Anaerobic dehalogenation of hydroxylated polychlorinated biphenyls by Desulfitobacterium dehalogenans

Unlocking the potential of resuscitation-promoting factor for enhancing anaerobic microbial dechlorination of polychlorinated biphenyls

Microbial dechlorination of polychlorinated biphenyls (PCBs) is limited by the slow growth rate and low activity of dechlorinators. Resuscitation promoting factor (Rpf) of Micrococcus luteus, has been demonstrated to accelerate the enrichment of highly active PCB-dechlorinating cultures. However, it remains unclear whether the addition of Rpf can further improve the dechlorination performance of anaerobic dechlorination cultures. In this study, the effect of Rpf on the performance of TG4, an enriched PCB-dechlorinating culture obtained by Rpf amendment, for reductive dechlorination of four typical PCB congeners (PCBs 101, 118, 138, 180) was evaluated. The results indicated that Rpf significantly enhanced the dechlorination of the four PCB congeners, with residual mole percentages of PCBs 101, 118, 138 and 180 in Rpf-amended cultures being 16.2-29.31 %, 13.3-20.1 %, 11.9-14.4 % and 9.4-17.3 % lower than those in the corresponding cultures without Rpf amendment after 18 days of incubation. Different models were identified as appropriate for elucidating the dechlorination kinetics of distinct PCB congeners, and it was observed that the dechlorination rate constant is significantly influenced by the PCB concentration. The supplementing Rpf did not obviously change dechlorination metabolites, and the removal of chlorines occurred mainly at para- and meta- positions. Analysis of microbial community and functional gene abundance suggested that Rpf-amended cultures exhibited a significant enrichment of Dehalococcoides, Dehalogenimonas and Desulfitobacterium, as well as non-dechlorinators belonging to Desulfobacterota and Bacteroidetes. These findings highlight the potential of Rpf as an effective additive for enhancing PCB dechlorination, providing new insights into the survival of functional microorganisms involved in anaerobic reductive dechlorination.

Dioxin-like compounds formation mediated by Fe3+-montmorillonite: The substituent effects of halophenols

Toxic dioxin or/and dioxin-like compounds could be naturally formed from the reaction of halophenols on Fe³⁺-montmorillonite minerals under ambient conditions. Given that the toxicities and productions of dioxin or/and dioxin-like compounds are largely determined by the number, species, and position of the carried halogen atoms, it is necessary to explore the substituent effects on the reaction of halophenols with Fe³⁺-montmorillonite. Herein, Fe³⁺-montmorillonite catalyzed polymerizations of six halophenols were examined in a wide range of relative humidity (10%∼80%) using combinations of mass spectrometry identifications and density functional theory calculations. Results show that both the position and species of the substituents substantially impact the reaction rate, product species, and transformation pathways. In general, regardless of humidity ortho-substituted chlorophenols are more reactive than meta-substituted chlorophenols, which is also supported by the density functional theory calculations indicating that the ortho positions are more likely to be attacked. Regarding substituent species, bromophenols are slightly more reactive and also more easily affected by humidities than chlorophenols, which is due to the weaker electron absorbing ability of the bromine atom than the chlorine atom. Hydroxylated polyhalogenated diphenyl ethers are more frequently detected polymerization products, although hydroxylated polyhalogenated biphenyls are greater quantity of products. Overall, this study provides useful information for understanding the natural formation of dioxin or/and dioxin-like compounds mediated by clay minerals and underlying reaction mechanisms.

PCB dechlorination hotspots and reductive dehalogenase genes in sediments from a contaminated wastewater lagoon

Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants that are distributed worldwide. Although industrial PCB production has stopped, legacy contamination can be traced to several different commercial mixtures (e.g., Aroclors in the USA). Despite their persistence, PCBs are subject to naturally occurring biodegradation processes, although the microbes and enzymes involved are poorly understood. The biodegradation potential of PCB-contaminated sediments in a wastewater lagoon located in Virginia (USA) was studied. Total PCB concentrations in sediments ranged from 6.34 to 12,700 mg/kg. PCB congener profiles in sediment sample were similar to Aroclor 1248; however, PCB congener profiles at several locations showed evidence of dechlorination. The sediment microbial community structure varied among samples but was dominated by Proteobacteria and Firmicutes. The relative abundance of putative dechlorinating Chloroflexi (including Dehalococcoides sp.) was 0.01-0.19% among the sediment samples, with Dehalococcoides sp. representing 0.6-14.8% of this group. Other possible PCB dechlorinators present included the Clostridia and the Geobacteraceae. A PCR survey for potential PCB reductive dehalogenase genes (RDases) yielded 11 sequences related to RDase genes in PCB-respiring Dehalococcoides mccartyi strain CG5 and PCB-dechlorinating D. mccartyi strain CBDB1. This is the first study to retrieve potential PCB RDase genes from unenriched PCB-contaminated sediments.

