Reductive dechlorination of polychlorinated biphenyls: Interactions of dechlorinating microorganisms with methanogens and sulfate reducers

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 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.

Microbial reductive dechlorination of pre-existing PCBs and spiked 2,3,4,5,6-pentachlorobiphenyl in anaerobic slurries of a contaminated sediment of Venice Lagoon (Italy)

“…2). A release of Br − was observed in these microcosms, and, on the basis of previous findings [18,24–26,29], suggests that BES inhibited PCB dechlorination by acting as the preferential electron acceptor the specialized of bacteria.…”

“…To determine the indigenous bacteria potentially involved in the sediment PCB transformation, BES, molybdate, and antibiotics were separately and jointly added to different sets of microcosms to inhibit specific indigenous microbial populations, thus indirectly determining their individual role in the process. BES and molybdate were separately applied to determine the role of methanogens and sulfate (SO 4 2− )‐reducing bacteria, respectively, in the process [3,18,23–25]. However, BES has also been proven to inhibit PCB dechlorination, as it can be used as preferential electron acceptor by the dechlorinating microorganisms [18,24,25].…”

“…BES and molybdate were separately applied to determine the role of methanogens and sulfate (SO 4 2− )‐reducing bacteria, respectively, in the process [3,18,23–25]. However, BES has also been proven to inhibit PCB dechlorination, as it can be used as preferential electron acceptor by the dechlorinating microorganisms [18,24,25]. To determine whether BES could also act as a competitive electron acceptor, the concentration of Br − , which is one of the BES‐reductive dehalogenation products, was monitored in the BES‐amended microcosms.…”

“…PCB‐reductive dechlorination has sometimes been ascribed to microorganisms not belonging to sulfidogenic and methanogenic bacteria clusters [4,25,27–30]. To gain more information on the dechlorinating microorganisms occurring in the microcosms, penicillin G and D ‐cycloserine, generally regarded as selective inhibitors for eubacteria [26], were also employed.…”

Enhancement of microbial PCB dechlorination by chlorobenzoates, chlorophenols and chlorobenzenes

“…It may be possible to overcome this problem of recalcitrance if the level of chlorination can be reduced. Investigations in recent years have demonstrated the wide‐spread occurrence of microorganisms in anaerobic sediments that can remove chlorines from PCBs [3–8]. Dechlorination is mainly observed at meta and para positions and is determined by the pattern of chlorine substitution on the biphenyl ring [3].…”