Anaerobic degradation of halogenated phenols by sulfate-reducing consortia

A comprehensive review of chlorophenols: Fate, toxicology and its treatment

Chlorophenols represent one of the most abundant families of toxic pollutants emerging from various industrial manufacturing units. The toxicity of these chloroderivatives is proportional to the number and position of chlorine atoms on the benzene ring. In the aquatic environment, these pollutants accumulate in the tissues of living organisms, primarily in fishes, inducing mortality at an early embryonic stage. Contemplating the behaviour of such xenobiotics and their prevalence in different environmental components, it is crucial to understand the methods used to remove/degrade the chlorophenol from contaminated environment. The current review describes the different treatment methods and their mechanism towards the degradation of these pollutants. Both abiotic and biotic methods are investigated for the removal of chlorophenols. Chlorophenols are either degraded through photochemical reactions in the natural environment, or microbes, the most diverse communities on earth, perform various metabolic functions to detoxify the environment. Biological treatment is a slow process because of the more complex and stable structure of pollutants. Advanced Oxidation Processes are effective in degrading such organics with enhanced rate and efficiency. Based on their ability to generate hydroxyl radicals, source of energy, catalyst type, etc., different processes such as sonication, ozonation, photocatalysis, and Fenton's process are discussed for the treatment or remediation efficiency towards the degradation of chlorophenols. The review entails both advantages and limitations of treatment methods. The study also focuses on reclamation of chlorophenol-contaminated sites. Different remediation methods are discussed to restore the degraded ecosystem back in its natural condition.

External potential regulated biocathode for enhanced removal of gaseous chlorobenzene in bioelectrchemical system

Microbial electrolysis cell (MEC) with a biocathode could provide extra reaction driving force for gaseous chlorobenzene (CB) removal. In this work, external potentials (-0.1 to -0.7 V vs. SHE) were applied to regulate the biocathodic activity. Results showed -0.3 V was the optimum potential, while the removal efficiency, dechlorination efficiency and Coulombic efficiency achieved 94%, 65%, and 89%, respectively. Electrochemical stimulation enriched dechlorination microorganisms (Achromobacter and Gordonia), and significantly improved CB mineralization efficiency, which was twice higher than that without additional potential at 300 mg m^-3 inlet concentration. Furthermore, electron transfer between biocathode and microorganisms was mainly through direct electron transfer (DET). A new integrated redox pathway for CB anaerobic degradation was proposed, in which CB was sequentially converted into 2-chlorophenol and 3-chlorocatechol, then dechlorinated to catechol, and finally mineralized into CO₂. Overall, this work provided an insight into gaseous CB bioelectrochemical degradation through the potential regulation.

Magnetite drives self-dechlorination of 4-chlorophenol in anoxic aquatic sediments

The lack of available electron donors is well known as a major factor limiting the efficiency of microbial dechlorination of 4-chlorophenol (4-CP) in anoxic aquatic sediments. Considering that Fe(III) minerals largely contained in sediments can especially enrich Fe(III)-reducing bacteria and unlock the ring-like intermediates produced by dechlorination of 4-CP via dissimilatory Fe(III) reduction, a strategy of self-dechlorination of 4-CP utilizing its metabolism intermediates such as short-fatty acids (SCFAs) as the endogenous electron donors with magnetite was proposed in this study. The results showed that the removal efficiency of 4-CP increased by 156-203% in magnetite-supplemented biotic groups compared with the magnetite-free biotic group. Liquid chromatography-mass spectrometer (LC-MS) and gas chromatography (GC) revealed the possible metabolic pathway of anoxic 4-CP degradation with magnetite: 4-CP→phenol→cyclohexene-1-carboxylic acid→2-hydroxycyclohexanecarboxylic acid→hexanoic acid/valeric acid→butyric/propionic acids→CO₂. High-throughput sequencing analysis showed that the abundance of functional bacteria, Desulfuromonas, Pseudomonas and Bacillus species, were increased by 1.38-1.97, 1.50-2.04, and 11.60-17.18 folds in magnetite-supplemented biotic groups, compared with the magnetite-free biotic groups. Analysis of Fe²⁺ concentration and cyclic voltammetry (CV) suggested that the potential Fe(III)/Fe(II) transformation occurred and proceeded the anoxic 4-CP degradation continuously.

