Importance of passive diffusion in the uptake of polychlorinated biphenylsby phagotrophic protozoa

Unicellular protozoan grazers represent a size class of organisms where a t ransition in the mechanism of chlorobiphenyl (CB) introduction, from diffus ion through surface membranes to ingestion of contaminated prey, could occu r. This study compares the relative importance of these two processes in th e overall uptake of polychlorinated biphenyls by protists. Uptake rates and steady-state concentrations were compared in laboratory cultures of grazin g and nongrazing protozoa. These experiments were conducted with a 10-mu m marine scuticociliate (Uronema sp.), bacterial prey (Halomonas halodurans), and a suite of 21 CB congeners spanning a range of aqueous solubilities. T he dominant pathway of CB uptake by both grazing and nongrazing protozoa wa s diffusion. Organic-carbon-normalized CB concentrations (in the protozoan cell) were equivalent in grazing and nongrazing protozoa for all congeners studied. Rate constants for uptake into and loss from the protozoan cell we re independently determined by using [3,3',4,4'-C-14]tetrachlorobiphenyl (I UPAC no. 77), 0.38 +/- 0.03 min(-1) and (1.1 +/- 0.1) x 10(-5) (g of organi c carbon)(-1) min(-1), respectively. Magnitudes of the uptake and loss proc esses were calculated and compared by using a numerical model. The model re sult was consistent with data from the bioaccumulation experiment and suppo rted the hypothesis that diffusive uptake is faster than ingestive uptake i n phagotrophic unicellular protozoa.