Influence of Substituents on Reductive Dehalogenation of 3-Chlorobenzoate Analogs

Influence of organic carbon and metal oxide phases on sorption of 2,4,6-trichlorobenzoic acid under oxic and anoxic conditions

Chlorobenzoic acids represent crucial recalcitrant metabolites in the environment; thus, the influence of soil components on the sorption of 2,4,6-trichlorobenzoic acid (TCB) under oxic and anoxic conditions was studied. The surficial physiognomies of untreated and isolated soil samples were studied using FTIR, XRD, specific surface area, and PZC determination. The roles of redox potential, dissolved organic carbon (DOC), and pH, particularly under anoxic condition, were appraised. Batch equilibrium adsorption studies on soils of variable Fe/Mn oxides and organic carbon showed that adsorption was low across all components (log Koc = 0.82-3.10 Lg(-1)). The sorption of 2,4,6-TCB was well described by the pseudo second-order kinetic model. The fluctuation of both redox potential and pH during anoxic experiment had a negative impact on the sorption, partitioning, and the oxidation of organic matter. Linear relationships were observed for Kd with both soil total organic carbon (TOC) and surface area (SA). The results showed the existence of DOC-mediated sorption of 2,4,6-TCB which seems to be enhanced at lower pH. The reductive dissolution, particularly of iron compounds, possibly impeded sorption of 2,4,6-TCB under anoxic condition. It could be inferred that habitats dominated by fluctuating oxygen concentrations are best suited for the development of environmental conditions capable of mineralizing 2,4,6-TCB and similar xenobiotics.

Dechlorination of polychlorinated biphenyl-contaminated soil via anaerobic composting with pig manure

Anaerobic dechlorination is an effective degradation pathway of higher chlorinated polychlorinated biphenyls (PCBs). The efficiency of anaerobic composting remediation of PCB-contaminated soil using pig manure was determined. The results show that the dechlorination of PCB-contaminated soil via anaerobic composting with pig manure is feasible. PCB concentration is the most critical factor. Elevated PCB concentrations can inhibit dechlorination but does not disrupt the anaerobic fermentation process. At 1 mg kg(-1) PCBs, the degradation rate of five or more chlorinated biphenyls is 43.8%. The highest dechlorination performance in this experiment was obtained when the soil-to-organic waste ratio, carbon-to-nitrogen ratio, moisture content, and PCB concentration were 2:3, 20, 60%, and 1 mg kg(-1), respectively.

Tetrachloroethene-dehalogenating bacteria

Tetrachloroethene is a frequent groundwater contaminant often persisting in the subsurface environments. It is recalcitrant under aerobic conditions because it is in a highly oxidized state and is not readily susceptible to oxidation. Nevertheless, at least 15 organisms from different metabolic groups, viz. halorespirators (9), acetogens (2), methanogens (3) and facultative anaerobes (2), that are able to metabolize tetrachloroethene have been isolated as axenic cultures to-date. Some of these organisms couple dehalo-genation to energy conservation and utilize tetrachloroethene as the only source of energy while others dehalogenate tetrachloroethene fortuitously. Halorespiring organisms (halorespirators) utilize halogenated organic compounds as electron acceptors in an anaerobic respiratory process. Different organisms exhibit differences in the final products of tetrachloroethene dehalogenation, some strains convert tetrachloroethene to trichloroethene only, while others also carry out consecutive dehalogenation to dichloroethenes and vinyl chloride. Thus far, only a single organism, 'Dehalococcoides ethenogenes' strain 195, has been isolated which dechlorinates tetrachloroethene all the way down to ethylene. The majority of tetrachloroethene-dehalogenating organisms have been isolated only in the past few years and several of them, i.e., Dehalobacter restrictus, Desulfitobacterium dehalogenans, 'Dehalococcoides ethenogenes', 'Dehalospirillum multivorans', Desulfuromonas chloroethenica, and Desulfomonile tiedjei, are representatives of new taxonomic groups. This contribution summarizes the available information regarding the axenic cultures of the tetrachloroethene-dehalogenating bacteria. The present knowledge about the isolation of these organisms, their physiological characteristics, morphology, taxonomy and their ability to dechlorinate tetrachloroethene is presented to facilitate a comprehensive comparison.

