The effect of protozoan grazers on the cycling of polychlorinated biphenyls (PCBs) in marine systems

Abstract

Processes affecting organic carbon distribution and composition can control the speciation of organic contaminants such as polychlorinated biphenyls (PCBs) and ultimately determine their residence time in a particular environment. In marine systems, the microbial loop influences organic carbon dynamics by recycling a significant fraction of dissolved and particulate organic matter. The goal of this thesis was to understand how these recycling processes affect chlorobiphenyl (CB) cycling in marine systems by monitoring CB dynamics among organic carbon pools represented by dissolved organic matter, bacterial prey and phagotrophic protozoan grazers. Initially, I studied the extent to which a protozoan grazer (Uronema sp. - 10μm ciliate) equilibrated with aqueous PCBs within 2-3 hours. Initial calculations predicted rapid equilibration via passive diffusion. Experimentally, no difference in equilibration time was noted between grazing and non-grazing protozoa, indicating that diffusion was the primary uptake pathway for these organisms. The results were extended to determine the transition size of an organism where the rates of diffusive and ingested uptake are equivalent (100-500μm). Disassociation rate constants were estimated for complexes of CB congeners and dissolved organic carbon (DOC). CB-DOC complexes enhanced the diffusive uptake rate constant for Tenax resin and, by inference, protozoan grazers. In the second phase of this work, concentrations of surfactants, organic carbon and cells were monitored over time in protozoan cultures. The effects of bacterial growth substrate and protozoan species were examined. Surfactants increased during protozoan exponential growth while total DOC concentrations decreased. Production of surface-active material in ciliate cultures was significantly higher than in flagellate cultures, and all protozoan cultures were higher than the bacterial control. Common headspace vessels were then used to compare and contrast the affinity of protozoan and bacterial culture filtrates (<0.2μm) for PCBs relative to a seawater control. Affinities were normalized to bulk DOC and surfactant concentrations to determine underlying relationships among these parameters. Values of equilibrium partition coefficients (Koc) ranged from 1046 in Vineyard Sound seawater to 105.4 and 105.5 in protist cultures, indicating that "grazer-enhanced" DOM was a better sorbent for PCBs than DOM in bacterial controls and Vineyard Sound seawater.