Evolution properties and dechlorination capacities of particulate organic matter from a landfill

Particulate organic matter (POM) includes humin and non-degradable residues, and the knowledge about its composition, evolution and environmental behavior is limited. The composition, evolution and its influence on dechlorination of the POM in landfill was studied. The results show that POM accounts for 27 %-57 % of the organic matter in landfill cell, which is mainly composed of protein-, fulvic- and humic-like components. Firmicutes and Proteobacteria were the main microorganisms driving the compositional evolution of POM during the landfilling process. The electron acceptance capacities (EAC) and electron donating capacities (EDC) of POM were in the range of 0.05-0.51 μmol/gC^-1 and 0.13-0.66 μmol/gC^-1, respectively, and the average EAC and EDC of POM in the intermediate and old stage of landfill were higher than those in the initial stage. The combined action of MR-1 and POM increased the degradation rate of PCP by 20 %-40 %, which was ascribed to the reduction capacities and electron transfer process of POM. POM derived from the intermediate and old stages promoted PCP dechlorination more effectively when compared with the initial stage due to its high electron transfer capacities (ETC), which are of great significance for soil in-situ bioremediation.

Genome-Guided Identification of Organohalide-Respiring Deltaproteobacteria from the Marine Environment

The marine environment is a major reservoir for both anthropogenic and natural organohalides, and reductive dehalogenation is thought to be an important process in the overall cycling of these compounds. Here we demonstrate that the capacity of organohalide respiration appears to be widely distributed in members of marine Deltaproteobacteria. The identification of reductive dehalogenase genes in diverse Deltaproteobacteria and the confirmation of their dehalogenating activity through functional assays and transcript analysis in select isolates extend our knowledge of organohalide-respiring Deltaproteobacteria diversity. The presence of functional reductive dehalogenase genes in diverse Deltaproteobacteria implies that they may play an important role in organohalide respiration in the environment.

Organohalide compounds are widespread in the environment as a result of both anthropogenic activities and natural production. The marine environment, in particular, is a major reservoir of organohalides, and reductive dehalogenation is thought to be an important process in the overall cycling of these compounds. Deltaproteobacteria are important members of the marine microbiota with diverse metabolic capacities, and reductive dehalogenation has been observed in some Deltaproteobacteria. In this study, a comprehensive survey of Deltaproteobacteria genomes revealed that approximately 10% contain reductive dehalogenase (RDase) genes, which are found within a common gene neighborhood. The dehalogenating potential of select RDase A-containing Deltaproteobacteria and their gene expression were experimentally verified. Three Deltaproteobacteria strains isolated from marine environments representing diverse species, Halodesulfovibrio marinisediminis, Desulfuromusa kysingii, and Desulfovibrio bizertensis, were shown to reductively dehalogenate bromophenols and utilize them as terminal electron acceptors in organohalide respiration. Their debrominating activity was not inhibited by sulfate or elemental sulfur, and these species are either sulfate- or sulfur-reducing bacteria. The analysis of RDase A gene transcripts indicated significant upregulation induced by 2,6-dibromophenol. This study extends our knowledge of the phylogenetic diversity of organohalide-respiring bacteria and their functional RDase A gene diversity. The identification of reductive dehalogenase genes in diverse Deltaproteobacteria and confirmation of their organohalide-respiring capability suggest that Deltaproteobacteria play an important role in natural organohalide cycling.

Chlorinated phenol treatment and in situ hydrogen peroxide production in a sulfate-reducing bacteria enriched bioelectrochemical system

Wastewaters are increasingly being considered as renewable resources for the sustainable production of electricity, fuels, and chemicals. In recent years, bioelectrochemical treatment has come to light as a prospective technology for the production of energy from wastewaters. In this study, a bioelectrochemical system (BES) enriched with sulfate-reducing bacteria (SRB) in the anodic chamber was proposed and evaluated for the biodegradation of recalcitrant chlorinated phenol, electricity generation (in the microbial fuel cell (MFC)), and production of hydrogen peroxide (H₂O₂) (in the microbial electrolysis cell (MEC)), which is a very strong oxidizing agent and often used for the degradation of complex organics. Maximum power generation of 253.5 mW/m², corresponding to a current density of 712.0 mA/m², was achieved in the presence of a chlorinated phenol pollutant (4-chlorophenol (4-CP) at 100 mg/L (0.78 mM)) and lactate (COD of 500 mg/L). In the anodic chamber, biodegradation of 4-CP was not limited to dechlorination, and further degradation of one of its metabolic products (phenol) was observed. In MEC operation mode, external voltage (0.2, 0.4, or 0.6 V) was added via a power supply, with 0.4 V producing the highest concentration of H₂O₂ (13.3 g/L-m² or 974 μM) in the cathodic chamber after 6 h of operation. Consequently, SRB-based bioelectrochemical technology can be applied for chlorinated pollutant biodegradation in the anodic chamber and either net current or H₂O₂ production in the cathodic chamber by applying an optimum external voltage.

