Determination of adsorbed and absorbed polychlorinated biphenyls (PCBs) in seawater microorganisms

To distinguish between adsorbed and absorbed PCBs in seawater microorganisms, 2,2′,4,4′,5,5′-hexachlorobiphenyl (IUPAC #153; HCB) was added to a pure culture of Chrysochromulina apheles Moestrup et H.A. Thomsen. After the addition of the HCB, the cells were immediately harvested onto 2 μm polycarbonate filters and rinsed with a gradient of ethanol concentrations. Rinsing with 40% ethanol (v/v) was found to remove 80% of the HCB, which was loosely adsorbed to the cell surfaces, but did not extract the interior of the cells, as tested by chlorophyll a analysis. This method was used in a time course experiment which estimated PCBs adsorption and absorption to different groups of plankton organisms. Three different 14C-PCBs, 4-chlorobiphenyl (IUPAC #3; MCB), 2,2′,5,5′-tetrachlorobiphenyl (IUPAC #52; TCB), and 2,2′,4,4′,5,5′-hexachlorobiphenyl (IUPAC #153, HCB), were incubated in seawater from the northern Baltic Sea during a spring bloom. Samples were taken every third day and separated by filtration into three fractions; 0.2–2 μm (bacteria), 2–10 μm (flagellates), and >10 μm (microplankton; phytoplankton and protozoa). Two subsamples were retained from each size fraction. One of the subsamples was left untreated, to obtain adsorbed plus absorbed PCB, while the other subsample was rinsed with 40% of ethanol, to obtain the absorbed PCB. The sorption was found to vary depending on the hydrophobicity of the compounds, the structure of the cell membranes, and the lipid content and composition of the cells. The absorption increased for the TCB and the HCB in the largest size fraction over time, which coincided with an increase of the neutral and non-polar lipids.