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.

Vertical profiles of polychlorinated dioxins and microbial biomass including "Dehalococcoides" populations in cores of sediment of Lake Suwa, Japan, were investigated. The core samples were analyzed in 3-cm intervals at 0-15 cm and 50 cm, where a sharp gradient of Eh from 5 to −110 mV with depth occurred. The concentration of polychlorinated dioxins was relatively constant at 0-15 cm, ranging from 7.6 to 8.3 ng (7.0-9.2 pg-TEQ [toxic equivalent]) g −1 dry wt, but decreased sharply at 50 cm. The total bacterial count was in the order of 10 8 to 10 9 g −1 dry wt, being highest at 3-6 cm and decreasing in the deeper sediment. A similar vertical profile was found for respiratory quinones with larger amounts of ubiquinones than menaquinones at 0-15 cm. Quantitative real-time PCR with a specific primer set showed that "Dehalococcoides" and its phylogenetic relatives occurred in the order of 10 4 g −1 (dry wt) at 0-12 cm but were absent at 50 cm. The amplified clones showed 91-100% similarity (mostly <94%) in sequence to a well-known dioxin-dechlorinating organism, "Dehalococcoides" sp. strain CBDB1. These results suggested that the surface sediment up to a depth of 12 cm provided favorable conditions for the growth and activity of both aerobic and anaerobic microorganisms including "Dehalococcoides". It is likely that a wide variety of "Dehalococcoides" and phylogenetic relatives thereof are omnipresent even in sediment with low levels of dioxins and play the primary role in dechlorinating organohalorides over a relatively wide range of Eh.

In order to obtain basic information toward the bioremediation of dioxin-polluted soil, microbial communities in farmland soils polluted with high concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were studied by quinone profiling as well as conventional microbiological methods. The concentration of PCDD/Fs in the polluted soils ranged from 36 to 4,980 pg toxicity equivalent quality (TEQ) g(-1) dry weight of soil. There was an inverse relationship between the levels of PCDD/Fs and microbial biomass as measured by direct cell counting and quinone profiling. The most abundant quinone type detected was either MK-6 or Q-10. In addition, MK-8, MK-8(H2), and MK-9(H8) were detected in significant amounts. Numerical analysis of quinone profiles showed that the heavily polluted soils (> or = 1,430 pg TEQ g(-1)) contained different community structures from lightly polluted soils (< or = 56 pg TEQ g(-1)). Cultivation of the microbial populations in the heavily polluted soils with dibenzofuran or 2-chlorodibenzofuran resulted in enrichment of Q-10-containing bacteria. When the heavily polluted soil was incubated in static bottles with autoclaved compost as an organic nutrient additive, the concentrations of PCDD/Fs in the soil were decreased by 22% after 3 months of incubation. These results indicate that dioxin pollution exerted a significant effect on microbial populations in soil in terms of quantity, quality, and activity. The in situ microbial populations in the dioxin-polluted soil were suggested to have a potential for the transformation of PCDD/Fs and oxidative degradation of the lower chlorinated ones thus produced.