2005 Volume 20 Issue 4 Pages 227-242
Semi-anaerobic microcosms containing different levels of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were constructed by seeding with different mass ratios of lake sediment and dioxin-contaminated soil and incubating with organic medium for 1 year. In all microcosms, PCDD/Fs were reduced as a first-order reaction with similar removal rate coefficients, and only trace amounts of less chlorinated congeners were produced as the intermediate and end products. This apparent complete dechlorination of PCDD/Fs seemed to be due to a combination of reductive dechlorination of PCDD/Fs and oxidative degradation of the dechlorinated products. Total cell counting, 16S rRNA gene clone library analyses and quinone profiling showed that the microcosms contained relatively constant total populations with members of the phyla Bacteroidetes, Firmicutes and Proteobacteria (especially "Deltaproteobacteria") as the major constituents, independent of pollution levels. Quantitative real-time PCR with a specific primer set showed that the population density of "Dehalococcoides" and its phylogenetic relatives was highly correlated with the concentration of PCDD/Fs present. Some "Dehalococcoides" strains were isolated from the microcosms by repeated enrichment with chloroaromatics as the terminal electron acceptor. However, these isolates did not match with the major "Dehalococcoides"-related clones directly PCR-amplified. The results of this study suggest that PCDD/Fs in natural environments under given conditions are transformed with similar half-reduction rates independent of their concentrations, and a wide variety of "Dehalococcoides"-related bacteria play the primary role in this process.
