Reductive dechlorination of 2,4-dichlorobenzoate to 4-chlorobenzoate and hydrolytic dehalogenation of 4-chloro-, 4-bromo-, and 4-iodobenzoate by Alcaligenes denitrificans NTB-1

Aminobacter sp. MSH1 Mineralizes the Groundwater Micropollutant 2,6-Dichlorobenzamide through a Unique Chlorobenzoate Catabolic Pathway

2,6-Dichlorobenzamide (BAM) is a major groundwater micropollutant posing problems for drinking water treatment plants (DWTPs) that depend on groundwater intake. Aminobacter sp. MSH1 uses BAM as the sole source of carbon, nitrogen, and energy and is considered a prime biocatalyst for groundwater bioremediation in DWTPs. Its use in bioremediation requires knowledge of its BAM-catabolic pathway, which is currently restricted to the amidase BbdA converting BAM into 2,6-dichlorobenzoic acid (2,6-DCBA) and the monooxygenase BbdD transforming 2,6-DCBA into 2,6-dichloro-3-hydroxybenzoic acid. Here, we show that the 2,6-DCBA catabolic pathway is unique and differs substantially from catabolism of other chlorobenzoates. BbdD catalyzes a second hydroxylation, forming 2,6-dichloro-3,5-dihydroxybenzoic acid. Subsequently, glutathione-dependent dehalogenases (BbdI and BbdE) catalyze the thiolytic removal of the first chlorine. The remaining chlorine is then removed hydrolytically by a dehalogenase of the α/β hydrolase superfamily (BbdC). BbdC is the first enzyme in that superfamily associated with dehalogenation of chlorinated aromatics and appears to represent a new subtype within the α/β hydrolase dehalogenases. The activity of BbdC yields a unique trihydroxylated aromatic intermediate for ring cleavage that is performed by an extradiol dioxygenase (BbdF) producing 2,4,6-trioxoheptanedioic acid, which is likely converted to Krebs cycle intermediates by BbdG.

Decoding microbial community intelligence through metagenomics for efficient wastewater treatment

Activated sludge, a microbial ecosystem at industrial wastewater treatment plants, is an active collection of diverse gene pool that creates the intelligence required for coexistence at the cost of pollutants. This study has analyzed one such ecosystem from a site treating wastewater pooled from over 200 different industries. The metagenomics approach used could predict the degradative pathways of more than 30 dominating molecules commonly found in wastewater. Results were extended to design a bioremediation strategy using 4-methylphenol, 2-chlorobenzoate, and 4-chlorobenzoate as target compounds. Catabolic potential required to degrade four aromatic families, namely benzoate family, PAH family, phenol family, and PCB family, was mapped. Results demonstrated a network of diverse genera, where a few phylotypes were seen to contain diverse catabolic capacities and were seen to be present in multiple networks. The study highlights the importance of looking more closely at the microbial community of activated sludge to harness its latent potential. Conventionally treated as a black box, the activated biomass does not perform at its full potential. Metagenomics allows a clearer insight into the complex pathways operating at the site and the detailed documentation of genes allows the activated biomass to be used as a bioresource.

Biodegradation of 3-chlorobenzoate by Pseudomonas putida 10.2

Pseudomonas putida 10.2, a 3-chlorobenzoate (3CBa)-degrading bacterium, was isolated from a soil sample obtained from an agricultural area in Chiang Mai, Thailand. This bacterium could degrade 2MM 3CBa very rapidly with the concomitant formation of chloride ion when grown in mineral salt-yeast extract medium. The presence of glucose, lactose and pyruvate in the medium reduced the capability of this bacterium to degrade 3CBa. Metabolites such as 3-chlorocatechol (3CC), catechol and cis,cis-muconic acid (muconate) could be detected in the growth medium or in cell suspensions when 3CBa was used as the substrate. Furthermore, when crude enzyme extract prepared from 3CBa-grown P. putida 10.2 was incubated with 3CC, catechol and muconate could be detected in the reaction mixtures. Thus, the biodegradation pathway of 3CBa by P. putida 10.2 was proposed to involve transformation of 3CBa to 3CC. The dehalogenation step is believed to involve removal of chloride from 3CC to form catechol, which is subsequently converted to muconate.

Influence of root exudates on the bacterial degradation of chlorobenzoic acids

Degradation of chlorobenzoic acids (e.g., products of microbial degradation of PCB) by strains of microorganisms isolated from PCB contaminated soils was assessed. From seven bulk-soil isolates two strains unique in ability to degrade a wider range of chlorobenzoic acids than others were selected, individually and even in a complex mixture of 11 different chlorobenzoic acids. Such a feature is lacking in most tested degraders. To investigate the influence of vegetation on chlorobenzoic acids degraders, root exudates of two plant species known for supporting PCB degradation in soil were tested. While with individual chlorobenzoic acids the presence of plant exudates leads to a decrease of degradation yield, in case of a mixture of chlorobenzoic acids either a change in bacterial degradation specificity, associated with 3- and 4-chlorobenzoic acid, or an extension of the spectrum of degraded chlorobenzoic acids was observed.

Reductive dehalogenation mediated initiation of aerobic degradation of 2-chloro-4-nitrophenol (2C4NP) by Burkholderia sp. strain SJ98

Burkholderia sp. strain SJ98 (DSM 23195) was previously isolated and characterized for degradation and co-metabolic transformation of a number nitroaromatic compounds. In the present study, we evaluated its metabolic activity on chlorinated nitroaromatic compounds (CNACs). Results obtained during this study revealed that strain SJ98 can degrade 2-chloro-4-nitrophenol (2C4NP) and utilize it as sole source of carbon, nitrogen, and energy under aerobic conditions. The cells of strain SJ98 removed 2C4NP from the growth medium with sequential release of nearly stoichiometric amounts of chloride and nitrite in culture supernatant. Under aerobic degradation conditions, 2C4NP was transformed into the first intermediate that was identified as p-nitrophenol by high-performance liquid chromatography, LCMS-TOF, and GC-MS analyses. This transformation clearly establishes that the degradation of 2C4NP by strain SJ98 is initiated by "reductive dehalogenation"; an initiation mechanism that has not been previously reported for microbial degradation of CNAC under aerobic conditions.