Specificity of reductive dehalogenation of substituted ortho-chlorophenols by Desulfitobacterium dehalogenans JW/IU-DC1

Resting cells of Desulfitobacterium dehalogenans JW/IU-DC1 growth with pyruvate and 3-chloro-4-hydroxyphenylacetate (3-Cl-4-OHPA) as the electron acceptor and inducer of dehalogenation reductively ortho-dehalogenate pentachlorophenol (PCP); tetrachlorophenols (TeCPs); the trichlorophenols 2,3,4-TCP, 2,3,6-TCP, and 2,4,6-TCP; the dichlorophenols 2,3-DCP, 2,4-DCP, and 2,6-DCP; 2,6-dichloro-4-R-phenols (2,6-DCl-4-RPs, where R is -H, -F, -Cl, -NO2, -CO2, or -COOCH3; 2-chloro-4-R-phenols (2-Cl-4-RPs, where R is -H, -F, -Cl, -Br, -NO2, -CO2-, -CH2CO2, or -COOCH3); and the bromophenols 2-BrP, 2,6-DBrP, and 2-Br-4ClP [corrected]. Monochlorophenols, the dichlorophenols 2,5-DCP, 3,4-DCP, and 3,5-DCP, the trichlorophenols 2,3,5-TCP, 2,4,5-TCP, and 3,4,5-TCP, and the fluorinated analog of 3-Cl-4-OHPA, 3-F-4-OHPA ("2-F-4-CH2CO2- P"), are not dehalogenated. A chlorine substituent in position 3 (meta), 4 (para), or 6 (second ortho) of the phenolic moiety facilitates ortho dehalogenation in position 2. Chlorine in the 5 (second meta) position has a negative effect on the dehalogenation rate or even prevents dechlorination in the 2 position. In general, 2,6-DCl-4-RPs are dechlorinated faster than the corresponding 2-Cl-4-RPs with the same substituent R in the 4 position. The highest dechlorination rate, however, was found for dechlorination of 2,3-DCP, with a maximal observed first-order rate constant of 19.4 h-1 g (dry weight) of biomass-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial dehalogenases: biochemistry, genetics, and biotechnological applications

This review is a survey of bacterial dehalogenases that catalyze the cleavage of halogen substituents from haloaromatics, haloalkanes, haloalcohols, and haloalkanoic acids. Concerning the enzymatic cleavage of the carbon-halogen bond, seven mechanisms of dehalogenation are known, namely, reductive, oxygenolytic, hydrolytic, and thiolytic dehalogenation; intramolecular nucleophilic displacement; dehydrohalogenation; and hydration. Spontaneous dehalogenation reactions may occur as a result of chemical decomposition of unstable primary products of an unassociated enzyme reaction, and fortuitous dehalogenation can result from the action of broad-specificity enzymes converting halogenated analogs of their natural substrate. Reductive dehalogenation either is catalyzed by a specific dehalogenase or may be mediated by free or enzyme-bound transition metal cofactors (porphyrins, corrins). Desulfomonile tiedjei DCB-1 couples energy conservation to a reductive dechlorination reaction. The biochemistry and genetics of oxygenolytic and hydrolytic haloaromatic dehalogenases are discussed. Concerning the haloalkanes, oxygenases, glutathione S-transferases, halidohydrolases, and dehydrohalogenases are involved in the dehalogenation of different haloalkane compounds. The epoxide-forming halohydrin hydrogen halide lyases form a distinct class of dehalogenases. The dehalogenation of alpha-halosubstituted alkanoic acids is catalyzed by halidohydrolases, which, according to their substrate and inhibitor specificity and mode of product formation, are placed into distinct mechanistic groups. beta-Halosubstituted alkanoic acids are dehalogenated by halidohydrolases acting on the coenzyme A ester of the beta-haloalkanoic acid. Microbial systems offer a versatile potential for biotechnological applications. Because of their enantiomer selectivity, some dehalogenases are used as industrial biocatalysts for the synthesis of chiral compounds. The application of dehalogenases or bacterial strains in environmental protection technologies is discussed in detail.

Mineralization of monofluorobenzoate by a diculture under sulfate-reducing conditions

A mesophilic, dehalogenating, sulfate-reducing diculture was isolated from an anaerobic lake sediment. One strain of the diculture is proposed to be an endospore-forming Desulfotomaculum species, the second strain was a vibrioid, motile and non-sporeforming species which is tentatively assigned to the genus Desulfovibrio. The diculture was able to mineralize 4- and 2-fluorobenzoate both isomers being incompletely oxidized with the release of acetate, which was subsequently used by both sulfate-reducing strains. Other electron donors used for growth included benzoate, 3- and 4-hydroxybenzoate, protocatechuate, catechol, phenol, 2,5-dimethoxyphenol, fatty acids up to C8, malate and pyruvate. The culture obtained from a freshwater habitat grew optimally at NaCl concentrations of 0.3-0.5 g l-1, 33-37 degrees C, and pH 7.4. Our experiments showed that certain fluorinated aromatic hydrocarbons could serve as sole sources of carbon and energy for sulfate-reducing bacteria.