Characterization of a Highly Enriched Microbial Consortium Reductively Dechlorinating 2,3-Dichlorophenol and 2,4,6-Trichlorophenol and the Corresponding cprA Genes from River Sediment

Anaerobic reductive dechlorination of 2,3-dichlorophenol (2,3DCP) and 2,4,6-trichlorophenol (2,4,6TCP) was investigated in microcosms from River Nile sediment. A stable sediment-free anaerobic microbial consortium reductively dechlorinating 2,3DCP and 2,4,6TCP was established. Defined sediment-free cultures showing stable dechlorination were restricted to ortho chlorine when enriched with hydrogen as the electron donor, acetate as the carbon source, and either 2,3-DCP or 2,4,6-TCP as electron acceptors. When acetate, formate, or pyruvate were used as electron donors, dechlorination activity was lost. Only lactate can replace dihydrogen as an electron donor. However, the dechlorination potential was decreased after successive transfers. To reveal chlororespiring species, the microbial community structure of chlorophenol-reductive dechlorinating enrichment cultures was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments. Eight dominant bacteria were detected in the dechlorinating microcosms including members of the genera Citrobacter, Geobacter, Pseudomonas, Desulfitobacterium, Desulfovibrio and Clostridium. Highly enriched dechlorinating cultures were dominated by four bacterial species belonging to the genera Pseudomonas, Desulfitobacterium, and Clostridium. Desulfitobacterium represented the major fraction in DGGE profiles indicating its importance in dechlorination activity, which was further confirmed by its absence resulting in complete loss of dechlorination. Reductive dechlorination was confirmed by the stoichiometric dechlorination of 2,3DCP and 2,4,6TCP to metabolites with less chloride groups and by the detection of chlorophenol RD cprA gene fragments in dechlorinating cultures. PCR amplified cprA gene fragments were cloned and sequenced and found to cluster with the cprA/pceA type genes of Dehalobacter restrictus.

Interactions of Gaseous 2-Chlorophenol with Fe3+-Saturated Montmorillonite and Their Toxicity to Human Lung Cells

The interactions of gaseous 2-chlorophenol with Fe³⁺-saturated montmorillonite particles in a gas-solid system were investigated to simulate the reactions of mineral dusts with volatile organic pollutants in the atmosphere. Results suggested that Fe³⁺-saturated montmorillonite mediated the dimerization of gaseous 2-chlorophenol to form hydroxylated polychlorinated biphenyl, hydroxylated polychlorinated diphenyl ether, and hydroxylated polychlorinated dibenzofuran. The toxicity of Fe³⁺-montmorillonite particles to A549 human lung epithelial cells before and after interaction with 2-chlorophenol was examined to explore their adverse impact on human health. Based on cell morphological analysis, cytotoxicity tests, and Fourier-transform infrared imaging spectra, surface-catalyzed reactions of Fe³⁺-montmorillonite with 2-chlorophenol increased the toxicity of montmorillonite particle on A549 cells. This was supported by increased cellular membrane permeability, the release of extracellular lactate dehydrogenase, and cell damages on cellular DNA, proteins, and lipids. Since mineral dusts are important components of particulate matter, our results help to understand the interactions of volatile organic pollutants with particulate matter in the atmosphere and their adverse impacts on human health.

Detoxification of hydroxylated polychlorobiphenyls by Sphingomonas sp. strain N-9 isolated from forest soil

To examine the biodegradation of hydroxylated polychlorobiphenyls (OH-PCBs), we isolated Sphingomonas sp. strain N-9 from forest soil using mineral salt medium containing 4-hydroxy-3-chlorobiphenyl (4OH-3CB) at the concentration of 10 mg/L. Following incubation with strain N-9, the concentration of 4OH-3CB decreased in inverse proportion to strain N-9 proliferation, and it was converted to 3-chloro-4-hydroxybenzoic acid (4OH-3CBA) after 1 day. We observed that strain N-9 efficiently degraded lowly chlorinated OH-PCBs (1-4 Cl), while highly chlorinated OH-PCBs (5-6 Cl) were less efficiently transformed. Additionally, strain N-9 degraded PCBs and OH-PCBs with similar efficiencies, and the efficiency of OH-PCB degradation was dependent upon the positional relationships between OH-PCB hydroxyl groups and chlorinated rings. OH-PCB biodegradation may result in highly toxic products, therefore, we evaluated the cytotoxicity of two OH-PCBs [4OH-3CB and 4-hydroxy-3,5-dichlorobiphenyl (4OH-3,5CB)] and their metabolites [4OH-3CBA and 3,5-chloro-4-hydroxybenzoic acid (4OH-3,5CBA)] using PC12 rat pheochromocytoma cells. Our results revealed that both OH-PCBs induced cell membrane damage and caused neuron-like elongations in a dose-dependent manner, while similar results were not observed for their metabolites. These results indicated that strain N-9 can convert OH-PCBs into chloro-hydroxybenzoic acids having lower toxicity.

