Enrichment and properties of a 1,2,4-trichlorobenzene-dechlorinating methanogenic microbial consortium

Radiolytic degradation of 1,2,4-trichlorobenzene (TCB) in some organic solvents by gamma rays: The kinetic properties of complete dechlorination of TCB and its pathway

This study investigates the degradation of TCB in methanol, ethanol, hexane, and benzene solutions using gamma radiolysis. Kinetic properties of TCB dechlorination and its pathway are examined, with TCB selected as a representative chlorinated organic compound. Chromatograms of irradiated samples and mass spectra of liquid-phase products are presented. The change in concentration of TCB, dichlorobenzenes (DCB), chlorobenzene (MCB), and benzene with absorbed doses are observed. The radiation-chemical yield (G values) of TCB in the solvents are calculated as 1.83, 2.56, 1.93, and 1.84 100eV-1 in methanol, ethanol, hexane, and benzene solutions, respectively. 100 % degradation of TCB by gamma irradiation is found to be efficient in polar solvents but leads to a wide variety of byproducts in low polar solvents, particularly the formation of polychlorinated biphenyls in TCB + benzene solutions, making benzene an incompatible medium. The main dechlorination pathway of TCB involves the formation of 1,4-DCB, MCB, and benzene. Environmental Implication. The gamma irradiation of chlorinated organic compounds, focusing on TCB as a model compound, was investigated due to its status as a hazardous material for the environment and living organisms. TCB is a byproduct of the dechlorination of certain chlorinated pesticides listed under the Stockholm Convention's Persistent Organic Pollutants (POPs) list, which prohibits their production and use. Gamma irradiation was found to be an effective method for the degradation of chlorinated compounds, achieving 100 % degradation during irradiation. The study underscores the potential of gamma irradiation as a viable approach for the treatment of chlorinated compounds, particularly in addressing environmental and health concerns associated with TCB and related compounds.

Evaluation of carbon isotope fractionation during anaerobic reductive dehalogenation of chlorinated and brominated benzenes

Compound specific stable isotope analysis (CSIA) has been established as a useful tool to evaluate in situ biodegradation. Here, CSIA was used to determine microbial dehalogenation of chloro- and bromobenzenes in microcosms derived from Hackensack River sediments. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) was used to measure carbon isotope fractionation during reductive dehalogenation of hexachlorobenzene (HCB), pentachlorobenzene (PeCB), 1,2,3,5-tetrachlorobenzene (TeCB), 1,2,3,5-tetrabromobenzene (TeBB), and 1,3,5-tribromobenzene (TriBB). Strong evidence of isotope fractionation coupled to dehalogenation was not observed in the substrate, possibly due to the low solubilities of the highly halogenated benzene substrates and a dilution of the isotope signal. Nonetheless, we could measure a depletion of the δ¹³C value in the dichlorobenzene product during dechlorination of HCB, the sequential depletion and enrichment of δ¹³C value for trichlorobenzene in TeCB dechlorinating cultures, and the enrichment of δ¹³C during debromination of TriBB. This indicates that a measurable isotope fractionation occurred during reductive dehalogenation of highly halogenated chloro- and bromobenzenes in aquatic sediments. Thus, although more quantitative measurements will be needed, the data suggests that CSIA may have application for monitoring in situ microbial reductive dehalogenation of highly halogenated benzenes.

Dechlorination of three tetrachlorobenzene isomers by contaminated harbor sludge-derived enrichment cultures follows thermodynamically favorable reactions

