POPULATION-DYNAMICS OF POLYCHLORINATED BIPHENYL-DECHLORINATING MICROORGANISMS IN CONTAMINATED SEDIMENTS

The growth dynamics of polychlorinated biphenyl (PCB)-dechlorinating m icroorganisms were determined for the first time, along with those of sulfate reducers and methanogens, by using the most-probable-number te chnique, The time course of Aroclor 1248 dechlorination mirrored the g rowth of dechlorinators; dechlorination ensued when the dechlorinating population increased by 2 orders of magnitude from 2.5 x 10(5) to 4.6 x 10(7) cells g of sediment(-1), at a specific growth rate of 6.7 day (-1) between 2 and 6 weeks, During this period, PCB-dechlorinating mic roorganisms dechlorinated Aroclor 1248 at a rate of 3.9 x 10(-8) mol o f Cl g of sediment(-1) day(-1), reducing the average number of Cl mole cules per biphenyl from 3.9 to 2.8, The growth yield was 4.2 x 10(13) cells mol of Cl dechlorinated(-1). Once dechlorination reached a plate au, after 6 weeks, the number of dechlorinators began to decrease, On the other hand, dechlorinators inoculated into PCB-free sediments decr eased over time from their initial level, suggesting that PCBs are req uired for their selective enrichment, The numbers of sulfate reducers and methanogens increased in both PCB-free and contaminated sediments, showing little difference between them, The maximum population size o f sulfate reducers was about an order of magnitude higher than that of dechlorinators, whereas that of methanogens was slightly less, Unlike those of dechlorinators, however, numbers of both sulfate reducers an d methanogens remained high even when dechlorination ceased, The resul ts of this study imply that PCB concentrations may have to exceed a ce rtain threshold to maintain the growth of PCB dechlorinators.