The growth dynamics of polychlorinated biphenyl (PCB)-dechlorinating microorganisms were determined for the first time, along with those of sulfate reducers and methanogens, by using the most-probable-number technique. The time course of Aroclor 1248 dechlorination mirrored the growth of dechlorinators; dechlorination ensued when the dechlorinating population increased by 2 orders of magnitude from 2.5 ؋ 10 5 to 4.6 ؋ 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 microorganisms dechlorinated Aroclor 1248 at a rate of 3.9 ؋ 10 ؊8 mol of Cl g of sediment ؊1 day ؊1 , reducing the average number of Cl molecules per biphenyl from 3.9 to 2.8. The growth yield was 4.2 ؋ 10 13 cells mol of Cl dechlorinated ؊1 . Once dechlorination reached a plateau, after 6 weeks, the number of dechlorinators began to decrease. On the other hand, dechlorinators inoculated into PCB-free sediments decreased over time from their initial level, suggesting that PCBs are required 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 of 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 and methanogens remained high even when dechlorination ceased. The results of this study imply that PCB concentrations may have to exceed a certain threshold to maintain the growth of PCB dechlorinators.

Abstract-The potential role of methanogens and sulfate reducers in polychlorinated biphenyl (PCB) dechlorination was investigated using the specific inhibitors 2-bromoethanesulfonate for methanogens and molybdate for sulfate reducers and the most probable number technique. Sediments were spiked with Aroclor 1248 (300 g/g). The inhibition of sulfate reducers did not have any effect on Aroclor 1248 dechlorination or on the size of dechlorinating populations, which indicates that they were not directly involved. On the other hand, the inhibition of methanogens reduced the rate and extent of dechlorination but did not affect the size of dechlorinating populations. These results indicate that there are at least two different dechlorinating populations-one of which requires the presence of methanogens for dechlorination and the other of which can dechlorinate independently. In the absence of methanogenesis, meta-substituted congeners, such as 25-2Ј5Ј-, 23-2Ј5Ј-, and 25-2Ј-chlorobiphenyls, were not dechlorinated. When both inhibitors were added, there was no dechlorination or growth of dechlorinating microorganisms, probably because of the growth inhibition by the high redox potential stemming from the absence of the two major respirators or because of the combined effects of inhibitors themselves.

The potential role of methanogens and sulfate reducers in polychlorinated biphenyl (PCB) dechlorination was investigated using the specific inhibitors 2‐bromoethanesulfonate for methanogens and molybdate for sulfate reducers and the most probable number technique. Sediments were spiked with Aroclor® 1248 (300 μg/g). The inhibition of sulfate reducers did not have any effect on Aroclor 1248 dechlorination or on the size of dechlorinating populations, which indicates that they were not directly involved. On the other hand, the inhibition of methanogens reduced the rate and extent of dechlorination but did not affect the size of dechlorinating populations. These results indicate that there are at least two different dechlorinating populations—one of which requires the presence of methanogens for dechlorination and the other of which can dechlorinate independently. In the absence of methanogenesis, meta‐substituted congeners, such as 25‐2′5′‐, 23‐2′5′‐, and 25‐2′‐chlorobiphenyls, were not dechlorinated. When both inhibitors were added, there was no dechlorination or growth of dechlorinating microorganisms, probably because of the growth inhibition by the high redox potential stemming from the absence of the two major respirators or because of the combined effects of inhibitors themselves.