Hydroxylated polychlorinated biphenyls in the environment: sources, fate, and toxicities

Hydroxylated polychlorinated biphenyls (OH-PCBs) are produced in the environment by the oxidation of PCBs through a variety of mechanisms, including metabolic transformation in living organisms and abiotic reactions with hydroxyl radicals. As a consequence, OH-PCBs have been detected in a wide range of environmental samples, including animal tissues, water, and sediments. OH-PCBs have recently raised serious environmental concerns because they exert a variety of toxic effects at lower doses than the parent PCBs and they are disruptors of the endocrine system. Although evidence about the widespread dispersion of OH-PCBs in various compartments of the ecosystem has accumulated, little is currently known about their biodegradation and behavior in the environment. OH-PCBs are, today, increasingly considered as a new class of environmental contaminants that possess specific chemical, physical, and biological properties not shared with the parent PCBs. This article reviews recent findings regarding the sources, fate, and toxicities of OH-PCBs in the environment.

Anaerobic naphthalene degradation by Gram-positive, iron-reducing bacteria

An anaerobic naphthalene-degrading culture (N49) was enriched with ferric iron as electron acceptor. A closed electron balance indicated the total oxidation of naphthalene to CO(2). In all growing cultures, the concentration of the presumed central metabolite of naphthalene degradation, 2-naphthoic acid, increased concomitantly with growth. The first metabolite of anaerobic methylnaphthalene degradation, naphthyl-2-methyl-succinic acid, was not identified in culture supernatants, which does not support a methylation to methylnaphthalene as the initial activation reaction of naphthalene, but rather a carboxylation, as proposed for other naphthalene-degrading cultures. Substrate utilization tests revealed that the culture was able to grow on 1-methyl-naphthalene, 2-methyl-naphthalene, 1-naphthoic acid or 2-naphthoic acid, whereas it did not grow on 1-naphthol, 2-naphthol, anthracene, phenanthrene, indane and indene. Terminal restriction fragment length polymorphism and 16S rRNA gene sequence analyses revealed that the microbial community of the culture was dominated by one bacterial microorganism, which was closely related (99% 16S sequence similarity) to the major organism in the iron-reducing, benzene-degrading enrichment culture BF [ISME J (2007) 1: 643; Int J Syst Evol Microbiol (2010) 60: 686]. The phylogenetic classification supports a new candidate species and genus of Gram-positive spore-forming iron-reducers that can degrade non-substituted aromatic hydrocarbons. It furthermore indicates that Gram-positive microorganisms might also play an important role in anaerobic polycyclic aromatic hydrocarbon-degradation.

Hydroxylated metabolites of 4-monochlorobiphenyl and its metabolic pathway in whole poplar plants

4-Monochlorobiphenyl (CB3), mainly an airborne pollutant, undergoes rapid biotransformation to produce hydroxylated metabolites (OH-CB3s). However, up to now, hydroxylation of CB3 has not been studied in living organisms. In order to explore the formation of hydroxylated metabolites of CB3 in whole plants, poplars (Populus deltoides x nigra, DN34) were exposed to CB3 for 10 days. Poplars are a model plant with complete genomic sequence, and they are widely used in phytoremediation. Results showed poplar plants can metabolize CB3 into OH-CB3s. Three monohydroxy metabolites, including 2'-hydroxy-4-chlorobiphenyl (2'OH-CB3), 3'-hydroxy-4-chlorobiphenyl (3'OH-CB3), and 4'-hydroxy-4-chlorobiphenyl (4'OH-CB3), were identified in hydroponic solution and in different parts of the poplar plant. The metabolite 4'OH-CB3 was the major product. In addition, there were two other unknown monohydroxy metabolites of CB3 found in whole poplar plants. Based on their physical and chemical properties, they are likely to be 2-hydroxy-4-chlorobiphenyl (2OH-CB3) and 3-hydroxy-4-chlorobiphenyl (3OH-CB3). Compared to the roots and leaves, the middle portion of the plant (the middle wood and bark) had higher concentrations of 2'OH-CB3, 3'OH-CB3, and 4'OH-CB3, which suggests that these hydroxylated metabolites of CB3 are easily translocated in poplars from roots to shoots. The total masses of 2'OH-CB3, 3'OH-CB3, and 4'OH-CB3 in whole poplar plants were much higher than those in solution, strongly suggesting that it is mainly the poplar plant itself which metabolizes CB3 to OH-CB3s. Finally, the data suggest that the metabolic pathway be via epoxide intermediates.

Biodegradation of aromatic compounds: current status and opportunities for biomolecular approaches

Biodegradation can achieve complete and cost-effective elimination of aromatic pollutants through harnessing diverse microbial metabolic processes. Aromatics biodegradation plays an important role in environmental cleanup and has been extensively studied since the inception of biodegradation. These studies, however, are diverse and scattered; there is an imperative need to consolidate, summarize, and review the current status of aromatics biodegradation. The first part of this review briefly discusses the catabolic mechanisms and describes the current status of aromatics biodegradation. Emphasis is placed on monocyclic, polycyclic, and chlorinated aromatic hydrocarbons because they are the most prevalent aromatic contaminants in the environment. Among monocyclic aromatic hydrocarbons, benzene, toluene, ethylbenzene, and xylene; phenylacetic acid; and structurally related aromatic compounds are highlighted. In addition, biofilms and their applications in biodegradation of aromatic compounds are briefly discussed. In recent years, various biomolecular approaches have been applied to design and understand microorganisms for enhanced biodegradation. In the second part of this review, biomolecular approaches, their applications in aromatics biodegradation, and associated biosafety issues are discussed. Particular attention is given to the applications of metabolic engineering, protein engineering, and "omics" technologies in aromatics biodegradation.