Characterization of multiple chlorobenzoic acid-degrading organisms from pristine and contaminated systems: mineralization of 2,4-dichlorobenzoic acid

Multiple bacterial strains with CBA metabolic properties were isolated using a simple selective strategy. Phylogenetic analysis of the 16S rRNA gene sequences grouped them into two main clusters consisting of four bacterial phyla and belonging to 17 genera. Whereas growth was more frequent with 2-CBA (∼68%), 50% grew on 4-CBA and ∼7% utilized 3-CBA. One third of the strains exhibited 2,4-dichlorobenzoic acid (2,4-diCBA) catabolic function and were mainly representatives of α-, β- and γ-Proteobacteria. In batch experiments, growth was concomitant with substrate disappearance and near-stoichiometric release of chloride. Doubling times for 2,4-diCBA degradation doubled those determined for mono-substituted CBAs. Out of the six 2,4-diCBA degraders submitted for enzyme assays, significant induction of catechol 1,2-dioxygenase types I and II activities in cell-free extracts were found in four while protocatechuate 3,4-dioxygenase activity was detected in the remaining two. Activities in CBA-grown cells were 20 orders-of-magnitude higher than those grown on benzoic acid.

Degradation of 4-Chlorobenzoate by Facultatively Alkalophilic Arthrobacter sp. Strain SB8

A facultative alkalophile capable of utilizing 4-chlorobenzoate (4-CBA), strain SB8, was isolated from soil with an alkaline medium (pH 10.0) containing the haloaromatic compound as the carbon source. The strain, identified as an Arthrobacter sp., showed rather extensive 4-CBA-degrading ability. 4-CBA utilization by the strain was possible in the alkaline medium containing up to 10 g of the compound per liter. The 4-CBA-dechlorinating activity of resting cells was almost completely uninhibited by substrate concentrations up to 150 mM. The bacterium dehalogenated 4-CBA in the initial stage of the degradation and metabolized the compound via 4-hydroxybenzoate and protocatechuate. O(2) was needed for 4-CBA dechlorination by resting cells but not by cell extracts. O(2) was inhibitory to the 4-CBA dechlorination activity of cell extracts. These facts suggest dechlorination of 4-CBA by halide hydrolysis and an energy requirement for the transport of 4-CBA into cells.

Uptake of Benzoic Acid and Chloro-Substituted Benzoic Acids by Alcaligenes denitrificans BRI 3010 and BRI 6011

The mechanism of uptake of benzoic and 2,4-dichlorobenzoic acid (2,4-DCBA) by Alcaligenes denitrificans BRI 3010 and BRI 6011 and Pseudomonas sp. strain B13, three organisms capable of degrading various isomers of chlorinated benzoic acids, was investigated. In all three organisms, uptake of benzoic acid was inducible. For benzoic acid uptake into BRI 3010, monophasic saturation kinetics with apparent K(infm) and V(infmax) values of 1.4 (mu)M and 3.2 nmol/min/mg of cell dry weight, respectively, were obtained. For BRI 6011, biphasic saturation kinetics were observed, suggesting the presence of two uptake systems for benzoic acid with distinct K(infm) (0.72 and 5.3 (mu)M) and V(infmax) (3.3 and 4.6 nmol/min/mg of cell dry weight) values. BRI 3010 and BRI 6011 accumulated benzoic acid against a concentration gradient by a factor of 8 and 10, respectively. A wide range of structural analogs, at 50-fold excess concentrations, inhibited benzoic acid uptake by BRI 3010 and BRI 6011, whereas with B13, only 3-chlorobenzoic acid was an effective inhibitor. For BRI 3010 and BRI 6011, the inhibition by the structural analogs was not of a competitive nature. Uptake of benzoic acid by BRI 3010 and BRI 6011 was inhibited by KCN, by the protonophore 3,5,3(prm1), 4(prm1)-tetrachlorosalicylanilide (TCS), and, for BRI 6011, by anaerobiosis unless nitrate was present, thus indicating that energy was required for the uptake process. Uptake of 2,4-DCBA by BRI 6011 was constitutive and saturation uptake kinetics were not observed. Uptake of 2,4-DCBA by BRI 6011 was inhibited by KCN, TCS, and anaerobiosis even if nitrate was present, but the compound was not accumulated intracellularly against a concentration gradient. Uptake of 2,4-DCBA by BRI 6011 appears to occur by passive diffusion into the cell down its concentration gradient, which is maintained by the intracellular metabolism of the compound. This process could play an important role in the degradation of xenobiotic compounds by microorganisms.

Microbial degradation of the benzonitrile herbicides dichlobenil, bromoxynil and ioxynil in soil and subsurface environments--insights into degradation pathways, persistent metabolites and involved degrader organisms

The benzonitriles dichlobenil, bromoxynil and ioxynil are important broad-spectrum or selective herbicides used in agriculture, orchards and public areas worldwide. The dichlobenil metabolite 2,6-dichlorobenzamide is the most frequently encountered groundwater contaminant in Denmark, which suggests that the environmental fate of these three structurally related benzonitrile herbicides should be addressed in detail. This review summarises the current knowledge on microbial degradation of dichlobenil, bromoxynil and ioxynil with particular focus on common features of degradation rates and pathways, accumulation of persistent metabolites and diversity of the involved degrader organisms.