Effects of Polychlorinated Biphenyl Congener Concentration and Sediment Supplementation on Rates of Methanogenesis and 2,3,6-Trichlorobiphenyl Dechlorination in an Anaerobic Enrichment

We have employed a method of enrichment that allows us to significantly increase the rate of reductive polychlorinated biphenyl (PCB) dechlorination. This method shortens the time required to investigate the effects that culture conditions have on dechlorination and provides an estimate of the potential activity of the PCB-dechlorinating anaerobes. The periodic supplementation of sterile sediment and PCB produced an enhanced, measurable, and sustained rate of dechlorination. We observed volumetric rates of the dechlorination of 2,3,6-trichlorobiphenyl (2,3,6-CB) to 2,6-dichlorobiphenyl (2,6-CB) of more than 300 μmol liter -1 day -1 when the cultures were supplemented daily. A calculation of this activity that is based on an estimate of the number of dechlorinating anaerobes present indicates that 1.13 pmol of 2,3,6-CB was dechlorinated to 2,6-CB day -1 bacterial cell -1 . This rate is similar to that of the reductive dechlorination of 3-chlorobenzoate by Desulfomonile tiedjei. Methanogenesis declined from 585.3 to 125.9 μmol of CH 4 liter -1 day -1 , while dechlorination increased from 8.2 to 346.0 μmol of 2,3,6-CB dechlorinated to 2,6-CB liter -1 day -1 .

Biology, ecology, and biotechnological applications of anaerobic bacteria adapted to environmental stresses in temperature, pH, salinity, or substrates

Anaerobic bacteria include diverse species that can grow at environmental extremes of temperature, pH, salinity, substrate toxicity, or available free energy. The first evolved archaebacterial and eubacterial species appear to have been anaerobes adapted to high temperatures. Thermoanaerobes and their stable enzymes have served as model systems for basic and applied studies of microbial cellulose and starch degradation, methanogenesis, ethanologenesis, acetogenesis, autotrophic CO2 fixation, saccharidases, hydrogenases, and alcohol dehydrogenases. Anaerobes, unlike aerobes, appear to have evolved more energy-conserving mechanisms for physiological adaptation to environmental stresses such as novel enzyme activities and stabilities and novel membrane lipid compositions and functions. Anaerobic syntrophs do not have similar aerobic bacterial counterparts. The metabolic end products of syntrophs are potent thermodynamic inhibitors of energy conservation mechanisms, and they require coordinated consumption by a second partner organism for species growth. Anaerobes adapted to environmental stresses and their enzymes have biotechnological applications in organic waste treatment systems and chemical and fuel production systems based on biomass-derived substrates or syngas. These kinds of anaerobes have only recently been examined by biologists, and considerably more study is required before they are fully appreciated by science and technology.

Contamination and restoration of groundwater aquifers

Humans are exposed to chemicals in contaminated groundwaters that are used as sources of drinking water. Chemicals contaminate groundwater resources as a result of waste disposal methods for toxic chemicals, overuse of agricultural chemicals, and leakage of chemicals into the subsurface from buried tanks used to hold fluid chemicals and fuels. In the process, both the solid portions of the subsurface and the groundwaters that flow through these porous structures have become contaminated. Restoring these aquifers and minimizing human exposure to the parent chemicals and their degradation products will require the identification of suitable biomarkers of human exposure; better understandings of how exposure can be related to disease outcome; better understandings of mechanisms of transport of pollutants in the heterogeneous structures of the subsurface; and field testing and evaluation of methods proposed to restore and cleanup contaminated aquifers. In this review, progress in these many different but related activities is presented.

Subculturing of a polychlorinated biphenyl-dechlorinating anaerobic enrichment on solid media

An anaerobic culture capable of dechlorinating polychlorinated biphenyls was subcultured under strict anaerobic conditions on solid media containing sterilized river sediment. The dechlorination activity was transferred as a bacterial colony on a solid medium three times. After two transfers on solid medium, the culture was no longer methanogenic but still dechlorinated a mixture of tri- and tetrachlorobiphenyls. This demonstrates that anaerobic bacteria are responsible for the polychlorinated biphenyl dechlorination and can be grown without polychlorinated biphenyl on solid media.

Introduction of a de novo bioremediation ability, aryl reductive dechlorination, into anaerobic granular sludge by inoculation of sludge with Desulfomonile tiedjei

Methanogenic upflow anaerobic granular-sludge blanket (UASB) reactors treat wastewaters at a high rate while simultaneously producing a useful product, methane; however, recalcitrant environmental pollutants may not be degraded. To impart 3-chlorobenzoate (3-CB)-dechlorinating ability to UASB reactors, we inoculated granular sludge in UASB reactors with either a pure culture of Desulfomonile tiedjei (a 3-CB-dechlorinating anaerobe) or a three-member consortium consisting of D. tiejei, a benzoate degrader, and an H2-utilizing methanogen. No degradation occurred in an uninoculated control reactor which was started with the same granular sludge, but inoculated reactors and granules from the inoculated UASB systems rapidly transformed 3-CB (54 mumol/day/g of granule biomass). After several months at a hydraulic retention time of 0.5 day, much shorter than the generation time of D. tiedjei, the reactors still dechlorinated 3-CB. This indicated that the bacteria were immobilized in the reactor granules, and by using an antibody probe for D. tiedjei, we demonstrated that this microorganism had colonized the sludge granules. These results represent the first addition of a pure culture or a defined microbial mixture to a viable waste treatment process to introduce a specific de novo degradative pathway into a granular-sludge consortium.

