Dechlorination of PCDD/FS and Carbon Stable Isotope Fractionation

Of PCDDs by Dehalococcoides Ethenogenes Strain 195

Donna E. Fennell, (1) Fang Liu,(1) Ivonne Nijenhuis (2) and Hans H. Richnow (2)

1) Rutgers, the State University of New Jersey, Department of Environmental Sciences, New Brunswick, NJ, 08901, USA, 732-932-8750, 732-932-8644, fennell@envsci.rutgers.edu, fangliu@eden.rutgers.edu

2) UFZ-Centre for Environmental Research Leipzig-Halle, Department of Isotope Biogeochemistry,  Leipzig, Germany, Tel+49 341 235 2351, Fax+49 341 235 2492, ivonne.nijenhuis@ufz.de, hans.richnow@ufz.de

The dehalorespiring bacterium, Dehalococcoides ethenogenes strain 195 grows with chlorinated ethenes and dechlorinates a variety of other chlorinated compounds, including polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). We have shown that D. ethenogenes strain 195 dechlorinates selected PCDD/F congeners including 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD), 1,2,3,4-tetrachlorodibenzofuran and 1,2,3,4,7,8-hexachlorodibenzofuran.  Dechlorination of the tetra-chlorinated ring proceeds through a lateral dechlorination (removal of a 2 or 3 position chlorine) followed by a peri dechlorination step (removal of a 1 or 4 position chlorine).  The dechlorination results in production of less toxic congeners.  During this study we also examined carbon stable isotope (¹³C/¹²C) fractionation of PCDDs during dechlorination by a mixed culture containing D. ethenogenes strain 195. The culture was inoculated into mineral medium in 160 mL serum bottles and spiked with 1,2,3,4-TeCDD or 1,2,3,4-TeCDD together with tetrachloroethene (PCE) as a co-substrate. 1,2,3,4-TeCDD was dechlorinated to 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TriCDD) and further to 1,3-dichlorodibenzo-p-dioxin (1,3-DCDD). The carbon stable isotope ratios of 1,2,3,4-TeCDD, 1,2,4-TriCDD and 1,3-DCDD were measured in samples taken throughout the incubation period. Gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) was used to determine the carbon stable isotope composition. The isotope ratio of 1,2,3,4-TeCDD was enriched from about -29.5 to -26.5‰ during dechlorination. Unexpectedly, 1,2,4-TriCDD obtained an isotope composition between -29 to -25.5‰ and thus the product was isotopically enriched compared to the substrate. Upon further dehalogenation, 1,3-DCDD was found with an isotope signature of -31 to -28 ‰. This product was depleted in ¹³C compared to the educt. Similar patterns in isotope signatures were observed in both treatments and from replicate bottles.  1,2,4-TriCDD had the highest ¹³C/¹²C isotope ratio, 1,3-DCDD had the lowest ¹³C/¹²C isotope ratio and the 1,2,3,4-TeCDD ¹³C/¹²C isotope ratio was somewhere between them.  Dehalococcoides are widely distributed in the environment.  Dechlorination carried out by these and closely related organisms may detoxify sediments.  Further studies are needed to determine if carbon stable isotope fractionation could be a useful tool for assessing PCDD dechlorination in the environment.