Enhanced reductive dechlorination of polychlorinated biphenyl impacted sediment by bioaugmentation with a dehalorespiring bacterium

Abstract

Anaerobic reductive dehalogenation of commercial PCBs such as Aroclor 1260 has a critical role of transforming highly chlorinated congeners to less chlorinated congeners that are then susceptible to aerobic degradation. The efficacy of bioaugmentation with the dehalorespiring bacterium Dehalobium chlorocoercia DF1 was tested in 2-L laboratory mesocosms containing sediment contaminated with weathered Aroclor 1260 (1.3 ppm) from Baltimore Harbor, MD. Total penta- and higher chlorinated PCBs decreased by approximately 56% (by mass) in bioaugmented mesocosms after 120 days compared with no activity observed in unamended controls. Bioaugmentation with DF-1 enhanced the dechlorination of doubly flanked chlorines and stimulated the dechlorination of single flanked chlorines as a result of an apparent synergistic effect on the indigenous population. Addition of granulated activated carbon had a slight stimulatory effect indicating that anaerobic reductive dechlorination of PCBs at low concentrations was not inhibited by a high background of inorganic carbon that could affect bioavailability. The total number of dehalorespiring bacteria was reduced by approximately half after 60 days. However, a steady state level was maintained that was greater than the indigenous population of putative dehalorespiring bacteria in untreated sediments and DF1 was maintained within the indigenous population after 120 days. The results of this study demonstrate that bioaugmentation with dehalorespiring bacteria has a stimulatory effect on the dechlorination of weathered PCBs and supports the feasibility of using in situ bioaugmentation as an environmentally less invasive and lower cost alternate to dredging for treatment of PCB impacted sediments.

Figure 1

PCB analysis by homolog at day 0 (white bars) and day 120 (black bars) after treatment with filter sterilized spent growth medium (A), sterilized spent growth medium and GAC (B), concentrated DF1 in growth medium inoculated directly into the sediment (C) and concentrated DF1 adsorbed onto GAC (D). Each bar represents the mean and standard deviation of three replicates samples.

Figure 2

Changes in Cl per biphenyl in mesocosms over time after treatment with sterilized spent growth medium (○), sterilized spent growth medium and GAC (□),DF1 inoculated directly into sediment (●), and DF1 adsorbed onto GAC inoculated into sediment (■). Each datum point represents the mean and standard deviation of three replicates samples.

Figure 3

Congeners showing greatest change in mesocosms receiving both GAC and DF1 at day 0 (□) and day 120 (■). An asterisk (*) indicates congeners that could be dechlorination substrates of DF1, or products from double flanked dechlorination by DF-1. Each bar represents the mean and standard deviation of three replicates samples.

Figure 4

Enumeration of putative dechlorinating Chloroflexi normalized to 16S rRNA gene copies/gram sediment at day 0 (□), day 60 () and day 120 (■). Treatments included spent medium only, spent medium and GAC, amendment with DF-1 by direct injection, and amendment with DF-1 adsorbed to GAC. Each bar represents the mean and standard deviation of three replicates samples.

Figure 5

DHPLC community analysis of putative dechlorinating Chloroflexi 16S rRNA genes in mesocosms at day 0 (bottom trace) and day 120 (top trace). A, addition of spent growth media alone; B, addition of spent growth media and GAC; C, addition of DF1; D, addition of DF1and GAC. Peak eluting at about 6.3 minutes (labeled 8/DF1 with arrow) in C and D confirmed DF1 16S rRNA gene by sequencing. Other phylotypes of putative dechlorinating Chloroflexi labeled 1, 2, 3, 4, 5, 6, or 7 are described in Table S2.