Microbial degradation pathways of the herbicide dichlobenil in soils with different history of dichlobenil-exposure

This is the first detailed study of metabolite production during degradation of the herbicide 2,6-dichlorobenzonitrile (dichlobenil). Degradation of dichlobenil and three potential metabolites: 2,6-dichlorobenzamide (BAM), 2,6-dichlorobenzoic acid (2,6-DCBA) and ortho-chlorobenzamide (OBAM) was studied in soils either previously exposed or not exposed to dichlobenil using a newly developed HPLC method. Dichlobenil was degraded in all four soils; BAM and 2,6-DCBA were only degraded in soils previously exposed to dichlobenil (100% within 35-56 days and 85-100% in 56 days, respectively), and OBAM in all four soils (25-33% removal in 48 days). BAM produced from dichlobenil was either hydrolyzed to 2,6-DCBA or dechlorinated to OBAM, which was further hydrolyzed to ortho-chlorobenzoic acid. BAM was rapidly mineralized in previously exposed soils only. All potential metabolites and the finding that BAM was a dead-end metabolite of dichlobenil in soils not previously exposed to dichlobenil needs to be included in risk assessments of the use of dichlobenil.

Cloning of the Arthrobacter sp. FG1 dehalogenase genes and construction of hybrid pathways in Pseudomonas putida strains

An Arthrobacter strain, able to utilize 4-chlorobenzoic acid as the sole carbon and energy source, was isolated and characterized. The first step of the catabolic pathway was found to proceed via a hydrolytic dehalogenation that leads to the formation of 4-hydroxybenzoic acid. The dehalogenase encoding genes (fcb) were sequenced and found highly homologous to and organized as those of other 4-chlorobenzoic acid degrading Arthrobacter strains. The fcb genes were cloned and successfully expressed in the heterologous host Pseudomonas putida PaW340 and P. putida KT2442 upper TOL, which acquired the ability to grow on 4-chlorobenzoic acid and 4-chlorotoluene, respectively. The cloned dehalogenase displayed a high specificity for para-substituted haloaromatics with affinity Cl > Br > I >> F, in the order.

Analysis of the C-terminal domain ofBurkholderiasp. strain LB400 BphK reveals a conserved motif that affects catalytic activity

The bphK gene encoding glutathione S-transferase (GST) is located in the bph operon (PCB co-metabolism) in Burkholderia sp. strain LB400 and the enzyme has recently been shown to have dechlorination activity in relation to 4-chlorobenzoate (4-CBA). Alignments using other glutathione S-transferase sequences found in PCB degradation operons identified a highly conserved region in the C-terminal domain of these enzymes that included a conserved motif implicated in protein folding in eukaryotic GSTs. Site-directed mutagenesis indicated that the region is indirectly involved in the catalytic activity and substrate specificity of BphK. Predicted hydrogen bond interactions involving Asp155 play an important role in the enzymatic properties of this glutathione S-transferase.

Chlorobenzoate-degrading bacteria in similar pristine soils exhibit different community structures and population dynamics in response to anthropogenic 2-, 3-, and 4-chlorobenzoate levels

A study was conducted to determine the diversity of 2-, 3-, and 4-chlorobenzoate (CB) degraders in two pristine soils with similar physical and chemical characteristics. Surface soils were collected from forested sites and amended with 500 microg of 2-, 3-, or 4-CB g(-1) soil. The CB levels and degrader numbers were monitored throughout the study. Degraders were isolated, grouped by DNA fingerprints, identified via 16S rDNA sequences, and screened for plasmids. The CB genes in selected degraders were isolated and/or sequenced. In the Madera soil, 2-CB and 4-CB degraded within 11 and 42 d, respectively, but 3-CB did not degrade. In contrast, 3-CB and 4-CB degraded in the Oversite soil within 14 and 28 d, respectively, while 2-CB did not degrade. Approximately 10(7) CFU g(-1) of degraders were detected in the Madera soil with 2-CB, and the Oversite soil with 3- and 4-CB. No degraders were detected in the Madera soil with 4-CB even though the 4-CB degraded. Nearly all of the 2-CB degraders isolated from the Madera soil were identified as a Burkholderia sp. containing chromosomally encoded degradative genes. In contrast, several different 3- and 4-CB degraders were isolated from the Oversite soil, and their populations changed as CB degradation progressed. Most of these 3-CB degraders were identified as Burkholderia spp. while the majority of 4-CB degraders were identified as Bradyrhizobium spp. Several of the 3-CB degraders contained the degradative genes on large plasmids, and there was variation between the plasmids in different isolates. When a fresh sample of Madera soil was amended with 50, 100, or 200 microg 3-CB g(-1), 3-CB degradation occurred, suggesting that 500 microg 3-CB g(-1) was toxic to the degraders. Also, different 3-CB degraders were isolated from the Madera soil at each of the three lower levels of 3-CB. No 2-CB degradation was detected in the Oversite soil even at lower 2-CB levels. These results indicate that the development of 2-, 3-, and 4-CB degrader populations is site-specific and that 2-, 3-, and 4-CB are degraded by different bacterial populations in pristine soils. These results also imply that the microbial ecology of two soils that develop under similar biotic and abiotic environments can be quite different.

BphK shows dechlorination activity against 4-chlorobenzoate, an end product of bph-promoted degradation of PCBs

A bphK gene encoding glutathione S-transferase (GST) activity is located in the bph operon in Burkholderia sp. strain LB400 but its role in polychlorinated biphenyl (PCB) metabolism is unknown. This gene was over-expressed in Escherichia coli and an in vivo assay based on growth of E. coli containing GST activity was used to identify potential novel substrates for this enzyme. Using this assay, 4-chlorobenzoate (4-CBA) was identified as a substrate for the BphK enzyme. High pressure liquid chromatography analysis and chloride ion detection showed removal of 4-CBA and an equivalent increase of chloride in cell extracts when incubated with this enzyme. These results would indicate that this BphK enzyme has dechlorination activity in relation to 4-CBA and may have a role in protection of other Bph enzymes against certain chlorinated metabolites of PCB degradation.

