Bioavailability of Pyrene Associated with Different Types of Protein Compounds: Direct Evidence for Its Uptake by Daphnia magna

The protein-like dissolved organic matter (DOM) is ubiquitous in aquatic environments. However, the bioavailability of protein-like DOM-associated hydrophobic organic compounds (HOCs) is not well-understood, and in particular, the direct evidence of their uptake by organisms is scarce. In the present work, tryptone (2000 Da), bovine serum albumin (BSA; 66 000 Da), and phycocyanin (120 000 Da) were chosen as model protein-like DOM, which were labeled by commercial fluorescein (cy5) to investigate the uptake mechanisms of protein compound-associated pyrene (a typical HOC) by Daphnia magna. The pyrene concentration in the tissues except the gut and immobilization of D. magna were detected to calculate the bioavailable fraction of protein compound-associated pyrene when the freely dissolved pyrene concentration was controlled through passive dosing devices. The results demonstrated that the tryptone could permeate cellular membrane and directly enter the tissues of D. magna from the exposure solutions, whereas BSA and phycocyanin might indirectly enter the tissues from the gut. A part of pyrene associated with protein compounds was bioavailable to D. magna; the order of their bioavailable fractions was trypone (54.6–58.1%) > phycocyanin (21.6–32.8%) > BSA (17.7–26.8%). The difference was principally related to the uptake mechanisms of pyrene associated with different types of protein. This work suggests that the protein compound-associated HOCs should be considered to evaluate the bioavailability and eco-environmental hazard of HOCs in natural waters.