"Dehalococcoides" sp. strain CBDB1 extensively dechlorinates the commercial polychlorinated biphenyl mixture aroclor 1260

"Dehalococcoides" sp. strain CBDB1 in pure culture dechlorinates a wide range of PCB congeners with three to eight chlorine substituents. Congener-specific high-resolution gas chromatography revealed that CBDB1 extensively dechlorinated both Aroclor 1248 and Aroclor 1260 after four months of incubation. For example, 16 congeners comprising 67.3% of the total PCBs in Aroclor 1260 were decreased by 64%. We confirmed the dechlorination of 43 different PCB congeners. The most prominent dechlorination products were 2,3',5-chlorinated biphenyl (25-3-CB) and 24-3-CB from Aroclor 1248 and 235-25-CB, 25-25-CB, 24-25-CB, and 235-236-CB from Aroclor 1260. Strain CBDB1 removed flanked para chlorines from 3,4-, 2,4,5-, and 3,4,5-chlorophenyl rings, primarily para chlorines from 2,3,4,5-chlorophenyl rings, primarily meta chlorines from 2,3,4- and 2,3,4,6-chlorophenyl rings, and either meta or para chlorines from 2,3,4,5,6-chlorophenyl rings. The site of attack on the 2,3,4-chorophenyl ring was heavily influenced by the chlorine configuration on the opposite ring. This dechlorination pattern matches PCB Process H dechlorination, which was previously observed in situ both in the Acushnet Estuary (New Bedford, MA) and in parts of the Hudson River (New York). Accordingly, we propose that Dehalococcoides bacteria similar to CBDB1 are potential agents of Process H PCB dechlorination in the environment. This is the first time that a complex naturally occurring PCB dechlorination pattern has been reproduced in the laboratory using a single bacterial strain.

Dual roles of an essential cysteine residue in activity of a redox-regulated bacterial transcriptional activator

CprK from Desulfitobacterium dehalogenans is the first characterized transcriptional regulator of anaerobic dehalorespiration and is controlled at two levels: redox and effector binding. In the reduced state and in the presence of chlorinated aromatic compounds, CprK positively regulates expression of the cpr gene cluster. One of the products of the cpr gene cluster is CprA, which catalyzes the reductive dehalogenation of chlorinated aromatic compounds. Redox regulation of CprK occurs through a thiol/disulfide redox switch, which includes two classes of cysteine residues. Under oxidizing conditions, Cys11 and Cys200 form an intermolecular disulfide bond, whereas Cys105 and Cys111 form an intramolecular disulfide. Here, we report that Cys11 is involved in redox inactivation in vivo. Upon replacement of Cys11 with serine, alanine, or aspartate, CprK loses its DNA binding activity. C11A is unstable; however, circular dichroism studies demonstrate that the stability and overall secondary structures of CprK and the C11S and C11D variants are similar. Furthermore, effector binding remains intact in the C11S and C11D variants. However, fluorescence spectroscopic results reveal that the tertiary structures of the C11S and C11D variants differ from that of the wild type protein. Thus, Cys11 plays a dual role as a redox switch and in maintaining the correct tertiary structure that promotes DNA binding.

Pathways for the anaerobic microbial debromination of polybrominated diphenyl ethers

The debromination pathways of seven polybrominated diphenyl ethers (PBDEs) by three different cultures of anaerobic dehalogenating bacteria were investigated using comprehensive two-dimensional gas chromatography (GC x GC). The congeners analyzed were the five major components of the industrially used octa-BDE mixture (octa-BDEs 196, 203, and 197, hepta-BDE 183, and hexa-BDE 153) as well as the two most commonly detected PBDEs in the environment, penta-BDE 99 and tetra-BDE 47. Among the dehalogenating cultures evaluated in this study were a trichloroethene-enriched consortium containing multiple Dehalococcoides species, and two pure cultures, Dehalobacter restrictus PER-K23 and Desulfitobacterium hafniense PCP-1. PBDE samples were analyzed by GC x GC coupled to an electron capture detector to maximize separation and identification of the product congeners. All studied congeners were debrominated to some extent by the three cultures and all exhibited similar debromination pathways with preferential removal of para and meta bromines. Debromination of the highly brominated congeners was extremely slow, with usually less than 10% of nM concentrations of PBDEs transformed after three months. In contrast, debromination of the lesser brominated congeners, such as penta 99 and tetra 47, was faster, with some cultures completely debrominating nM levels of tetra 47 within weeks.