The use of mutants to discern the degradation pathway of 3,4'-dichlorobiphenyl in Pseudomonas acidovorans M3GY

Abstract Pseudomonas acidovorans strain M3GY is a recombinant bacterium with the novel ability to utilize 3,4'-dichlorobiphenyl (3,4'-DCBP) as a growth substrate. This strain was previously shown to oxidize the 3'-ring and produce 4-chlorobenzoate (4-CBa) through the standard biphenyl pathway. Although 4-CBa was metabolized through the meta-fission pathway, the genes encoding the ortho-chlorocatechol pathway were retained. Nevertheless, neither 3-CBa nor 3-chlorocatechol (3-CC) were detected as intermediates during metabolism of 3,4'-DCBP, nor was 4-CBa utilized as a sole carbon source, by this strain. Two mutant strains were produced to resolve these anomalies. Mutant strain M3GY-9 was obtained by Tn5 insertion and selection for growth on biphenyl, and was unable to grow on 3-CBa. It accumulated 3-CC from 3,4'-DCBP when grown on biphenyl. Thus, M3GY attacks both rings, and the failure to isolate 3-CBa or 3-CC is due to rapid turnover by the enzymes of the ortho-chlorocatechol pathway in the wild-type strain. Mutant strain M3GY-1 grew on 4-CBa, unlike the wild-type strain. Washed cell suspensions of mutant strain MEGY-1 consumed 4-fluorobenzoate, 4-bromobenzoate, and, to a lesser extent 4-iodobenzoate. The mutation that resulted in the ability of mutant strain M3GY-I to effectively utilize 4-CBa as a sole carbon source was associated with a transport mechanism.

A novel ortho-dehalogenation reaction of 2-chlorocinnamic acid catalyzed by the pink yeast Rhodotorula rubra Y-1529

In the present study, a resting cells suspension of Rhodotorula rubra Y-1529 was shown to have the capacity to perform an ortho-dehalogenation reaction on 2-chlorocinnamic acid. The results from the biodegradation of U-[14C]benzoic acid, cinnamic acid, 3-chlorocinnamic acid and 4-chlorocinnamic acid suggest that the first step of the ortho-dehalogenation reaction occurred during the oxidation of the unsaturated C3 side chain of 2-chlorocinnamic acid to 2-chlorobenzoic acid. None of the 2-chlorobenzoic acid was found in the biodegradation system, suggesting that this step was a highly regulated step. After the side-chain oxidation reaction, the hydroxylation of the benzene ring was determined to be at the para-position first, followed by the meta-position. The occurrence of 3:4-position ring fission reactions and the production of the final product, CO2, was proven by the biodegradation of U-[14C] benzoic acid. This oxidative dehalogenation reaction catalyzed by R. rubra was found to be regiospecific for 2-chlorocinnamic acid; the chloride ion was probably removed after the ring fission reaction. A pathway of the ortho-dehalogenation reaction of 2-chlorocinnamic acid catalyzed by R. rubra was proposed based on these data.

Phylogenetic and phenotypic diversity of 4-chlorobenzoate-degrading bacteria isolated from soils

Twenty numerically dominant 4-chlorobenzoate (4-CBA)-degrading bacteria were isolated from agricultural soils. The isolates were able to utilize 4-CBA as a sole source of carbon and energy. A total of 65% of the isolates was identified to the species level by fatty acid methyl ester (FAME) analysis, and the isolates were strains of Micrococcus, Pseudomonas, Oerskovia, Cellulomonas, and Arthrobacter species. The chromosomal DNA patterns of the isolates obtained by polymerase chain reaction (PCR) amplification of repetitive extragenic palindromic (REP) sequences were distinct from each other. Most of the isolates grew rapidly in 4-CBA medium, but their substrate utilization capabilities were generally restricted. Plasmid DNAs were detected from 55% of the isolates, and one strain, HR7, was shown to have self-transmissible, 4-CBA degradative plasmids. 4-CBA degradative enzymes were inducible by the presence of 4-CBA and most of the isolates appeared to mineralize it through 4-hydroxybenzoate rather than 4-chlorocatechol.

Chemoselective nitro group reduction and reductive dechlorination initiate degradation of 2-chloro-5-nitrophenol by Ralstonia eutropha JMP134

Ralstonia eutropha JMP134 utilizes 2-chloro-5-nitrophenol as a sole source of nitrogen, carbon, and energy. The initial steps for degradation of 2-chloro-5-nitrophenol are analogous to those of 3-nitrophenol degradation in R. eutropha JMP134. 2-Chloro-5-nitrophenol is initially reduced to 2-chloro-5-hydroxylaminophenol, which is subject to an enzymatic Bamberger rearrangement yielding 2-amino-5-chlorohydroquinone. The chlorine of 2-amino-5-chlorohydroquinone is removed by a reductive mechanism, and aminohydroquinone is formed. 2-Chloro-5-nitrophenol and 3-nitrophenol induce the expression of 3-nitrophenol nitroreductase, of 3-hydroxylaminophenol mutase, and of the dechlorinating activity. 3-Nitrophenol nitroreductase catalyzes chemoselective reduction of aromatic nitro groups to hydroxylamino groups in the presence of NADPH. 3-Nitrophenol nitroreductase is active with a variety of mono-, di-, and trinitroaromatic compounds, demonstrating a relaxed substrate specificity of the enzyme. Nitrosobenzene serves as a substrate for the enzyme and is converted faster than nitrobenzene.