Dechlorination patterns of three tetrachlorobenzene isomers, 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-TeCB, were studied in anoxic microcosms derived from contaminated harbor sludge. The removal of doubly, singly, and un-flanked chlorine atoms was noted in 1,2,3,4- and 1,2,3,5-TeCB fed microcosms, whereas only singly flanked chlorine was removed in 1,2,4,5-TeCB microcosms. The thermodynamically more favorable reactions were selectively followed by the enriched cultures with di- and/or mono-chlorobenzene as the main end products of the reductive dechlorination of all three isomers. Based on quantitative PCR analysis targeting 16S rRNA genes of known organohalide-respiring bacteria, the growth of Dehalococcoides was found to be associated with the reductive dechlorination of all three isomers, while growth of Dehalobacter, another known TeCB dechlorinator, was only observed in one 1,2,3,5-TeCB enriched microcosm among biological triplicates. Numbers of Desulfitobacterium and Geobacter as facultative dechlorinators were rather stable suggesting that they were not (directly) involved in the observed TeCB dechlorination. Bacterial community profiling suggested bacteria belonging to the phylum Bacteroidetes and the order Clostridiales as well as sulfate-reducing members of the class Deltaproteobacteria as putative stimulating guilds that provide electron donor and/or organic cofactors to fastidious dechlorinators. Our results provide a better understanding of thermodynamically preferred TeCB dechlorinating pathways in harbor environments and microbial guilds enriched and active in anoxic TeCB dechlorinating microcosms.

Reductive dehalogenation activity of indigenous microorganism in sediments of the Hackensack River, New Jersey

Organohalogen pollutants are of concern in many river and estuarine environments, such as the New York-New Jersey Harbor estuary and its tributaries. The Hackensack River is contaminated with various metals, hydrocarbons and halogenated organics, including polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins. In order to examine the potential for microbial reductive dechlorination by indigenous microorganisms, sediment samples were collected from five different estuarine locations along the Hackensack River. Hexachlorobenzene (HCB), hexabromobenzene (HBB), and pentachloroaniline (PCA) were selected as model organohalogen pollutants to assess anaerobic dehalogenating potential. Dechlorinating activity of HCB and PCA was observed in sediment microcosms for all sampling sites. HCB was dechlorinated via pentachlorobenzene (PeCB) and trichlorobenzene (TriCB) to dichlorobenzene (DCB). PCA was dechlorinated via tetrachloroaniline (TeCA), trichloroanilines (TriCA), and dichloroanilines (DCA) to monochloroaniline (MCA). No HBB debromination was observed over 12 months of incubation. However, with HCB as a co-substrate slow HBB debromination was observed with production of tetrabromobenzene (TeBB) and tribromobenzene (TriBB). Chloroflexi specific 16S rRNA gene PCR-DGGE followed by sequence analysis detected Dehalococcoides species in sediments of the freshwater location, but not in the estuarine site. Analysis targeting 12 putative reductive dehalogenase (rdh) genes showed that these were enriched concomitant with HCB or PCA dechlorination in freshwater sediment microcosms.

Polyphasic characterization of an anaerobic hexachlorobenzene-dechlorinating microbial consortium with a wide dechlorination spectrum for chlorobenzenes

An anaerobic consortium that was capable of reductively dechlorinating hexachlorobenzene (HCB) to benzene was enriched from contaminated sediment. The consortium was capable of dechlorinating all chlorobenzene isomers except 1,4-dichlorobenzene. Singly and doubly flanked chlorines, as well as unflanked meta-substituted chlorines, were dechlorinated, although doubly flanked chlorines were preferred. Formate, acetate and lactate (but not ethanol) could be utilized as optimum electron donors for reductive dechlorination. Alternative electron acceptors, including nitrate and sulfate, completely inhibited HCB degradation, whereas amorphous iron oxide (FeOOH) did not suppress dechlorination activity. No degradation was found in chloramphenicol-treated consortium; however, vancomycin, molybdate, and 2-bromoethanesulfonate did not inhibit HCB dechlorination. The results of inhibitory treatments suggested that the dechlorinators were non-sulfate-reducing gram-negative or vancomycin resistant gram-positive bacteria. In addition to physiological characterization, analyses of 16S rRNA gene library of the consortium and quantitative PCR of 16S rRNA genes suggested that Dehalococcoides sp. was involved in the reductive dechlorination of HCB, and Geobacter sp. may serve as a dechlorinating candidate.