The Desulfitobacterium genus

Desulfitobacterium spp. are strictly anaerobic bacteria that were first isolated from environments contaminated by halogenated organic compounds. They are very versatile microorganisms that can use a wide variety of electron acceptors, such as nitrate, sulfite, metals, humic acids, and man-made or naturally occurring halogenated organic compounds. Most of the Desulfitobacterium strains can dehalogenate halogenated organic compounds by mechanisms of reductive dehalogenation, although the substrate spectrum of halogenated organic compounds varies substantially from one strain to another, even with strains belonging to the same species. A number of reductive dehalogenases and their corresponding gene loci have been isolated from these strains. Some of these loci are flanked by transposition sequences, suggesting that they can be transmitted by horizontal transfer via a catabolic transposon. Desulfitobacterium spp. can use H2 as electron donor below the threshold concentration that would allow sulfate reduction and methanogenesis. Furthermore, there is some evidence that syntrophic relationships occur between Desulfitobacterium spp. and sulfate-reducing bacteria, from which the Desulfitobacterium cells acquire their electrons by interspecies hydrogen transfer, and it is believed that this relationship also occurs in a methanogenic consortium. Because of their versatility, desulfitobacteria can be excellent candidates for the development of anaerobic bioremediation processes. The release of the complete genome of Desulfitobacterium hafniense strain Y51 and information from the partial genome sequence of D. hafniense strain DCB-2 will certainly help in predicting how desulfitobacteria interact with their environments and other microorganisms, and the mechanisms of actions related to reductive dehalogenation.

Transcriptional Activation of Dehalorespiration

Desulfitobacterium dehalogenans can use chlorinated aromatics including polychlorinated biphenyls as electron acceptors in a process called dehalorespiration. Expression of the cpr gene cluster involved in this process is regulated by CprK, which is a member of the CRP/FNR (cAMP-binding protein/fumarate nitrate reduction regulatory protein) family of helix-turn-helix transcriptional regulators. High affinity interaction of the chlorinated aromatic compound with the effector domain of CprK triggers binding of CprK to an upstream target DNA sequence, which leads to transcriptional activation of the cpr gene cluster. When incubated with oxygen or diamide, CprK undergoes inactivation; subsequent treatment with dithiothreitol restores activity. Using mass spectrometry, this study identifies two classes of redox-active thiol groups that form disulfide bonds upon oxidation. Under oxidative conditions, Cys105, which is conserved in FNR and most other CprK homologs, forms an intramolecular disulfide bond with Cys111, whereas an intermolecular disulfide bond is formed between Cys11 and Cys200. SDS-PAGE and site-directed mutagenesis experiments indicate that the Cys11/Cys200 disulfide bond links two CprK subunits in an inactive dimer. Isothermal calorimetry and intrinsic fluorescence quenching studies show that oxidation does not change the affinity of CprK for the effector. Therefore, reversible redox inactivation is manifested at the level of DNA binding. Our studies reveal a strategy for limiting expression of a redox-sensitive pathway by using a thiol-based redox switch in the transcription factor.

Occurrence and expression ofcrdAandcprA5encoding chloroaromatic reductive dehalogenases inDesulfitobacteriumstrains

Desulfitobacterium hafniense PCP-1 (formerly frappieri PCP-1) has two reductive dehalogenases (RDases) that have been characterized. One is a membrane-associated 2,4,6-trichlorophenol RDase, which is encoded by crdA, and the other is a 3,5-dichlorophenol RDase encoded by cprA5. In this report, we determined the occurrence of these two RDase genes in seven other Desulfitobacterium strains. The presence or absence of these two RDases may explain the differences in the spectrum of halogenated compounds by these Desulfitobacterium strains. crdA gene sequences were found in all of the tested strains. It was expressed in strain PCP-1 regardless of the absence or presence of chlorophenols in the culture medium. crdA was also expressed in D. hafniense strains DCB-2 and TCE-1. cprA5 was detected only in D. hafniense strains PCP-1, TCP-A, and DCB-2. In these strains, cprA5 transcripts were detected only in the presence of chlorophenols. We also examined the expression of putative cprA RDases (cprA2, cprA3, and cprA4) that were shown to exist in the D. hafniense DCB-2 genome. RT-PCR experiments showed that cprA2, cprA3, and cprA4 were expressed in D. hafniense strains PCP-1, DCB-2, and TCP-A in the presence of chlorophenols. However, contrary to cprA5, these three genes were also expressed in the absence of halogenated compounds in the culture medium.Key words: reductive dehalogenase, Desulfitobacterium, gene family, gene expression.

