Bioaccumulation of 2,2′,5,5′-tetrachlorobiphenyl and pyrene by picoplankton (Synechococcus leopoliensis, cyanophyceae):: Influence of variable humic acid concentrations and pH

Natural dissolved organic matter (DOM) controls the aqueous phase partitioning of hydrophobic organic contaminants (HOCs), where the sum of aqueous HOCs is distributed between the bound and free forms of HOCs, that is [HOC-DOM] tt [HOC] + [DOM]. The hypothesis that the bioavailability of aqueous HOCs can be attributed solely to the concentration of the free form of HOCs was tested. Bioavailability was measured as accumulation of [C-14]-PCB (IUPAC 52) and [C-14]-pyrene over 48 h by the phytoplanktonic cyanobacterium Synechococcus leopoliensis. The phytoplankton were exposed within dialysis sacs to freely dissolved HOC at concentrations that were similar in all sacs (freely dissolved PCB similar to 125 ng/L, freely dissolved pyrene similar to 55 ng/L). However, humic acid concentrations were varied among the sacs (Suwannee River humic acid [HA] = 0, 2, 5, or 20 mg/L). The experiment was designed to vary the concentrations of HOC-HA and HA while keeping the concentration of freely dissolved HOCs at the same levels in all the sacs. Sorption mechanisms were probed by manipulating the pH of the exposure medium; the hydrophobicities of the cell surface and the humic acid are pH-sensitive, whereas the freely dissolved concentration of HOCs is pH-invariant. Bioaccumulation was predominantly controlled by the freely dissolved concentration of HOC and was greater at pH 4.3 than at pH 7.3. Although sorption of the PCB-HA and pyrene-HA complexes by S. leopoliensis occurred, this mechanism did not contribute significantly to the total accumulation of PCB or pyrene. These results suggest that the accumulation of HOCs by planktonic microorganisms can be predicted on the basis of the concentration of freely dissolved HOC molecules.