Kinetics of Biphenyl and Polychlorinated Biphenyl Metabolism in Soil

The metabolism of ¹⁴C-labeled PCBs (polychlorinated biphenyls), which comprised the Aroclor 1242 mixture, was greatly enhanced by the addition of biphenyl (BP) to soil. After 49 days, only 25 to 35% of the original PCBs remained in the soil, and 48 to 49% was converted to ¹⁴CO₂ (including soil carbonates) in treatments enriched with BP; by contrast, 92% of the PCBs remained and less than 2% was converted to ¹⁴CO₂ in the unenriched control. Although the mineralization of PCBs in soils inoculated with Acinetobacter strain P6 was not greater than that in uninoculated BP-enriched soils, the initial and maximum mineralization rates and the disappearance of more highly chlorinated PCBs were greater with Acinetobacter strain P6. The mineralization of BP was consistent with kinetic models based upon linear-no growth and exponential growth; lower cell densities (<10⁶/g) of BP-oxidizing bacteria gave a better fit for exponential growth, whereas the highest cell density (10⁹/g) gave a better fit for linear-no growth. The numbers of BP-oxidizing bacteria declined exponentially upon depletion of the substrate. Since the mineralization of the chlorinated cometabolites was brought about by microorganisms (commensals) other than BP oxidizers, ¹⁴CO₂ production could not be fit to either of the two growth models. However, ¹⁴CO₂ production from the highest-density inoculum could be fit to a first-order (no-growth) sequential-reaction series. Although the population dynamics of the commensals could not be determined, the rate-limiting step in the cometabolic-commensal metabolism of PCBs to CO₂ had to be the initial oxidation, since the rate of ¹⁴CO₂ production was directly related to the population density of BP oxidizers.