Penta- and 2,4,6-tri-chlorophenol biodegradation during municipal solid waste anaerobic digestion

In this study isotopic tracing using (13)C labelled pentachlorophenol (PCP) and 2,4,6-trichlorophenol (2,4,6-TCP) is proposed as a tool to distinguish the loss of PCP and 2,4,6-TCP due to biodegradation from other physical processes. This isotopic approach was applied to accurately assess in situ PCP and 2,4,6-TCP degradation under methanogenic conditions in several microcosms made up of household waste. These microcosms were incubated in anaerobic conditions at 35°C (mesophilic) and 55°C (thermophilic) without agitation. The volume of biogas produced (CH4 and CO2), was followed for a period of 130 days. At this stage of stable methanogenesis, (13)C6-PCP and (13)C6-2,4,6-TCP were introduced anaerobically in microcosms and its monitoring at mesophilic and thermophilic conditions was performed in parallel by gas chromatography mass spectrometry (GC-MS) and gas chromatography isotope-ratio mass spectrometry (GC-IRMS). This study proved the almost total dechlorination of bioavailable PCP and 2,4,6-TCP into 4-CP at 35°C. Nevertheless, high rate adsorption in particular materials of the two compounds was observed. Furthermore, Carbon-13 Nuclear Magnetic Resonance ((13)C-NMR) Spectroscopy analysis of (13)C labelled 2,4,6-TCP mesophilic incubations showed the partial mineralization of 4-CP at 35°C to acetate and then to HCO(3-). Consequently, NMR results confirm the biogas isotopic results indicating the mineralization of (13)C labelled 2,4,6-TCP into (13)C (CH4 and CO2). Concerning (13)C labelled PCP mesophilic incubations, the isotopic composition of the biogas still natural until the day 262. In contrast, no dechlorination was observed at 55°C. Thus PCP and 2,4,6-TCP were persistent in thermophilic conditions.

9-N-Alkylaminomethylanthracene probes for selective fluorescence sensing of pentafluorophenol

9-N-Alkylaminomethylanthracenes (1 and 2) were synthesised for halophenol sensing, and their selectivity and sensitivity towards pentafluorophenol (PFP) in ethanol were investigated.

Anaerobic 4-chlorophenol mineralization in an enriched culture under iron-reducing conditions

We enriched an anaerobic, soil-free 4-chlorophenol (4-CP)-degrading culture under iron-reducing conditions. The [ring-(14)C(U)]4-CP tracer experiment showed that 65 μM 4-CP mineralized to CO2 and CH4 through phenol, 4-hydroxybenzoate, and benzoate intermediates over 60 days. 16S rRNA gene analyses suggested the involvement of Dehalobacterium in the 4-CP dechlorination in the culture.

Bacterial degradation of chlorophenols and their derivatives

Chlorophenols (CPs) and their derivatives are persistent environmental pollutants which are used in the manufacture of dyes, drugs, pesticides and other industrial products. CPs, which include monochlorophenols, polychlorophenols, chloronitrophenols, chloroaminophenols and chloromethylphenols, are highly toxic to living beings due to their carcinogenic, mutagenic and cytotoxic properties. Several physico-chemical and biological methods have been used for removal of CPs from the environment. Bacterial degradation has been considered a cost-effective and eco-friendly method of removing CPs from the environment. Several bacteria that use CPs as their sole carbon and energy sources have been isolated and characterized. Additionally, the metabolic pathways for degradation of CPs have been studied in bacteria and the genes and enzymes involved in the degradation of various CPs have been identified and characterized. This review describes the biochemical and genetic basis of the degradation of CPs and their derivatives.