Construction and characterization of two recombinant bacteria that grow on ortho- and para-substituted chlorobiphenyls

Cloning and expression of the aromatic ring dehalogenation genes in biphenyl-growing, polychlorinated biphenyl (PCB)-cometabolizing Comamonas testosteroni VP44 resulted in recombinant pathways allowing growth on ortho- and para-chlorobiphenyls (CBs) as a sole carbon source. The recombinant variants were constructed by transformation of strain VP44 with plasmids carrying specific genes for dehalogenation of chlorobenzoates (CBAs). Plasmid pE43 carries the Pseudomonas aeruginosa 142 ohb genes coding for the terminal oxygenase (ISPOHB) of the ortho-halobenzoate 1,2-dioxygenase, whereas plasmid pPC3 contains the Arthrobacter globiformis KZT1 fcb genes, which catalyze the hydrolytic para-dechlorination of 4-CBA. The parental strain, VP44, grew only on low concentrations of 2- and 4-CB by using the products from the fission of the nonchlorinated ring of the CBs (pentadiene) and accumulated stoichiometric amounts of the corresponding CBAs. The recombinant strains VP44(pPC3) and VP44(pE43) grew on, and completely dechlorinated high concentrations (up to 10 mM), of 4-CBA and 4-CB and 2-CBA and 2-CB, respectively. Cell protein yield corresponded to complete oxidation of both biphenyl rings, thus confirming mineralization of the CBs. Hence, the use of CBA dehalogenase genes appears to be an effective strategy for construction of organisms that will grow on at least some congeners important for remediation of PCBs.

Isolation of Alcaligenes sp. strain L6 at low oxygen concentrations and degradation of 3-chlorobenzoate via a pathway not involving (chloro)catechols

Isolations of 3-chlorobenzoate (3CBA)-degrading aerobic bacteria under reduced O2 partial pressures yielded organisms which metabolized 3CBA via the gentisate or the protocatechuate pathway rather than via the catechol route. The 3CBA metabolism of one of these isolates, L6, which was identified as an Alcaligenes species, was studied in more detail. Resting-cell suspensions of L6 pregrown on 3CBA oxidized all known aromatic intermediates of both the gentisate and the protocatechuate pathways. Neither growth on nor respiration of catechol could be detected. Chloride production from 3CBA by L6 was strictly oxygen dependent. Cell-free extracts of 3CBA-grown L6 cells exhibited no catechol dioxygenase activity but possessed protocatechuate 3,4-dioxygenase, gentisate dioxygenase, and maleylpyruvate isomerase activities instead. In continuous culture with 3CBA as the sole growth substrate, strain L6 demonstrated an increased oxygen affinity with decreasing steady-state oxygen concentrations.

NADPH-dependent reductive ortho dehalogenation of 2,4-dichlorobenzoic acid in Corynebacterium sepedonicum KZ-4 and Coryneform bacterium strainNTB-1 via 2,4-dichlorobenzoyl coenzyme A

Corynebacterium sepedonicum KZ-4, described earlier as a strain capable of growth on 2,4-dichlorobenzoate (G.M. Zaitsev and Y.N. Karasevich, Mikrobiologiya 54:356-369, 1985), is known to metabolize this substrate via 4-hydroxybenzoate and protocatechuate, and evidence consistent with an initial reductive dechlorination step to form 4-chlorobenzoate was found in another coryneform bacterium, strain NTB-1 (W.J.J. van den Tweel, J.B. Kok, and J.A.M. de Bont, Appl. Environ. Microbiol. 53:810-815, 1987). 2-Chloro-4-fluorobenzoate was found to be converted stoichiometrically to 4-fluorobenzoate by resting cells of strain KZ-4, compatible with a reductive process. Experiments with cell extracts demonstrated that Mg - ATP and coenzyme A (CoA) were required to stimulate reductive dehalogenation, consistent with the intermediacy of 2-chloro-4-fluoro-benzoyl-CoA and 2,4-dichlorobenzoyl-CoA thioesters. 2,4-Dichlorobenzoyl-CoA was shown to be converted to 4-chlorobenzoyl-CoA in a novel NADPH-dependent reaction in extracts of both KZ-4 and NTB-1. In addition to the ligase and reductive dehalogenase activities, hydrolytic 4-chlorobenzoyl-CoA dehalogenase and thioesterase activities, 4-hydroxybenzoate 3-monooxygenase, and protocatechuate 3,4-dioxygenase activities were demonstrated to be present in the soluble fraction of KZ-4 extracts following ultracentrifugation. We propose that the pathway for 2,4-dichlorobenzoate catabolism in strains KZ-4 and NTB-1 involves formation of 2,4-dichlorobenzoyl-CoA, NADPH-dependent ortho dehalogenation yielding 4-chlorobenzoyl-CoA, hydrolytic removal of chlorine from the para position to generate 4-hydroxybenzoyl-CoA, hydrolysis to form 4-hydroxybenzoate, oxidation to yield protocatechuate, and oxidative ring cleavage.

Initial hydrogenation and extensive reduction of substituted 2,4-dinitrophenols

Rhodococcus erythropolis HL 24-1 isolated as a 2,4-dinitrophenol-degrading organism can utilize 2-chloro-4,6-dinitrophenol as the sole nitrogen, carbon, and energy source under aerobic conditions. This compound is metabolized with liberation of stoichiometric amounts of chloride and nitrite. Under anaerobic conditions, 2,4-dinitrophenol was transiently accumulated in the culture fluid, indicating a reductive elimination of chloride. During aerobic bioconversion of 2-amino-4,6-dinitrophenol by R. erythropolis HL 24-1, a reductive elimination of nitrite leading to 2-amino-6-nitrophenol was observed. Elimination of chloride or nitrite by the initial formation of a hydride Meisenheimer complex is discussed. A methyl group in the ortho position of the 2,4-dinitrophenol gives rise to an extensive reduction of the aromatic ring under aerobic conditions. Thus, 2-methyl-4,6-dinitrophenol was shown to be converted to the two diastereomers of 4,6-dinitro-2-methylhexanoate as dead-end metabolites which were identified by spectroscopic methods.

