Microbial Dechlorination of 2,3,5,6-Tetrachlorobiphenyl under Anaerobic Conditions in the Absence of Soil or Sediment

Bacterial enrichment cultures developed with Baltimore Harbor (BH) sediments were found to reductively dechlorinate 2,3,5,6-tetrachlorobiphenyl (2,3,5,6-CB) when incubated in a minimal estuarine medium containing short-chain fatty acids under anaerobic conditions with and without the addition of sediment. Primary enrichment cultures formed both meta and ortho dechlorination products from 2,3,5,6-CB. The lag time preceding dechlorination decreased from 30 to less than 20 days as the cultures were sequentially transferred into estuarine medium containing dried, sterile BH sediment. In addition, only ortho dechlorination was observed following transfer of the cultures. Sequential transfer into medium without added sediment also resulted in the development of a strict ortho-dechlorinating culture following a lag of more than 100 days. Upon further transfer into the minimal medium without sediment, the lag time decreased to less than 50 days. At this stage all cultures, regardless of the presence of sediment, would produce 2,3,5-CB and 3,5-CB from 2,3,5,6-CB. The strict ortho-dechlorinating activity in the sediment-free cultures has remained stable for more than 1 year through several transfers. These results reveal that the classical microbial enrichment technique using a minimal medium with a single polychlorinated biphenyl (PCB) congener selected for ortho dechlorination of 2,3,5,6-CB. Furthermore, this is the first report of sustained anaerobic PCB dechlorination in the complete absence of soil or sediment.Anaerobic dechlorination of polychlorinated biphenyls (PCBs) has been demonstrated in situ and with laboratory microcosms containing sediment (reviewed in reference 1a). However, sustained PCB dechlorination has never been shown to occur in the absence of soil or sediments. Morris et al. (6) demonstrated a sediment requirement for the stimulation of PCB dechlorination within freshwater sediment slurries. Wu and Wiegel have recently described PCB-dechlorinating enrichments which required soil for the successful transfer of PCB-dechlorinating activity (9). In addition, no anaerobic microorganisms that dechlorinate PCBs have been isolated or characterized, and this may be due in part to the soil or sediment requirement. The inability to isolate dechlorinating organisms or maintain dechlorination without sediment has limited biogeochemical and physiological investigations into the mechanisms of PCB dechlorination.Dechlorination (ortho, meta, and para) of single PCB congeners has been observed following anaerobic incubation of Baltimore Harbor (BH) sediment under estuarine or marine conditions (2). While sediments from several sites within BH are contaminated with PCBs (1, 5), background contamination of sediment is not necessarily a prerequisite for the development of PCB dechlorination in laboratory microcosms. Wu et al. (8) recently demonstrated meta and ortho dechlorination of Aroclor 1260 when it was added to the same BH sediments. These results showed that more than one dechlorinating activity could be developed with these sediments. It has been proposed that discrete microbial populations are responsible for specific PCB dechlorinations (1a). Consistent with this idea, the ortho dechlorination observed with BH sediments may be catalyzed by discrete microbial populations. In addition, these organisms may be able to couple PCB dechlorination with growth. Therefore we have attempted to select for ortho PCB-dechlorinating organisms by enrichment under minimal conditions with high levels of 2,3,5,6-tetrachlorobiphenyl. We also speculated that given the proper conditions, a PCB-dechlorinating population could be maintained in an actively dechlorinating state in the absence of sediment. Here we report that a distinct PCB-dechlorinating activity, namely, ortho dechlorination, was selected for through sequential transfer initiated with sediments from BH and sustained in the absence of soil or sediment. This is the first report of sustained anaerobic PCB-dechlorinating activity in the total absence of sediment.