Insoluble Fe-humic acid complex as a solid-phase electron mediator for microbial reductive dechlorination

We report that the insoluble Fe-HA complex, which was synthesized with both commercial Aldrich humic acid (HA) and natural HA, functions as a solid-phase electron mediator (EM) for the anaerobic microbial dechlorination of pentachlorophenol. Spectroscopic characterizations and sequential Fe extraction demonstrated that the Fe-HA complex was predominated with Na4P2O7-labile Fe (represented as the organically bound Fe fraction) and poorly ordered Fe fraction (the fraction left in the residue after the sequential extraction), which were associated with different possible binding processes with carboxylate and phenolic groups. The change in the electron-mediating activity caused by Fe extraction indicated that the electron-mediating function of the Fe-HA complex is attributable to the Na4P2O7-labile Fe fraction. The Fe-HA complex also accelerated the microbial reduction of Fe(III) oxide, which suggested the presence of multiple electron-mediating functions in the complex. The electron shuttle assay showed that the Fe-HA complex had an electron-accepting capacity of 0.82 mequiv g(-1) dry Fe-HA complex. The presence of redox-active moieties in the Fe-HA complex was verified by cyclic voltammetry analysis of the sample after electrical reduction, with a redox potential estimated at 0.02 V (vs a standard hydrogen electrode).

Dehalogenation of chlorobenzenes, dichlorotoluenes, and tetrachloroethene by three Dehalobacter spp

Three enrichment cultures containing Dehalobacter spp. were developed that dehalogenate each of the dichlorobenzene (DCB) isomers to monochlorobenzene (MCB), and the strains using 1,2-DCB (12DCB1) or 1,3-DCB (13DCB1) are now considered isolated, whereas the strain using 1,4-DCB (14DCB1) is considered highly enriched. In this study, we examined the dehalogenation capability of each strain to use chlorobenzenes with three or more chlorines, tetrachloroethene (PCE), or dichlorotoluene (DCT) isomers. Strain 12DCB1 preferentially dehalogenated singly flanked chlorines, but not doubly flanked or unflanked chlorines. It dehalogenated pentachlorobenzene to MCB with little buildup of intermediates. Strain 13DCB1, which could use either 1,3-DCB or 1,2-DCB, demonstrated the widest dehalogenation spectrum of electron acceptors tested, and dehalogenated every chlorobenzene isomer except 1,4-DCB. Notably, strain 13DCB1 dehalogenated the recalcitrant 1,3,5-trichlorobenzene isomer to MCB, and qPCR of 16S rRNA genes indicated that strain 13DCB1 grew. Strain 14DCB1 exhibited the narrowest range of substrate utilization, but was the only strain to dehalogenate para-substituted chlorines. Strains 12DCB1 and 13DCB1 dehalogenated PCE to cis-dichloroethene, and all strains dehalogenated 3,4-DCT to monochlorotoluene. These findings show that Dehalobacter spp., like Dehalococcoides spp., are versatile dehalogenators and should be considered when determining the fate of chlorinated organics at contaminated sites.

A humin-dependent Dehalobacter species is involved in reductive debromination of tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant on the market. It has been detected in various environmental samples, and a growing body of evidence has demonstrated its toxic effects on living organisms. In this study, we report the enrichment and phylogenetic identification of bacteria that debrominate TBBPA to bisphenol A in the presence of humin. Incubation experiments indicated that humin was required for this debromination activity. Of the five compounds examined for inclusion in the TBBPA-debrominating culture, formate was the optimal electron donor. A 16S rRNA gene library showed that the culture was dominated by three known dehalogenator genera: Dehalobacter, Geobacter, and Sulfurospirillum. Further investigation indicated that Dehalobacter was responsible for the debromination of TBBPA. PCR-denaturing gradient gel electrophoresis analysis showed that Dehalobacter grew in the culture by utilizing TBBPA. Moreover, the copy number of the Dehalobacter 16S rRNA genes increased by about two orders of magnitude in the cultures without the addition of TBBPA, whereas it increased by approximately four orders of magnitude when TBBPA was present. The incubation experiments showed that Dehalobacter was reliant on humin for its debromination activity, indicating a new type of metabolism in Dehalobacter that is linked to humin respiration.