Purification and characterization of a novel 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of Desulfomonile tiedjei DCB-1

Although reductive dehalogenation by anaerobic microorganisms offers great potential for the degradation of halocarbons, little is known about the biochemical mechanisms involved. It has previously been demonstrated that the dehalogenase activity involved in 3-chlorobenzoate dehalogenation by Desulfomonile tiedjei DCB-1 is present in the membrane fraction of the cell extracts. We report herein the purification of a 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of D. tiedjei DCB-1. The dehalogenase activity was monitored by the conversion of 3-chlorobenzoate to benzoate with reduced methyl viologen as a reducing agent. The membrane fraction of the cell extracts was obtained by ultracentrifugation, and the membrane proteins were solubilized with either the detergent CHAPS (3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate) or Triton X-100 in the presence of glycerol. The solubilized dehalogenase was purified by ammonium sulfate fractionation and a combination of anion exchange, hydroxyapatite, and hydrophobic interaction chromatographies. This procedure yielded about 7% of the total dehalogenase activity with a 120-fold increase in specific activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified dehalogenase consisted of two subunits with molecular weights of 64,000 and 37,000. The enzyme converted 3-chlorobenzoate to benzoate at its highest specific activity in 10 mM potassium phosphate buffer (pH 7.2) at 38 degrees C. The enzyme was yellow and probably a heme protein. The enzyme had an adsorbance peak at 408 nm. The dithionite-reduced enzyme displayed absorbance peaks at 416, 522, and 550 nm. The dithionite-reduced enzyme was able to complex with carbon monoxide. The nature of the heme chromophore is currently unknown.

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.

Obligately anaerobic bacteria in biotechnology

New obligately anaerobic bacteria are being discovered at an accelerating rate and it is becoming very evident that the diversity of anoxic biotransformations has been greatly underestimated. Furthermore, among contemporary anaerobes there are many that thrive in extreme environments including, for example, an impressive array of both archaebacterial and eubacterial hyperthermophiles. Free energy for growth and reproduction may be conserved not only via fermentations but also by anoxygenic photophosphorylation and other modes of creating transmembrane proton potential. Thus forms of anaerobic respiration in which various inorganic oxidants (or indeed carbon dioxide) serve as terminal electron acceptors have greatly extended the natural habitats in which such organisms may predominate. Anaerobic bacteria are, however, often found in nature as members of close microbial communities (consortia) that, although sustained by syntrophic and other relations between component species, are liable to alter their composition and character in response to environmental changes, e.g., availability of terminal oxidants. It follows that the biotechnological exploitation of obligately anaerobic bacteria must be informed by knowledge both of their biochemical capacities and of their normal environmental roles. It is against this background that illustrative examples of the activities of anaerobic bacteria are considered under three heads: 1. Biodegradation/Bioremediation, with special reference to the anaerobic breakdown of aromatic and/or halogenated organic substances; 2. Biosynthesis/Bioproduction, encompassing normal and modified fermentations; and 3. Biotransformations, accomplished by whole or semipermeabilized organisms or by enzymes derived therefrom, with particular interest attaching to the production of chiral compounds by a number of procedures, including electromicrobial reduction.

Pseudomonas aeruginosa 142 uses a three-component ortho-halobenzoate 1,2-dioxygenase for metabolism of 2,4-dichloro- and 2-chlorobenzoate

Cell extracts of Pseudomonas aeruginosa 142, which was previously isolated from a polychlorinated biphenyl-degrading consortium, were shown to degrade 2,4-dichlorobenzoate, 2-chlorobenzoate, and a variety of other substituted ortho-halobenzoates by a reaction that requires oxygen, NADH, Fe(II), and flavin adenine dinucleotide. By using extracts that were chromatographically depleted of chlorocatechol and catechol 1,2-dioxygenase activities, products of the initial reaction with 2,4- or 2,5-dichlorobenzoate and 2-chlorobenzoate were identified by mass spectrometry as 4-chlorocatechol and catechol. In contrast to the well-characterized benzoate dioxygenases or the recently described 2-halobenzoate 1,2-dioxygenase from P. cepacia 2CBS (S. Fetzner, R. Müller, and F. Lingens, J. Bacteriol. 174:279-290, 1992) that possess two protein components, the P. aeruginosa enzyme was resolved by ion-exchange chromatography into three components, each of which is required for activity. To verify the distinct nature of this enzyme, we purified, characterized, and identified one component as a ferredoxin (M(r), approximately 13,000) containing a single [2Fe-2S] Rieske-type cluster (electron paramagnetic resonance spectroscopic values of gx = 1.82, gy = 1.905, and gz = 2.02 in the reduced state) that is related in sequence to ferredoxins found in the naphthalene and biphenyl three-component dioxygenase systems. By analogy to these enzymes, we propose that the P. aeruginosa ferredoxin serves as an electron carrier between an NADH-dependent ferredoxin reductase and the terminal component of the ortho-halobenzoate 1,2-dioxygenase. The broad specificity and high regiospecificity of the enzyme make it a promising candidate for use in the degradation of mixtures of chlorobenzoates.