Biodegradation of xenobiotics by anaerobic bacteria

Xenobiotic biodegradation under anaerobic conditions such as in groundwater, sediment, landfill, sludge digesters and bioreactors has gained increasing attention over the last two decades. This review gives a broad overview of our current understanding of and recent advances in anaerobic biodegradation of five selected groups of xenobiotic compounds (petroleum hydrocarbons and fuel additives, nitroaromatic compounds and explosives, chlorinated aliphatic and aromatic compounds, pesticides, and surfactants). Significant advances have been made toward the isolation of bacterial cultures, elucidation of biochemical mechanisms, and laboratory and field scale applications for xenobiotic removal. For certain highly chlorinated hydrocarbons (e.g., tetrachlorethylene), anaerobic processes cannot be easily substituted with current aerobic processes. For petroleum hydrocarbons, although aerobic processes are generally used, anaerobic biodegradation is significant under certain circumstances (e.g., O(2)-depleted aquifers, oil spilled in marshes). For persistent compounds including polychlorinated biphenyls, dioxins, and DDT, anaerobic processes are slow for remedial application, but can be a significant long-term avenue for natural attenuation. In some cases, a sequential anaerobic-aerobic strategy is needed for total destruction of xenobiotic compounds. Several points for future research are also presented in this review.

Anaerobic microbial dehalogenation

The natural production and anthropogenic release of halogenated hydrocarbons into the environment has been the likely driving force for the evolution of an unexpectedly high microbial capacity to dehalogenate different classes of xenobiotic haloorganics. This contribution provides an update on the current knowledge on metabolic and phylogenetic diversity of anaerobic microorganisms that are capable of dehalogenating--or completely mineralizing--halogenated hydrocarbons by fermentative, oxidative, or reductive pathways. In particular, research of the past decade has focused on halorespiring anaerobes, which couple the dehalogenation by dedicated enzyme systems to the generation of energy by electron transport-driven phosphorylation. Significant advances in the biochemistry and molecular genetics of degradation pathways have revealed mechanistic and structural similarities between dehalogenating enzymes from phylogenetically distinct anaerobes. The availability of two almost complete genome sequences of halorespiring isolates recently enabled comparative and functional genomics approaches, setting the stage for the further exploitation of halorespiring and other anaerobic dehalogenating microbes as dedicated degraders in biological remediation processes.

Regulation of anaerobic dehalorespiration by the transcriptional activator CprK

Desulfomonile, Desulfitobacterium, and Dehalobacter are anaerobic microbes that can derive energy from the reductive dehalogenation of chlorinated organic compounds, many of which are environmental pollutants. There is very little information about how anaerobic dehalorespiration is regulated. An open reading frame within the Desulfitobacterium dehalogenans chlorophenol reductase (cpr) gene cluster (cprK) was proposed to be a transcriptional regulatory protein (Smidt, H., van Leest, M., van der Oost, J., and deVos, W. M. (2000) J. Bacteriol. 182, 5683-5691). We have cloned, actively overexpressed in Escherichia coli, and purified to homogeneity the D. dehalogenans CprK. The results of electrophoretic mobility shift assays, DNA footprinting studies, and promoter-lac fusion experiments indicate that CprK is a transcriptional activator of the cpr gene cluster. CprK binds 3-chloro-4-hydroxyphenylacetate (CHPA) with high affinity (K(d) = 3.5 mum, determined by isothermal titration calorimetry), which promotes its specific interaction with a DNA sequence (TTAAT-N4-ACTAA) located upstream of the -35 and -10 promoter regions of several cpr genes and activates transcription of these genes. Binding to the upstream "box" sequence increases the affinity of CprK for CHPA by approximately 10-fold (K(d) = 0.4 mum, determined by electrophoretic mobility shift assays). Chlorophenylacetate, which lacks the ortho-hydroxy group, and hydroxyphenylacetate, lacking the chlorine group, do not activate transcription or promote DNA binding, even at millimolar concentrations, at least 1000-fold higher than the K(d) value for CHPA. Lacking metals, CprK is oxygen-sensitive. Oxidation by diamide, which converts thiols to the disulfide, inactivates CprK, and reduction of the oxidized protein by dithiothreitol fully restores DNA binding, indicating that CprK is redox-regulated and is active only when reduced. This is the first reported characterization of a transcriptional regulator of anaerobic dehalorespiration.

Fungal laccase-catalyzed degradation of hydroxy polychlorinated biphenyls

Hydroxy polychlorinated biphenyls (hydroxy PCBs) are toxic metabolites of PCBs. Their toxicity such as strong endocrine disruption demands effective remediation methods. Laccases from Trametes versicolor and Pleurotus ostreatus were tested to degrade hydroxy PCBs. Optimum pHs for both enzymes were around 4.0. Laccase from T. versicolor degrades hydroxy PCBs more rapidly than that from P. ostreatus. The enzymatic activities remained little changes in up to 10% organic solvents, but decreased rapidly in more than 10% acetone, acetonitrile or DMSO. Degradation rate constants decreased with increase of chlorination and no degradation was observed with tetra-, penta- and hexa-chloro hydroxy PCBs in non-mediated reactions. However, the tetra- to hexa-chloro hydroxy PCBs were degraded by laccase from T. versicolor in the presence of the mediator 2,2,6,6-tetramethylpiperidine-N-oxy radical. The other mediators, 4-ethyl-2-methoxyphenol, 2,2'-azino-bis(3-ethylbenzthiazoline sulfonic acid) diammonium salt and 1-hydroxybenzotriazole and humic acid, also enhanced degradation of all the hydroxy PCBs except 4-hydroxy-2',3,3',4',5,5'-hexachlorobiphenyl. The results showed that 3-hydroxy biphenyl was more resistant to laccase degradation than 2- or 4-hydroxy analogues. Significant linear-correlations (coefficient of determination, r2 = 0.9097 and 0.8186 for laccases from P. ostreatus and T. versicolor, respectively) were found between the ionization potentials and the removal rate constants of hydroxy PCBs.