Pathway-dependent isotope fractionation during aerobic and anaerobic degradation of monochlorobenzene and 1,2,4-trichlorobenzene

Stable carbon isotope fractionation is a valuable tool for monitoring natural attenuation and to establish the fate of groundwater contaminants. In this study, we measured carbon isotope fractionation during aerobic and anaerobic degradation of two chlorinated benzenes: monochlorobenzene (MCB) and 1,2,4-trichlorobenzene (1,2,4-TCB). MCB isotope fractionation was measured in anaerobic methanogenic microcosms, while 1,2,4-TCB isotope experiments were carried out in both aerobic and anaerobic microcosms. Large isotope fractionation was observed in both the anaerobic microcosm experiments. Enrichment factors (ε) for anaerobic reductive dechlorination of MCB and 1,2,4-TCB were -5.0‰ ± 0.2‰ and -3.0‰ ± 0.4‰, respectively. In contrast, no significant isotope fractionation was found during aerobic microbial degradation of 1,2,4-TCB. The cleavage of a C-Cl σ bond occurs during anaerobic reductive dechlorination of MCB and 1,2,4-TCB, while no σ bond cleavage is involved during aerobic degradation via dioxygenase. The difference in isotope fractionation for aerobic versus anaerobic biodegradation of MCB and 1,2,4-TCB can be explained by the difference in the initial step of aerobic versus anaerobic biodegradation pathways.

Bacterial cultures preferentially removing singly flanked chlorine substituents from chlorobenzenes

The wide though not ubiquitous distribution of chlorobenzene-dechlorinating bacteria in anaerobic sludge from German sewage plants is demonstrated. The model substrates 1,2,3- and 1,2,4-trichlorobenzene (TCB) were dechlorinated to dichlorobenzenes (DCBs) and monochlorobenzene (MCB) via distinct pathways. For easy visualization and differentiation of the pathways, a novel plotting method was developed. While many of the cultures showed a dechlorination pattern similar to that previously found for Dehalococcoides species, removing doubly flanked rather than singly flanked chlorine substituents from TCBs, some cultures formed 1,2-DCB from 1,2,3-TCB and/or 1,3-DCB from 1,2,4-TCB. Stable cultures preferentially catalyzing the removal of singly flanked chlorines were obtained by repeated subcultivation in sediment-free synthetic medium. This dechlorination pattern is potentially of great benefit for remediation as the accumulation of persistent intermediates such as 1,3,5-TCB from highly chlorinated compounds can be avoided. In addition, the cultures dechlorinated 1,3,5-TCB, pentachlorobenzene (PeCB), and hexachlorobenzene (HCB). Nested PCR demonstrated the presence of low numbers of Dehalococcoides species. However, the observed insensitivity of the dechlorinating bacteria in our cultures to oxygen and sensitivity to vancomycin is not in accordance with the reported properties of Dehalococcoides species, suggesting that other bacteria than Dehalococcoides catalyzed the removal of singly flanked chlorines from TCB.

Hexachlorobenzene dechlorination by indigenous sediment microorganisms

Indigenous microbes from the sediments, whether contaminated with hexachlorobenzene (HCB) or not, could dechlorinate HCB effectively without any acclimation and supplemental nourishment. Temperature seriously affected the HCB-dechlorination: within the measured 15-45 degrees C span, the optimum range was between 30 and 35 degrees C. Sulfate-reducing bacteria (SRB), denitrifiers, and acetogens might not be directly involved in the HCB dechlorination. However, the SRB retarded subsequent dechlorination of pentachlorobenzene to tetra- and trichlorobenzenes. Some vancomycin-resistant gram-positive bacteria and methanogens were most likely to be the HCB-dechlorinators. The dechlorination followed the Michaelis-Menten behavior with the k'(m) and K(HCB) between 0.45-0.73 mg L(-1)day(-1) and 3.2-17.2 mg L(-1), respectively. These findings suggest a potential HCB treatment and cleanup for wastewater and contaminated site.

Projection of landfill stabilization period by time series analysis of leachate quality and transformation trends of VOCs

