Carbon isotope fractionation during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin by a Dehalococcoides-containing culture

Carbon isotope fractionation was observed during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) by a mixed culture containing Dehalococcoides ethenogenes strain 195. Fractionation was examined when 1,2,3,4-TeCDD was added as the only chlorinated compound and when 1,2,3,4-TeCDD was added with a known growth substrate, tetrachloroethene (PCE). The 1,2,3,4-TeCDD was dechlorinated to 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD) which was enriched in 13C relative to 1,2,3,4-TeCDD with isotope separation factors, εC, of 1.3 ± 0.2‰ and 1.7 ± 0.4‰ (average ± 95% confidence interval (CI)) in cultures with and without PCE, respectively. The 1,2,4-TrCDD was further dechlorinated to 1,3-dichlorodibenzo-p-dioxin (1,3-DCDD) which was depleted in 13C relative to 1,2,4-TrCDD with εC of −2.4 ± 0.4‰ and −2.9 ± 0.8‰ (average ± 95% CI) in cultures with and without PCE, respectively. This demonstrates carbon isotope fractionation during sequential reductive dechlorination of PCDDs, where isotope fractionation during dechlorination of the intermediate was substantial and a 13C depleted lightly chlorinated PCDD congener was ultimately formed during dechlorination of more highly chlorinated PCDD congeners. Despite reproducible, statistically significant differences between isotope compositions of the parent, 1,2,3,4-TeCDD and daughter, 1,2,4-TrCDD and 1,3-DCDD congeners in triplicate bottles of both treatments, fractionation factors for 1,2,3,4-TeCDD could not be determined for all replicates by regression analysis of the plot of the Rayleigh equation. It is possible that dissolution of 1,2,3,4-TeCDD imposed a kinetic limitation on dechlorination, thus masking isotope fractionation during its dechlorination.