Phylogenetic Characterization of a Polychlorinated-Dioxin- Dechlorinating Microbial Community by Use of Microcosm Studies

Abstract

Microcosms capable of reductive dechlorination of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were constructed in glass bottles by seeding them with a polluted river sediment and incubating them anaerobically with an organic medium. All of the PCDD/F congeners detected were equally reduced without the accumulation of significant amounts of less-chlorinated congeners as the intermediate or end products. Alternatively, large amounts of catechol and salicylic acid were produced in the upper aqueous phase. Thus, the dechlorination of PCDD/Fs and the oxidative degradation of the dechlorinated products seemed to take place simultaneously in the microcosm. Denaturing gel gradient electrophoresis and clone library analyses of PCR-amplified 16S rRNA genes from the microcosm showed that members of the phyla Firmicutes, Proteobacteria, and Bacteroidetes predominated. A significant number of Chloroflexi clones were also detected. Quantitative real-time PCR with specific primer sets showed that the 16S rRNA genes of a putative dechlorinator, "Dehalococcoides," and its relatives accounted for 0.1% of the total rRNA gene copies of the microcosm. Most of the clones thus obtained formed a cluster distinct from the typical "Dehalococcoides" group. Quinone profiling indicated that ubiquinones accounted for 18 to 25% of the total quinone content, suggesting the coexistence and activity of ubiquinone-containing aerobic bacteria. These results suggest that the apparent complete dechlorination of PCDD/Fs found in the microcosm was due to a combination of the dechlorinating activity of the "Dehalococcoides"-like organisms and the oxidative degradation of the dechlorinated products by aerobic bacteria with aromatic hydrocarbon dioxygenases.

Characterization of Rhizobium naphthalenivorans sp. nov. with special emphasis on aromatic compound degradation and multilocus sequence analysis of housekeeping genes

“…of PCDD/Fs Yoshida et al, 2005). In these microcosms, all congeners of PCDD/Fs were totally reduced without signifi cant accumulation of fewer chlorinated congeners.…”

“…One of the possible explanations for this phenomenon was that anaerobic dehalorespiring bacteria and aerobic aromatic-hydrocarbon-degrading bacteria coexisted and played roles in reductive dechlorination of PCDD/Fs and oxidative degradation of fewer chlorinated and dechlorinated products, respectively. Indeed, the microcosms yielded high population densities of Dehalococcoides and its relatives within the phylum Chlorofl exi as potent dechlorinators and lower levels of aromatic-hydrocarbon-degrading bacteria, as revealed by culture-independent molecular approaches and cultivation methods Kaiya et al, 2012;Yoshida et al, 2005).…”

Genotypic responses of bacterial community structure to a mixture of wastewater-borne PAHs and PBDEs in constructed mangrove microcosms

“…Likewise, the mono-to penta-BDEs were found to be degraded by a variety of aerobic bacteria containing aromatic hydrocarbon dioxygenases, e.g., Sphingomonas sp. PH-07, Rhodococcus jostii RHA1, Burkholderia aenovorans LB400 and Pseudonocardia dioxanivorans CB1190 [14,53,54].…”

Fungal enzyme production and biodegradation of polychlorinated dibenzo-p-dioxins and dibenzofurans in contaminated sawmill soil

“…In nature, dehalogenase and dioxygenase enzymes might also work sequentially. Yoshida et al (2005) studied the degradation of chlorinated dioxins in polluted river sediments under anaerobic conditions. The best results were obtained at the surface of the sediment layer, because lower chlorinated congeners formed from reductive dechlorination were subsequently oxidized by aerobic bacteria.…”