Microbial reductive dehalogenation

A wide variety of compounds can be biodegraded via reductive removal of halogen substituents. This process can degrade toxic pollutants, some of which are not known to be biodegraded by any other means. Reductive dehalogenation of aromatic compounds has been found primarily in undefined, syntrophic anaerobic communities. We discuss ecological and physiological principles which appear to be important in these communities and evaluate how widely applicable these principles are. Anaerobic communities that catalyze reductive dehalogenation appear to differ in many respects. A large number of pure cultures which catalyze reductive dehalogenation of aliphatic compounds are known, in contrast to only a few organisms which catalyze reductive dehalogenation of aromatic compounds. Desulfomonile tiedjei DCB-1 is an anaerobe which dehalogenates aromatic compounds and is physiologically and morphologically unusual in a number of respects, including the ability to exploit reductive dehalogenation for energy metabolism. When possible, we use D. tiedjei as a model to understand dehalogenating organisms in the above-mentioned undefined systems. Aerobes use reductive dehalogenation for substrates which are resistant to known mechanisms of oxidative attack. Reductive dehalogenation, especially of aliphatic compounds, has recently been found in cell-free systems. These systems give us an insight into how and why microorganisms catalyze this activity. In some cases transition metal complexes serve as catalysts, whereas in other cases, particularly with aromatic substrates, the catalysts appear to be enzymes.

Microbial reductive dehalogenation

A wide variety of compounds can be biodegraded via reductive removal of halogen substituents. This process can degrade toxic pollutants, some of which are not known to be biodegraded by any other means. Reductive dehalogenation of aromatic compounds has been found primarily in undefined, syntrophic anaerobic communities. We discuss ecological and physiological principles which appear to be important in these communities and evaluate how widely applicable these principles are. Anaerobic communities that catalyze reductive dehalogenation appear to differ in many respects. A large number of pure cultures which catalyze reductive dehalogenation of aliphatic compounds are known, in contrast to only a few organisms which catalyze reductive dehalogenation of aromatic compounds. Desulfomonile tiedjei DCB-1 is an anaerobe which dehalogenates aromatic compounds and is physiologically and morphologically unusual in a number of respects, including the ability to exploit reductive dehalogenation for energy metabolism. When possible, we use D. tiedjei as a model to understand dehalogenating organisms in the above-mentioned undefined systems. Aerobes use reductive dehalogenation for substrates which are resistant to known mechanisms of oxidative attack. Reductive dehalogenation, especially of aliphatic compounds, has recently been found in cell-free systems. These systems give us an insight into how and why microorganisms catalyze this activity. In some cases transition metal complexes serve as catalysts, whereas in other cases, particularly with aromatic substrates, the catalysts appear to be enzymes.

Anaerobic transformation and toxicity of trichlorophenols in a stable enrichment culture

The transformation and toxicity of trichlorophenols (TCPs) were studied with a methanogenic enrichment culture derived from sewage sludge. Transformation of TCPs rapidly resumed after heating of the culture at *) degrees C for 1 h, suggesting that the dechlorinating bacteria are spore-forming anaerobes. 2,4,6-TCP was rapidly dechlorinated via 2,4-dichlorophenol to 4-chlorophenol. During the transformation of 2,4,6-TCP, the most probable number of dechlorinating bacteria increased by 4 orders of magnitude. The most extensive dechlorination was observed in media with complex carbon sources such as yeast extract, peptone, and Casamino Acids, but glucose, galactose, and lactose were also used by the consortium. Experiments using chloramphenicol indicated that the reductive dechlorination of 2,4,6-TCP was regulated by an inducible enzyme system. The highest initial concentration at which dechlorination of 2,4,6-TCP was observed was 400 microM. 2,4,5-TCP and 3,4,5-TCP were dechlorinated to, respectively, 3,4-dichlorophenol and 3-chlorophenol at initial concentrations of less than or equal to 40 microM. Toxicity for the acid-producing and methanogenic bacteria in the consortium was a function of chemical structure, as the inhibition of these activities increased from 2,4,6-TCP, via 2,4,5-TCP, to 3,4,5,-TCP.