Epigenetic toxicity of hydroxylated biphenyls and hydroxylated polychlorinated biphenyls on normal rat liver epithelial cells

2-Biphenylol, 3-biphenylol, 2,2'-biphenyldiol, 3,3'-biphenyldiol, 3-chloro-2-biphenylol, and 4,4'-dichloro-3-biphenylol were evaluated using the scrape-loading/dye transfer (SL/ DT) technique to determine in vitro gap junctional intercellular communication (GJIC) in a normal rat liver epithelial cell line as a measure of the epigenetic toxicity. Cytotoxicity was determined using the neutral red uptake assay. A dose range of 0-300 microM was examined. Only 3,3'-biphenyldiol and 4,4'-dichloro-3-biphenylol induced cytotoxicity within the tested dose ranges. Noncytotoxic doses were selected for evaluation of epigenetic toxicity. 4,4'-Dichloro-3-biphenylol was most inhibitory to GJIC at the lowest dose. The cytotoxicity and GJIC inhibitory effects observed for 4,4'-dichloro-3-biphenylol might be, although not exclusively, a consequence of the lipophilic nature of this chemical. 3-Chloro-2-biphenylol was least inhibitory to GJIC. 3-Chloro-2-biphenylol was less inhibitory to GJIC than 2-biphenylol because of the presence of the chlorine functional group, which appears to attenuate the toxic effect of the ortho-hydroxyl group. Although cells were capable of complete recovery of GJIC after removal of each of the chemicals, only with 2,2'-biphenyldiol and 4,4,'-dichloro-3-biphenylol did the cells demonstrate partial recovery without removal of the chemical. The more noncoplanar conformation of 2,2'-biphenyldiol and 2-biphenylol might explain their more inhibitory behavior in comparison to 3,3'-biphenyldiol and 3-biphenylol, respectively.

The Many Faces of Vitamin B12: Catalysis by Cobalamin-Dependent Enzymes

Vitamin B12 is a complex organometallic cofactor associated with three subfamilies of enzymes: the adenosylcobalamin-dependent isomerases, the methylcobalamin-dependent methyltransferases, and the dehalogenases. Different chemical aspects of the cofactor are exploited during catalysis by the isomerases and the methyltransferases. Thus, the cobalt-carbon bond ruptures homolytically in the isomerases, whereas it is cleaved heterolytically in the methyltransferases. The reaction mechanism of the dehalogenases, the most recently discovered class of B12 enzymes, is poorly understood. Over the past decade our understanding of the reaction mechanisms of B12 enzymes has been greatly enhanced by the availability of large amounts of enzyme that have afforded detailed structure-function studies, and these recent advances are the subject of this review.

Environmental ice photochemistry: monochlorophenols

Photolysis of 2- and 4-chlorophenol samples in water ice of the initial concentrations 10(-7) to 10(-2) mol L(-1) is reported. Major phototransformations appeared to be based on the coupling reactions due to chlorophenol aggregation at the grain boundaries of the polycrystalline state. The main products, chlorobiphenyldiols, belong to the family of phenolic halogenated compounds (such as hydroxylated polychlorobiphenyls) that are known xenobiotics found in nature. No photosolvolysis products, that is products from intermolecular reactions between organic and water molecules, were observed at temperatures below -10 degrees C. Raising the temperature to -5 degrees C caused a moderate photosolvolytic activity in the case of 4-chlorophenol (formation of hydroquinone), in contrast to 2-chlorophenol which was almost exclusively transformed into pyrocatechol. It is suggested that photosolvolysis above this temperature occurs in a liquid or quasi-liquid layer that covers the ice crystal surfaces. The results support our model in which significant amounts of some persistent, bioaccumulative, and toxic compounds may be generated by photochemistry of primary pollutants in cold ecosystems and in the upper atmosphere, and may be subsequently released to the environment.

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.

Sequential anaerobic-aerobic treatment of soil contaminated with weathered Aroclor 1260

Soil contaminated with weathered Aroclor 1260 was bioremediated by sequential anaerobic and aerobic laboratory-scale treatment. The initial concentration was 59 microg of PCBs/g of soil. Following 4 months of anaerobic treatment with an enrichment culture, all of the major components in Aroclor 1260 were completely or partially transformed to less chlorinated PCB congeners. The major products of reductive dechlorination were 24-24-tetrachlorobiphenyl and 24-26-tetrachlorobiphenyl, and the average chlorine substituents per PCB molecule decreased from 6.4 to 5.2. The molar concentration of PCBs did not decrease during the anaerobic treatment. All of the major products formed during the anaerobic treatmentwere degraded in the subsequent aerobic treatment using Burkholderia sp. strain LB400. After 28 days of the aerobic treatment, the concentration of PCBs was reduced to 20 ug/g of soil. PCBs were not significantly removed in aerobic treatments unless they were bioaugmented with LB400. Also, PCB degradation was not detected in soil bioaugmented with LB400 without prior anaerobic treatment. These results confirm the potential for extensive biological destruction of highly chlorinated, weathered PCB congeners in soil.

