Effect of environmental conditions on polychlorinated biphenyl transformations and bacterial communities in a river sediment

The aim of this study was to evaluate polychlorinated biphenyl (PCB) removal in relation to the associated bacterial community composition in Ohio River sediments (USA) using field and laboratory approaches. PCB removal was studied in different sediment layers applying dialysis equilibrators in the field for 4 months. In parallel, dissolved chemical constituents and bacterial community composition using the 16S rDNA cloning–sequencing approach were determined within the appropriate sediment layers. The effects of several bioremediation treatments on PCB removal and bacterial community composition were evaluated in laboratory experiments. Sediments spiked with 245-2’4’5’ hexachlorobiphenyl were incubated under different conditions for up to 250 days, including alternating anaerobic–aerobic conditions, amendments with electron donors, FeSO4, and incubations at different temperatures (10°C, 25°C, 40°C). PCB levels remained stable in the sediment layers under field conditions. Surface sediments (0–26 cm) had elevated levels of nitrate+nitrite and sulfate, while subsurface sediments (27–35 cm) were characterized by high concentrations of ammonium, methane, and dissolved organic carbon. Oxic/anoxic conditions were also reflected in the bacterial community, with aerobic bacteria in the Bulkholderiales order enriched in the surface sediments, and anaerobic bacteria in the Clostridiales, Syntrophobacteriales, and Desulfobacterales orders more prevalent in the subsurface sediments. Although the chemical status was favorable and the potential degraders were present, PCB transformation did not take place under the prevailing environmental conditions. Under laboratory conditions, PCB transformation was positively influenced by lowered redox conditions with or without amendments with FeSO4, electron donors, or alternating anaerobic–aerobic conditions. The main PCB transformation process in all anaerobic treatments at 25°C was reductive dechlorination of 245-2’4’5’ hexachlorobiphenyl to 24-2’4’5’ pentachlorobiphenyl and 24-2’4’ tetrachlorobiphenyl, which occurred at rates between 60–85% in 177 days. Exposing sediments to air for an additional period of 100 days in the sequential anaerobic–aerobic treatment did not result in additional mineralization of PCBs. Temperature, electron donors, and redox conditions significantly affected the abundance of bacteria in the Bacteroidales, Hydrogenophilales, Pseudomonales, Myxococcales, Syntrophobacterales, Acidobacterales, Caldilineales, and Clostridiales orders. The PCB dechlorinator Dehalococcoides and other dechlorinating bacteria were detected in anaerobic sediments, including Desulfitobacterium, Desulfuromonas, Anaeromyxobacter, Geobacter, and Desulfomonile. The data suggested that shifts in the bacterial communities due to abiotic conditions affected PCB transformation, either directly (degraders) or indirectly (bacterial syntrophic associations with degraders). The data indicated that elevated redox status and low temperature conditions accounted for stable PCB concentrations under field conditions in Ohio River sediments. In laboratory experiments, PCB removal increased under anaerobic conditions at 25°C, but was not affected by other varying conditions. Depending on the environmental conditions, several bacterial groups involved in the stepwise degradation of natural organic matter to dechlorinating substrates acetate and hydrogen were prevalent in sediments, including Clostridiales, Caldilineales, Syntrophobacterales, Bacteroidales, and Acidobacteriales. Measurements of the prevailing bacterial community composition and chemical–physical properties important for PCB degradation could be valuable prior to developing PCB bioremediation strategies in sediments.