Hydrogen sulfide production by sulfate-reducing bacteria utilizing additives eluted from plastic resins

In the present study it was demonstrated that organic additives eluted from plastic resins could be utilized as substrates by sulfate-reducing bacteria. Two laboratory-scale experiments, a microcosm experiment and a leaching experiment, were conducted using polyvinyl chloride (PVC) as a model plastic resin. In the former experiment, the conversion of sulfate to sulfide was evident in microcosms that received plasticized PVC as the sole carbon source, but not in those that received PVC homopolymer. Additionally, dissolved organic carbon accumulated only in microcosms that received plasticized PVC, indicating that the dissolved organic carbon originated from additives. In the leaching experiment, phenol and bisphenol A were found in the leached solutions. These results suggest that the disposal of waste plastics in inert waste landfills may result in the production of H(2)S.

Performance and microbial community analysis of two-stage process with extreme thermophilic hydrogen and thermophilic methane production from hydrolysate in UASB reactors

The two-stage process for extreme thermophilic hydrogen and thermophilic methane production from wheat straw hydrolysate was investigated in up-flow anaerobic sludge bed (UASB) reactors. Specific hydrogen and methane yields of 89 ml-H(2)/g-VS (190 ml-H(2)/g-sugars) and 307 ml-CH(4)/g-VS, respectively were achieved simultaneously with the overall VS removal efficiency of 81% by operating with total hydraulic retention time (HRT) of 4 days . The energy conversion efficiency was dramatically increased from only 7.5% in the hydrogen stage to 87.5% of the potential energy from hydrolysate, corresponding to total energy of 13.4 kJ/g-VS. Dominant hydrogen-producing bacteria in the H(2)-UASB reactor were Thermoanaerobacter wiegelii, Caldanaerobacter subteraneus, and Caloramator fervidus. Meanwhile, the CH(4)-UASB reactor was dominated with methanogens of Methanosarcina mazei and Methanothermobacter defluvii. The results from this study suggest the two stage anaerobic process can be effectively used for energy recovery and for stabilization of hydrolysate at anaerobic conditions.

A review of anaerobic treatment of saline wastewater

Large volumes of saline (> 2% w/v NaCl) wastewaters are discharged from many industries; e.g. seafood processing, textile dyeing, oil and gas production, tanneries and drinking water treatment processes. Although anaerobic treatment would be the most cost-effective and sustainable technology for the treatment of many of these saline wastewaters, the salinity is considered to be inhibitory to anaerobic biological treatment processes. The recent applications of salt-tolerant cultures for the treatment of wastewaters from seafood processing and ion-exchange processes suggest that biological systems can be used to treat salty wastewaters. Additionally, organisms capable of anaerobic degradation of contaminants in saline solutions have been observed in marine sediments and have been characterized during the last two decades. This manuscript provides a review of the recent research on anaerobic treatment of saline wastewater and bacterial consortia capable of the anaerobic degradation of pollutants in saline solutions, documenting that the biological treatment of saline wastewaters is promising.

Reductive dechlorination of chlorophenols in estuarine sediments of Lake Shinji and Lake Nakaumi

Dechlorination of all mono- and dichlorophenol isomers in anaerobic sediment samples of estuarine Lake Shinji and Lake Nakaumi was examined to characterize the chlorophenol-dechlorinating microbial communities in the environments with different salinity levels. Dechlorination was observed only in 2-chlorophenol (2-CP), 3-chlorophenol (3-CP) and 2,6-dichlorophenol (2,6-DCP), and in 2-CP and 2,6-DCP in the Lake Shinji and Nakaumi sediment, respectively. In the sediment of Lake Shinji, the highest activity was observed without adding sodium chloride and sulfate, whereas in the Lake Nakaumi sediment, the highest activity was at 0.7 % of sodium chloride and 6.0 mM of sodium sulfate. The chlorophenols were degraded to benzoate via phenol in both sediments under methanogenic conditions. Benzoate then disappeared from the cultures. All microbial consortia enriched with each monochlorophenol dechlorinated 2-CP, but showed different substrate specificities for dichlorophenols as follows: 2-CP-enriched consortium dechlorinated 2,3-dichlorophenol and 2,6-DCP, 3-CP-enriched consortium dechlorinated all dichlorophenol isomers, and 4-chlorophenol-enriched consortium dechlorinated 2,4-dichlorophenol and 2,6-DCP. Maintenance of the population by halorespiration was suggested in the dechlorination of 2-CP.

