-associated pyrene. The results indicated that DOM promoted the bioavailability of pyrene when the <i>C</i><sub>free</sub> 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 <i>D. magna</i>, and the desorption or release of pyrene from DOM in the gut of <i>D. magna</i>. 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.