Variation in chlorobenzoate catabolism by Pseudomonas putida P111 as a consequence of genetic alterations

Pseudomonas putida P111 is able to utilize a broad range of monochlorinated, dichlorinated, and trichlorinated benzoates. The involvement of two separate dioxygenases was noted from data on plasmid profiles and DNA hybridization. The benzoate dioxygenase, which converts 3-chlorobenzoate (3-CB), 4-CB, and benzoate to the corresponding catechols via reduction of a dihydrodiol, was shown to be chromosomally coded. The chlorobenzoate-1,2-dioxygenase that converts ortho-chlorobenzoates to the corresponding catechols without the need of a functional dioldehydrogenase was shown to be encoded on plasmid pPB111 (75 kb). Cured strains were unable to utilize ortho-chlorobenzoates for growth. DNA hybridization data indicated that catabolism of the corresponding chlorocatechols was coded on chromosomal genes. Maintenance of plasmid pPB111 was dependent on the presence of ortho-chlorobenzoates in the growth media. A unique variant of P111 (P111D), able to grow on 3,5-dichlorobenzoate (3,5-DCB), was obtained by continuous subculturing from media containing progressively lower and higher concentrations of 3-CB and 3,5-DCB, respectively. The low frequency of segregants able to grow on 2,5-DCB, 2,3-DCB, and 2,3, 5-trichlorobenzoate was evident by lag periods greater than 200 h. Continued subculture on 3,5-DCB resulted in the formation of new plasmid pPH111 (120 kb), which was homologous to pPB111. A probe from the clc operon, which encodes for the chlorocatechol pathway, hybridized to plasmid pPH111 and to the chromosome of the wild-type strain P111 but not to its plasmid pPB111 nor to the chromosome of strain P111A, which had lost the ability to utilize chlorobenzoates.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific removal of chlorine from the ortho-position of halogenated benzoic acids by reductive dechlorination in anaerobic enrichment cultures

Anaerobic enrichment cultures catalysing the reductive dechlorination of chlorinated benzoic acids were obtained from three fresh-water sediments collected from seven different locations. Sub-cultures from these enrichments specifically removed ortho-substituted chlorine from 2,3,6-, 2,3,5- and 2,4,6-trichlorobenzoic acid, yielding chloride and 2,5-, 3,5-, and 2,4-dichlorobenzoic acids, respectively. These reductive dehalogenations were stimulated by the addition of benzoate and/or volatile organic acids. In one of these enrichments dehalogenation of ortho- and/or para-chlorine substituents was also observed from 2,3-, 2,4-, 2,5-, and 3,4-dichlorobenzoic acid, yielding 3- and 4-chlorobenzoate. Removal of meta-chlorines was not observed in any of the enrichments.

Degradation of 2,4,5-trichlorophenoxyacetic acid by a Nocardioides simplex culture

A Nocardioides simplex strain 3E was isolated which totally dechlorinated 2,4,5-trichlorophenoxyacetic acid and was capable of its utilization as the sole source of carbon. The mechanism of 2,4,5-trichlorophenoxyacetic acid degradation by this strain was investigated. Chloroaromatic metabolites that occur in the lag, exponential and stationary growth phases of the strain Nocardioides simplex 3E were isolated and identified bases on a combination of TLC, GC-MS and HPLC data. Decomposition of 2,4,5-trichlorophenoxyacetic acid at the initial stage was shown to proceed by two pathways: via the splitting of the two-carbon fragment to yield 2,4,5-trichlorophenol and the reductive dechlorination to produce 2,4-dichlorophenoxyacetic acid. Hydrolytic dechlorination of 2,4,5-trichlorophenoxyacetic acid was found to yield dichlorohydroxyphenoxyacetic acid, thus pointing to the possible existence of a third branch at the initial stage of degradation of the xenobiotic. 2,4,5-Trichlorophenol and 2,4-dichlorophenoxyacetic acid produced during the metabolism of 2,4,5-trichlorophenoxyacetic acid and in experiments with resting cells are utilized by the strain Nocardioides simplex 3E as growth substrates.

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.

Microbial breakdown of halogenated aromatic pesticides and related compounds

Considerable progress has been made in the last few years in understanding the mechanisms of microbial degradation of halogenated aromatic compounds. Much is already known about the degradation mechanisms under aerobic conditions, and metabolism under anaerobiosis has lately received increasing attention. The removal of the halogen substituent is a key step in degradation of halogenated aromatics. This may occur as an initial step via reductive, hydrolytic or oxygenolytic mechanisms, or after cleavage of the aromatic ring at a later stage of metabolism. In addition to degradation, several biotransformation reactions, such as methylation and polymerization, may take place and produce more toxic or recalcitrant metabolites. Studies with pure bacterial and fungal cultures have given detailed information on the biodegradation pathways of several halogenated aromatic compounds. Several of the key enzymes have been purified or studied in cell extracts, and there is an increasing understanding of the organization and regulation of the genes involved in haloaromatic degradation. This review will focus on the biodegradation and biotransformation pathways that have been established for halogenated phenols, phenoxyalkanoic acids, benzoic acids, benzenes, anilines and structurally related halogenated aromatic pesticides. There is a growing interest in developing microbiological methods for clean-up of soil and water contaminated with halogenated aromatic compounds.

Degradation of halogenated aromatic compounds

Due to their persistence, haloaromatics are compounds of environmental concern. Aerobically, bacteria degrade these compounds by mono- or dioxygenation of the aromatic ring. The common intermediate of these reactions is (halo)catechol. Halocatechol is cleaved either intradiol (ortho-cleavage) or extradiol (meta-cleavage). In contrast to ortho-cleavage, meta-cleavage of halocatechols yields toxic metabolites. Dehalogenation may occur fortuitously during oxygenation. Specific dehalogenation of aromatic compounds is performed by hydroxylases, in which the halo-substituent is replaced by a hydroxyl group. During reductive dehalogenation, haloaromatic compounds may act as electron-acceptors. Herewith, the halosubstituent is replaced by a hydrogen atom.

Enzymic Dehalogenation of 4-Chlorobenzoyl Coenzyme A in Acinetobacter sp. Strain 4-CB1

4-Chlorobenzoate degradation in cell extracts of Acinetobacter sp. strain 4-CB1 occurs by initial synthesis of 4-chlorobenzoyl coenzyme A (4-chlorobenzoyl CoA) from 4-chlorobenzoate, CoA, and ATP. 4-Chlorobenzoyl CoA is dehalogenated to 4-hydroxybenzoyl CoA. Following the dehalogenation reaction, 4-hydroxybenzoyl CoA is hydrolyzed to 4-hydroxybenzoate and CoA. Possible roles for the CoA moiety in the dehalogenation reaction are discussed.