Radioiodination of humic substances

The known IODO-GEN™-method [1] was adapted for radiolabeling of humic and fulvic acids with131I. The water insoluble oxidizing agent 1,3,4,6tetrachloro-3α,6α-diphenylglycoluril (IODO-GEN™) forms an iodous ion species (I+), which undergoes an electrophilic I/H-substitution on aromatic moieties of the humic and fulvic acids. This method offers mild conditions with a lesser extent of oxidative alterations of the target molecule, accompanied by an easy handling due to the virtual water-insolubility of the oxidizing agent. The method was optimized and different techniques were tested for the purification of the radioiodinated humic material. The yield of the labeling procedure varies between 45 and 75% depending on the provenance of the humic material and the applied purification method. A specific activity up to 40 MBq/mg was achieved. Furthermore, the known inherent photo-susceptibility of the iodinated humic substance and the influence of reducing agents were verified. An additional release of131I up to 20% and up to 35%, respectively were observed.

Fe(III)-enhanced anaerobic transformation of 2,4-dichlorophenoxyacetic acid by an iron-reducing bacterium Comamonas koreensis CY01

This work studied the ability of Comamonas koreensis CY01 to reduce Fe(III) (hydr)oxides by coupling the oxidation of electron donors and the enhanced biodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by the presence of Fe(III) (hydr)oxides. The experimental results suggested that strain CY01 can utilize ferrihydrite, goethite, lepidocrocite or hematite as the terminal electron acceptor and citrate, glycerol, glucose or sucrose as the electron donor. Strain CY01 could transform 2,4-D to 4-chlorophenol through reductive side-chain removal and dechlorination. Under the anaerobic conditions, Fe(III) reduction and 2,4-D biodegradation by strain CY01 occurred simultaneously. The presence of Fe(III) (hydr)oxides would significantly enhance 2,4-D biodegradation, probably due to the fact that the reactive mineral-bound Fe(II) species generated from Fe(III) reduction can abiotically reduce 2,4-D. This is the first report of a strain of C. koreensis capable of reducing Fe(III) (hydr)oxides and 2,4-D, which extends the diversity of iron-reducing bacteria associated with dechlorination.

An integrated anaerobic/aerobic bioprocess for the remediation of chlorinated phenol-contaminated soil and groundwater

An investigation of biodegradation of chlorinated phenol in an anaerobic/aerobic bioprocess environment was made. The reactor configuration used consisted of linked anaerobic and aerobic reactors, which served as a model for a proposed bioremediation strategy. The proposed strategy was studied in two reactors before linkage. In the anaerobic compartment, the transformation of the model contaminant, 2,4,6-trichlorophenol (2,4,6-TCP), to lesser-chlorinated metabolites was shown to occur during reductive dechlorination under sulfate-reducing conditions. The consortium was also shown to desorb and mobilize 2,4,6-TCP in soils. This was followed, in the aerobic compartment, by biodegradation of the pollutant and metabolites, 2,4-dichlorophenol, 4-chlorophenol, and phenol, by immobilized white-rot fungi. The integrated process achieved elimination of the compound by more than 99% through fungal degradation of metabolites produced in the dechlorination stage. pH correction to the anaerobic reactor was found to be necessary because acidic effluent from the fungal reactor inhibited sulfate reduction and dechlorination.

Activity of Desulfitobacterium sp. strain Viet1 demonstrates bioavailability of 2,4-dichlorophenol previously sequestered by the aquatic plant Lemna minor

Aquatic plants take up and sequester organic contaminants such as chlorophenols through incorporation in cell wall materials and storage in vacuoles. The ultimate fate of plant-sequestered chlorophenols, however, remains unclear. This research investigated 2,4-dichlorophenol (2,4-DCP) sequestration by the aquatic plant Lemna minor and evaluated contaminant release and bioavailability after plant death and cellular disruption. 14C-labeled 2,4-DCP was used to establish that contaminant removed from the aqueous phase was retained internal to L. minor. An assay with Desulfitobacterium sp. strain Viet1 was used to assess the readily bioavailable fraction of plant-sequestered 2,4-DCP and plant metabolites of 2,4-DCP. In plant-free systems, strain Viet1 dechlorinated 2,4-DCP to stoichiometric amounts of 4-chlorophenol (4-CP) as a stable and quantifiable end product. Anaerobic microcosms containing inactivated L. minor, which had accumulated 3.8 micromol of 2,4-DCP equivalents/g of plant material (fresh weight) during a preceding aerobic exposure, were inoculated with strain Viet1. After 118 d of incubation with strain Viet1, 43.5% (+/-1.4%) of the contaminant was recovered as 4-CP, indicating a large portion of plant-sequestered 2,4-DCP was bioavailable for dechlorination by strain Viet1. In contrast, 4-CP formation was not observed in autoclaved microcosms, and only 26.1% (+/-1.0%) of plant-sequestered 2,4-DCP was recovered in the aqueous phase. These findings demonstrate contaminant cycling between plants and microorganisms, and emphasize that understanding the mechanisms and pathways of contaminant sequestration by plants is critical for predicting long-term contaminant fate.