The purpose of this study was to evaluate suitability of using the time series analysis for selected leachate quantity and quality parameters to forecast the duration of post closure period of a closed landfill. Selected leachate quality parameters (i.e., sodium, chloride, iron, bicarbonate, total dissolved solids (TDS), and ammonium as N) and volatile organic compounds (VOCs) (i.e., vinyl chloride, 1,4-dichlorobenzene, chlorobenzene, benzene, toluene, ethyl benzene, xylenes, total BTEX) were analyzed by the time series multiplicative decomposition model to estimate the projected levels of the parameters. These parameters were selected based on their detection levels and consistency of detection in leachate samples. In addition, VOCs detected in leachate and their chemical transformations were considered in view of the decomposition stage of the landfill. Projected leachate quality trends were analyzed and compared with the maximum contaminant level (MCL) for the respective parameters. Conditions that lead to specific trends (i.e., increasing, decreasing, or steady) and interactions of leachate quality parameters were evaluated. Decreasing trends were projected for leachate quantity, concentrations of sodium, chloride, TDS, ammonia as N, vinyl chloride, 1,4-dichlorobenzene, benzene, toluene, ethyl benzene, xylenes, and total BTEX. Increasing trends were projected for concentrations of iron, bicarbonate, and chlorobenzene. Anaerobic conditions in landfill provide favorable conditions for corrosion of iron resulting in higher concentrations over time. Bicarbonate formation as a byproduct of bacterial respiration during waste decomposition and the lime rock cap system of the landfill contribute to the increasing levels of bicarbonate in leachate. Chlorobenzene is produced during anaerobic biodegradation of 1,4-dichlorobenzene, hence, the increasing trend of chlorobenzene may be due to the declining trend of 1,4-dichlorobenzene. The time series multiplicative decomposition model in general provides an adequate forecast for future planning purposes for the parameters monitored in leachate. The model projections for 1,4-dichlorobenzene were relatively less accurate in comparison to the projections for vinyl chloride and chlorobenzene. Based on the trends observed, future monitoring needs for the selected leachate parameters were identified.

Dechlorination of 1,2,3- and 1,2,4-trichlorobenzene by the white-rot fungus Trametes versicolor

The degradation of 1,2,3-, 1,3,5- and 1,2,4-trichlorobenzene (TCB) by the white-rot fungus Trametes versicolor was studied. Time course experiments showed a degradation rate of 2.27 and 2.49 nmol d(-1)mg(-1) dry weight of biomass during the first 4d of incubation in cultures spiked with 6 mg L(-1) of 1,2,3- and 1,2,4-TCB, respectively. A high percent of degradation of 91.1% (1,2,3-TCB) and 79.6 (1,2,4-TCB) was obtained after 7d. However, T. versicolor was not able to degrade 1,3,5-TCB under the conditions tested. For a range of concentrations of 1,2,4-TCB between 6.5 and 23 mg L(-1), a complete dechlorination of the molecule was observed. Cytochrome P450 monooxygenase appears to be involve in the first step of 1,2,4-TCB degradation, as evidenced by marked inhibition of both dechlorination and degradation of 1,2,4-TCB in the presence of the known cyt P450 inhibitors 1-aminobenzotriazole and piperonyl butoxide. Four intermediates formed from 1,2,4-TCB degradation were detected the second day of incubation, which did not appear the seventh day: 2,3,5-trichloromuconate, its corresponding carboxymethylenebutenolide, 2- or 5-chloro-4-oxo-2-hexendioic acid and 2- or 5-chloro-5-hydroxy-4-oxo-2-pentenoic acid. Based on these results, a degradation pathway of 1,2,4-TCB through cyt P450 monooxygenase and epoxide hydrolase was proposed.

Reductive dehalogenation of dichlorobenzenes and monochlorobenzene to benzene in microcosms

Anaerobic microcosms were constructed using sediments from a historically chlorobenzene-contaminated site and were provided with yeast extract as an electron donor. In these methanogenic microcosms, all three isomers of dichlorobenzene (DCB) were reductively dehalogenated to monochlorobenzene (MCB) when added together or individually, with 1,2-DCB dehalogenation being the most rapid and 1,4-DCB the slowest. When nearly all of the DCBs were consumed, benzene was detected and its accumulation was concomitant with MCB disappearance. Small amounts of toluene were also detected along with benzene. Subsequent MCB doses were also converted to benzene, and benzene reached levels in excess of 5000 micromol/L in some microcosms. An initial DCB dose stimulated, and in some cases was necessary for, MCB dehalogenation. Subsequent doses of DCB or MCB were dehalogenated more rapidly than previous ones, consistent with a growth-related process. Addition of a ca. 4% inoculum from microcosms that had consumed DCBs or MCB stimulated DCB and MCB dehalogenation in fresh microcosms, also indicative of growth and suggests thatthe chlorobenzene-dehalogenating microorganisms in these microcosms are candidates for bioaugmentation at anaerobic DCB or MCB contaminated sites. These studies add to evidence that benzene production from chlorobenzenes needs to be considered when modeling processes at contaminated sites.