Characterization of the B12- and iron-sulfur-containing reductive dehalogenase from Desulfitobacterium chlororespirans

The United Nations and the U.S. Environmental Protection Agency have identified a variety of chlorinated aromatics that constitute a significant health and environmental risk as "priority organic pollutants," the so-called "dirty dozen." Microbes have evolved the ability to utilize chlorinated aromatics as terminal electron acceptors in an energy-generating process called dehalorespiration. In this process, a reductive dehalogenase (CprA), couples the oxidation of an electron donor to the reductive elimination of chloride. We have characterized the B12 and iron-sulfur cluster-containing 3-chloro-4-hydroxybenzoate reductive dehalogenase from Desulfitobacterium chlororespirans. By defining the substrate and inhibitor specificity for the dehalogenase, the enzyme was found to require an hydroxyl group ortho to the halide. Inhibition studies indicate that the hydroxyl group is required for substrate binding. The carboxyl group can be replaced by other functionalities, e.g. acetyl or halide groups, ortho or meta to the chloride to be eliminated. The purified D. chlororespirans enzyme could dechlorinate an hydroxylated PCB (3,3',5,5'-tetrachloro-4,4'-biphenyldiol) at a rate about 1% of that with 3-chloro-4-hydroxybenzoate. Solvent deuterium isotope effect studies indicate that transfer of a single proton is partially rate-limiting in the dehalogenation reaction.

Transcriptional regulation of the cpr gene cluster in ortho-chlorophenol-respiring Desulfitobacterium dehalogenans

To characterize the expression and possible regulation of reductive dehalogenation in halorespiring bacteria, a 11.5-kb genomic fragment containing the o-chlorophenol reductive dehalogenase-encoding cprBA genes of the gram-positive bacterium Desulfitobacterium dehalogenans was subjected to detailed molecular characterization. Sequence analysis revealed the presence of eight designated genes with the order cprTKZEBACD and with the same polarity except for cprT. The deduced cprC and cprK gene products belong to the NirI/NosR and CRP-FNR families of transcription regulatory proteins, respectively. CprD and CprE are predicted to be molecular chaperones of the GroEL type, whereas cprT may encode a homologue of the trigger factor folding catalysts. Northern blot analysis, reverse transcriptase PCR, and primer extension analysis were used to elucidate the transcriptional organization and regulation of the cpr gene cluster. Results indicated halorespiration-specific transcriptional induction of the monocistronic cprT gene and the biscistronic cprBA and cprZE genes. Occasional read-through at cprC gives rise to a tetracistronic cprBACD transcript. Transcription of cprBA was induced 15-fold upon addition of the o-chlorophenolic substrate 3-chloro-4-hydroxyphenylacetic acid within 30 min with concomitant induction of dehalogenation activity. Putative regulatory protein binding motifs that to some extent resemble the FNR box were identified in the cprT-cprK and cprK-cprZ intergenic regions and the promoter at cprB, suggesting a role for FNR-like CprK in the control of expression of the cprTKZEBACD genes.

Microbial reductive dehalogenation of polychlorinated biphenyls

Under anaerobic conditions, microbial reductive dechlorination of polychlorinated biphenyls (PCBs) occurs in soils and aquatic sediments. In contrast to dechlorination of supplemented single congeners for which frequently ortho dechlorination has been observed, reductive dechlorination mainly attacks meta and/or para chlorines of PCB mixtures in contaminated sediments, although in a few instances ortho dechlorination of PCBs has been observed. Different microorganisms appear to be responsible for different dechlorination activities and the occurrence of various dehalogenation routes. No axenic cultures of an anaerobic microorganism have been obtained so far. Most probable number determinations indicate that the addition of PCB congeners, as potential electron acceptors, stimulates the growth of PCB-dechlorinating microorganisms. A few PCB-dechlorinating enrichment cultures have been obtained and partially characterized. Temperature, pH, availability of naturally occurring or of supplemented carbon sources, and the presence or absence of H(2) or other electron donors and competing electron acceptors influence the dechlorination rate, extent and route of PCB dechlorination. We conclude from the sum of the experimental data that these factors influence apparently the composition of the active microbial community and thus the routes, the rates and the extent of the dehalogenation. The observed effects are due to the specificity of the dehalogenating bacteria which become active as well as changing interactions between the dehalogenating and non-dehalogenating bacteria. Important interactions include the induced changes in the formation and utilization of H(2) by non-dechlorinating and dechlorinating bacteria, competition for substrates and other electron donors and acceptors, and changes in the formation of acidic fermentation products by heterotrophic and autotrophic acidogenic bacteria leading to changes in the pH of the sediments.