Effect of cosubstrates on the dechlorination of selected chlorophenolic compounds by Rhodococcus erythropolis 1CP

Effects of aromatic and nonaromatic cosubstrates on the rate of 2,4-dichlorophenol degradation by R. erythropolis 1CP were studied under growth and nongrowth conditions. Glucose and maltose were found to accelerate 2,4-dichlorophenol (initial conc. 50 mg/l) dechlorination from 11 days to 24 and 20 h, respectively. The stimulating effect was observed within the range of glucose concentration of 0.5-2.0 g/l in the medium. Ascorbic acid at a concentration of 10-70 mg/l as a nongrowth substrate also increased the decomposition of 2,4-dichlorophenol, though at a lower degree than glucose. Experiments with washed cells showed the limiting stage of chlorophenol degradation by R. erythropolis 1CP to be, possibly, aromatic ring dechlorination after its hydroxylation. As an additional source of electrons, glucose introduced into suspensions of washed cells grown on 3-hydroxybenzoate made it possible to eliminate the bottleneck in 2,4-dichlorophenol metabolism by significantly accelerating dechlorination of 3,5-dichlorocatechol. Based on the experimental and literature data, the enzyme systems involved in 2,4-dichlorophenol degradation by the culture R. erythropolis 1CP are assumed to be nonspecific.

Evidence for 4-chlorobenzoic acid dehalogenation mediated by plasmids related to pSS50

The biodegradation of 4-chlorobiphenyl usually proceeds through the intermediate 4-chlorobenzoate. Few bacterial strains can degrade 4-chlorobiphenyl to 4-chlorobenzoate and 4-chlorobenzoate to CO2. This study demonstrates that the 4-chlorobiphenyl-degrading Alcaligenes sp. strain ALP83 can degrade 4-chlorobenzoate to 4-hydroxybenzoate. The dehalogenase activity is correlated with a 10-kb fragment carried on plasmid pSS70.

Dehalogenation of 4-chlorobenzoate by 4-chlorobenzoate dehalogenase from pseudomonas sp. CBS3: an ATP/coenzyme A dependent reaction

Pseudomonas sp. CBS3 was grown with 4-chlorobenzoate as sole source of carbon and energy. Freshly prepared cell-free extracts converted 4-chlorobenzoate to 4-hydroxybenzoate. After storage for 16 hours at 25 degrees C only about 50% of the initial activity was left. Treatment at 55 degrees C for 10 minutes, dialysis or desalting of the extracts by gel filtration caused a total loss of the activity of the 4-chlorobenzoate dehalogenase. The activity could be restored by the addition of ATP, coenzyme A and Mg2+. If one of these cofactors was missing, no dehalogenating activity was detectable. The amount of 4-hydroxybenzoate formed was proportional to the amount of ATP available in the test system whereas CoA served as a real coenzyme. A novel ATP/coenzyme A dependent reaction mechanism for the dehalogenation of 4-chlorobenzoate by 4-chlorobenzoate dehalogenase from Pseudomonas sp. CBS3 is proposed.

Biodegradation of halogenated organic compounds

In this review we discuss the degradation of chlorinated hydrocarbons by microorganisms, emphasizing the physiological, biochemical, and genetic basis of the biodegradation of aliphatic, aromatic, and polycyclic compounds. Many environmentally important xenobiotics are halogenated, especially chlorinated. These compounds are manufactured and used as pesticides, plasticizers, paint and printing-ink components, adhesives, flame retardants, hydraulic and heat transfer fluids, refrigerants, solvents, additives for cutting oils, and textile auxiliaries. The hazardous chemicals enter the environment through production, commercial application, and waste. As a result of bioaccumulation in the food chain and groundwater contamination, they pose public health problems because many of them are toxic, mutagenic, or carcinogenic. Although synthetic chemicals are usually recalcitrant to biodegradation, microorganisms have evolved an extensive range of enzymes, pathways, and control mechanisms that are responsible for catabolism of a wide variety of such compounds. Thus, such biological degradation can be exploited to alleviate environmental pollution problems. The pathways by which a given compound is degraded are determined by the physical, chemical, and microbiological aspects of a particular environment. By understanding the genetic basis of catabolism of xenobiotics, it is possible to improve the efficacy of naturally occurring microorganisms or construct new microorganisms capable of degrading pollutants in soil and aquatic environments more efficiently. Recently a number of genes whose enzyme products have a broader substrate specificity for the degradation of aromatic compounds have been cloned and attempts have been made to construct gene cassettes or synthetic operons comprising these degradative genes. Such gene cassettes or operons can be transferred into suitable microbial hosts for extending and custom designing the pathways for rapid degradation of recalcitrant compounds. Recent developments in designing recombinant microorganisms and hybrid metabolic pathways are discussed.

Biotransformation of halogenated compounds

As a result of natural production and contamination of the environment by xenobiotic compounds, halogenated substances are widely distributed in the biosphere. Concern arises as a result of the toxic, carcinogenic, and potential teratogenic nature of these substances. The biotransformations of such halogenated substances are reviewed, with particular emphasis on the biocatalytic cleavage of the carbon-halogen bonds. The physiology, biochemistry, and genetics of the biological system involved in the dehalogenation reactions are discussed for three groups of organohalogens: (1) the haloacids, (2) the haloaromatics, and (3) the haloalkanes. Finally, the biotechnological applications of these microbial transformations are discussed. This includes prospects for their future application in biosynthetic processes for the synthesis of halogenated intermediates or novel compounds and also the use of such systems for the detoxification and degradation of environmental pollutants.