Microbial degradation of chlorinated benzenes

Chlorinated benzenes are important industrial intermediates and solvents. Their widespread use has resulted in broad distribution of these compounds in the environment. Chlorobenzenes (CBs) are subject to both aerobic and anaerobic metabolism. Under aerobic conditions, CBs with four or less chlorine groups are susceptible to oxidation by aerobic bacteria, including bacteria (Burkholderia, Pseudomonas, etc.) that grow on such compounds as the sole source of carbon and energy. Sound evidence for the mineralization of CBs has been provided based on stoichiometric release of chloride or mineralization of (14)C-labeled CBs to (14)CO(2). The degradative attack of CBs by these strains is initiated with dioxygenases eventually yielding chlorocatechols as intermediates in a pathway leading to CO(2) and chloride. Higher CBs are readily reductively dehalogenated to lower chlorinated benzenes in anaerobic environments. Halorespiring bacteria from the genus Dehalococcoides are implicated in this conversion. Lower chlorinated benzenes are less readily converted, and mono-chlorinated benzene is recalcitrant to biotransformation under anaerobic conditions.

Assessment of in situ biodegradation of monochlorobenzene in contaminated groundwater treated in a constructed wetland

The degradation of monochlorobenzene (MCB) was assessed in a constructed wetland treating MCB contaminated groundwater using a detailed geochemical characterisation, stable isotope composition analysis and in situ microcosm experiments. A correlation between ferrous iron mobilisation, decreasing MCB concentration and enrichment in carbon isotope composition was visible at increasing distance from the inflow point, indicating biodegradation of MCB in the wetland. Additionally, in situ microcosm systems loaded with 13C-labelled MCB were deployed for the first time in sediments to investigate the biotransformation of MCB. Incorporation of 13C-labelled carbon derived from the MCB into bacterial fatty acids substantiated in situ degradation of MCB. The detection of 13C-labelled benzene indicated reductive dehalogenation of MCB. This integrated approach indicated the natural attenuation of the MCB in a wetland system. Further investigations are required to document and optimise the in situ biodegradation of MCB in constructed and natural wetland systems treating contaminated groundwater.

1,2,4-trichlorobenzene marine risk assessment with special emphasis on the Osparcom region North Sea

A risk assessment on 1,2,4-trichlorobenzene was carried out specifically for the marine environment according to the methodology laid down in the EU Risk Assessment Regulation 1488/94 and the Guidance Documents of the EU Existing Substances Regulation 793/93. The study consists of the collection and evaluation of data on effects and environmental concentrations from analytical monitoring programs in large rivers and estuaries in the North Sea area. The risk is indicated by comparing the predicted environmental concentration (PEC) with the predicted no-effect concentrations (PNEC) for the marine aquatic environment. A PNECwater) value of 0.3 microg/l and a PNECsed value of 38 microg/kgdw were derived from the results of toxicological studies in organisms representing three trophic levels, i.e. aquatic plants, invertebrates and fish. Based on monitoring data two situations are distinguished: a typical case and a worst case with a PECwater of <0.047 and 0.1 microg/l, respectively, and a PECsed of 40 and 90 microg/kgdw, respectively. The calculated PEC/PNEC ratios were 0.16 and 0.3 for water and 1 and 2.4 for sediment, respectively. It was concluded that no risks are expected for aquatic organisms. Based on the combination of worst-case assumptions risks to benthic organisms could not be fully excluded, but since all open uses of 1,2,4-trichlorobenzene will be ended following the EU risk assessment outcome of 2001 any potential risk is expected to be reduced accordingly. 1,2,4-trichlorobenzene is not considered toxic according to the EU criteria and the available data on persistence of 1,2,4-trichlorobenzene indicate a half-life in water of a few days and a significant biodegradation potential. The bioaccumulation potential is low to moderate with most BCF ratios for fish ranging from 600 to 1400 and one highest of 2020. Based on an extensive evaluation of persistence, biodegradation and bioaccumulation data it is concluded that 1,2,4-trichlorobenzene is not a PBT, since it does not fulfill any of the EU criteria. Biomagnification in the food chain is not expected due to the relatively high elimination rate constants.

