Interaction of polychlorinated biphenyls (PCB) with natural microbial populations in freshwater environments

The interaction between polychlorinated biphenyls (PCB) and natural aquatic microbial populations was investigated with respect to the impact of PCB on microbial growth and heterotrophic activity as well as the biodegradative fate of PCB. PCB substrates employed in these studies included Aroclor 1254, (¹⁴C)PCB (54% chlorine by weight), 2-chlorobiphenyl, and 2,4'-dichlorobiphenyl. Hexane batch extraction and Tenax-GC column extraction of PCB were evaluated for use with PCB biodegradation assays. Biodegradation was assessed in terms of the environmental removal of PCB from natural waters, PCB metabolism and mineralization, and uptake of PCB by naturally occurring heterotrophic bacteria. Laboratory and in situ degradation studies demonstrated significant temporal and spatial differences in PCB biodegradation among samples collected seasonally from an oligotrophic reservoir. PCB biodegradation was accomplished primarily through equilibrium partitioning-cellular uptake of PCB and decomposition of at least the lesser chlorinated biphenylic residues, indicated by the accummlation of chlorobenzoic acid and chlorobenzoyl formic acid as well as a dechlorinated acidic residue. Terminal decomposition of PCB to carbon dioxide was not an important component of Aroclor biodegradation as determined by a mineralization assay developed for low water soluble compounds. In virtually all cases, PCB stimulated the growth of heterotrophic reservoir bacteria at substrate concentrations ranging from approximately 4 ug 1^-1 to 100 ug 1^-1. Growth stimulation was accompanied by PCB induced production of extra-cellular slime which may relate to alterations of microbial community structure and PCB accammodation by heterotrophic microorganisms. The overall short-term effects of Aroclor (100 µg 1^-1) on the glucose uptake and mineralization activity of natural microbial populations was insignificant; however, significant stimulation and inhibition was observed on a sample to sample basis.

Freshwater, microbial populations are capable of degrading PCB mixtures, although at exceedingly slow rates. The extent of degradation is governed by a variety of interacting envirormental parameters; however, the size of the heterotrophic bacterial population and the ambient PCB concentration are dominant variables. Biodecomposition in nature conforms to established biphenyl catabolic pathways and recalcitrant metabolites can be expected to accumulate in the environment at low concentrations. Higher chlorinated mixtures, however, are not extensively metabolized by natural microbial communities; therefore, bioaccumulation and cellular binding are the likely primary paths of biodegradation.