Carbon/electron source dependence of polychlorinated biphenyl dechlorination pathways for anaerobic granules

The effect of acclimating anaerobic granules from commercial bioreactors with different carbon/electron sources on their ability to reductively dechlorinate a tri-(2,3,4-CB) and heptachlorobiphenyl (2,2',3,3',4,5,6-CB) was studied. The anaerobic granules were first grown in upflow anaerobic sludge blanket (UASB) reactors fed with two different mixtures of carbon/electron sources, i.e., propionate/butyrate/methanol and formate/methanol. Differences in dechlorination patterns for 2,2',3,3',4,5,6-CB were observed in batch experiments inoculated with granules from these two sets of UASB reactors. Variation of the carbon/electron source, during the dechlorination process, had no effect on the dechlorination pathway, but the extents and rates of dechlorination were highest for ethanol and formate and lowest for pyruvate fed batches. Pre-acclimation of different anaerobic sludges to polychlorinated biphenyls (PCBs) shortened the lag period, but did not influence the PCB dechlorination pathway. This is the first time that similar acclimation conditions for several anaerobic microbial communities prior to inoculation were reported to yield similar substrate specificities for the reductive dechlorination of specific PCB congeners. This research demonstrates a successful strategy for the development of biocatalysts to serve as the inoculum of partially decontaminated sites in order to provide microorganisms with specificities complementary to those of naturally occurring dechlorinators.

Stimulation of reductive dechlorination of hexachlorobenzene in soil by inducing the native microbial activity

The reductive dechlorination and behaviour of (14)C-hexachlorobenzene (HCB) was investigated in an arable soil. The activity of the native anaerobic microbial communities could be induced by saturating the soil with water. Under these conditions high rates of dechlorination were observed. After 20 weeks of incubation only 1% of the applied 14C-HCB could be detected in the fraction of extractable residues. Additional organic substances, like wheat straw and lucerne straw, however considerably delayed and reduced the dechlorination process in the soil. The decline of HCB was not only caused by dechlorination but also by the formation of non-extractable residues, whereby their amounts varied with time depending on the experimental conditions. Several dechlorination products were detected, indicating the following main HCB transformation pathway: HCB --> PCB --> 1,2,3,5-TeCB --> 1,3,5-TCB --> 1,3-DCB, with 1,3,5-TCB as main intermediate dechlorination product. The other TeCB-, TCB- and DCB-isomers were also detected in low amounts, showing the presence of more than one dechlorination pathway. Since the methane production rates were lowest when the dechlorination rates were highest, it can be assumed that methanogenic bacteria were not involved in the dechlorination process of HCB. The established 14C-mass balances show, that with increasing dechlorination and incubation times, the 14C-recoveries decreased.

Microbial transformation of chlorinated benzenes under anaerobic conditions

Chlorobenzenes are reductively dechlorinated by anaerobic bacterial cultures obtained from sediments and sludge. Recently a strain was isolated that couples reductive dechlorination of chlorobenzenes with energy conservation. The results reviewed in this article suggest that additional anaerobic bacteria, thriving by dehalogenation of chlorobenzenes or chlorobiphenylic compounds, can be isolated.

Identification of a microorganism that links its growth to the reductive dechlorination of 2,3,5,6-chlorobiphenyl

Anaerobic bacteria reductively dechlorinate polychlorinated biphenyls (PCBs) in aquatic sediments, but these microorganisms remain uncultured and, until now, unidentified. Through denaturing gradient gel electrophoresis (DGGE) of 16S rDNA from a highly enriched ortho-PCB dechlorinating culture, the growth of a single microorganism was shown to be dependent upon the presence and dechlorination of 2,3,5,6-tetrachlorobiphenyl. This is the first identification of a microorganism that catalyses the reductive dechlorination of a PCB. The organism, bacterium o-17, has high sequence similarity with the green non-sulphur bacteria and with a group that includes Dehalococcoides ethenogenes. Bacterium o-17 required acetate for dechlorination and growth. H2:CO2 (80:20 at 101 kPa) did not support dechlorination or growth of the dechlorinator. Archaeal 16S rDNA was not detected in actively dechlorinating bromoethanesulphonate-treated non-methanogenic cultures, which indicated that methanogenic Archaea were not required for dechlorination. The consistent association with dechlorinating activity combined with high similarity to other known dechlorinating microorganisms indicates that bacterium o-17 catalyses the reductive ortho-dechlorination of 2,3,5,6-tetrachlorobiphenyl.

Bacterial dehalorespiration with chlorinated benzenes

Chlorobenzenes are toxic, highly persistent and ubiquitously distributed environmental contaminants that accumulate in the food chain. The only known microbial transformation of 1,2,3,5-tetrachlorobenzene (TeCB) and higher chlorinated benzenes is the reductive dechlorination to lower chlorinated benzenes under anaerobic conditions observed with mixed bacterial cultures. The lower chlorinated benzenes can subsequently be mineralized by aerobic bacteria. Here we describe the isolation of the oxygen-sensitive strain CBDB1, a pure culture capable of reductive dechlorination of chlorobenzenes. Strain CBDB1 is a highly specialized bacterium that stoichiometrically dechlorinates 1,2,3-trichlorobenzene (TCB), 1,2,4-TCB, 1,2,3,4-TeCB, 1,2,3,5-TeCB and 1,2,4,5-TeCB to dichlorobenzenes or 1,3,5-TCB. The presence of chlorobenzene as an electron acceptor and hydrogen as an electron donor is essential for growth, and indicates that strain CBDB1 meets its energy needs by a dehalorespiratory process. According to their 16S rRNA gene sequences, strain CBDB1, Dehalococcoides ethenogenes and several uncultivated bacteria form a new bacterial cluster, of which strain CBDB1 is the first, so far, to thrive on a purely synthetic medium.

Phylogenetic analysis of an anaerobic, trichlorobenzene-transforming microbial consortium

A culture-independent phylogenetic survey for an anaerobic trichlorobenzene-transforming microbial community was carried out. Small-subunit rRNA genes were PCR amplified from community DNA by using primers specific for Bacteria or Euryarchaeota and were subsequently cloned. Application of a new hybridization-based screening approach revealed 51 bacterial clone families, one of which was closely related to dechlorinating Dehalobacter species. Several clone sequences clustered to rDNA sequences obtained from a molecular study of an anaerobic aquifer contaminated with hydrocarbons and chlorinated solvents (Dojka et al., Appl. Env. Microbiol. 64:3869-3877, 1998).

Physiological characterization of a bacterial consortium reductively dechlorinating 1,2,3- and 1,2,4-trichlorobenzene

A bacterial mixed culture reductively dechlorinating trichlorobenzenes was established in a defined, synthetic mineral medium without any complex additions and with pyruvate as the carbon and energy source. The culture was maintained over 39 consecutive transfers of small inocula into fresh media, enriching the dechlorinating activity. In situ probing with fluorescence-labeled rRNA-targeted oligonucleotide probes revealed that two major subpopulations within the microbial consortium were phylogenetically affiliated with a sublineage within the Desulfovibrionaceae and the gamma subclass of Proteobacteria. The bacterial consortium grew by fermentation of pyruvate, forming acetate, propionate, CO2, formate, and hydrogen. Acetate and propionate supported neither the reduction of trichlorobenzenes nor the reduction of sulfate when sulfate was present. Hydrogen and formate were used for sulfate reduction to sulfide. Sulfate strongly inhibited the reductive dechlorination of trichlorobenzenes. However, when sulfate was depleted in the medium due to sulfate reduction, dechlorination of trichlorobenzenes started. Similar results were obtained when sulfite was present in the cultures. Molybdate at a concentration of 1 mM strongly inhibited the dechlorination of trichlorobenzenes. Cultures supplied with molybdate plus sulfate did not reduce sulfate, but dechlorination of trichlorobenzenes occurred. Supplementation of electron-depleted cultures with various electron sources demonstrated that formate was used as a direct electron donor for reductive dechlorination, whereas hydrogen was not.