Metabolites detected during biodegradation of 13C6-benzene in nitrate-reducing and methanogenic enrichment cultures

The mechanism for anaerobic metabolism of benzene remains unknown. To date, there have been only a few studies reporting metabolites of anaerobic benzene biodegradation, in part because anaerobic benzene-degrading enrichment cultures are not very common and only two isolates have been characterized to date. Phenol and benzoate, metabolites consistent with benzene hydroxylation or benzene carboxylation, have been identified previously in mixed cultures, and more recently benzene methylation to toluene has been proposed as another possible mechanism for anaerobic benzene degradation. In this study, 13C6-benzene was added to nitrate-reducing and methanogenic enrichment cultures and specific 13C-labeled metabolites were monitored over time. The putative metabolites were detected by gas chromatography/mass spectrometry in ether extracts of 100-mL samples of culture taken at each time point. This method of analysis provided the sensitivity required to accurately quantify low concentrations of these compounds. In addition, benzoate trapping was used in an attemptto increase concentrations of upstream metabolites. In both cultures, in the presence and absence of unlabeled benzoate (trap), [ring-13C]-toluene and [ring-13C]benzoate were detected transiently during degradation. The data strongly support initial methylation of benzene to toluene, followed bytransformation to benzoate. Although benzene methylation has been proposed previously, this is the first direct evidence to supportthis pathway. In the methanogenic culture only, 13C6-phenol was also detected. The transient appearance of phenol, which appeared to be further transformed to benzoate, suggests that a pathway involving hydroxylation to phenol, as proposed in other studies, was also operative.

UASB performance for bleached and unbleached kraft pulp synthetic wastewater treatment

This paper presents the performance of two bench scale Upflow Anaerobic Sludge Blanket reactors (UASB) used for the treatment of synthetic substrates that simulate bleached and unbleached cellulose pulp plant wastewaters. The control reactor was fed with diluted black liquor and the treatment reactor with the same substrate plus a mixture of chlorinated organics. The total concentration of the chlorinated compounds was gradually increased from 2.5 mg l-1 to 15.0 mg l-1. The average COD removal efficiency during the last phase of the experiment was 81% in the control reactor and 76% in the treatment reactor. These results indicate the capability of UASB reactors to treat this kind of wastewater and the low impact of the chlorinated organics on COD removal efficiency. The minimum overall chlorinated organics removal efficiency was 71% and the maximum was 99.7%. The experiment indicates that under the conditions used in this research the presence of chlorinated organics does not negatively impact the treatment process.

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.

Dehalogenation of chlorinated aromatic compounds using a hybrid bioinorganic catalyst on cells of Desulfovibrio desulfuricans

A novel bioinorganic catalyst was obtained via reduction of Pd(II) to Pd0 on to the surface of cells of Desulfovibrio desulfuricans at the expense of H2. Palladised biomass, supplied with formate or H2 as an electron donor, catalysed the dehalogenation of 2-chlorophenol and polychlorinated biphenyls. In the example of 2,3,4,5-tetrachlorobiphenyl, the bioinorganic catalyst promoted a rate of chloride release of 9.33 +/- 0.17 nmol min(-1) mg (-1) and only approximately 5% of this value was obtained using chemically reduced or commercially available Pd0. In the case of 2,2',4,4',6,6'-hexachlorobiphenyl the rate was more than four orders of magnitude faster than the degradation reported using a sulfidogenic culture. Negligible chloride release occurred from any of the chloroaromatic compounds using biomass alone, or from palladised biomass challenged with hexane carrier solvent only. Analysis of the spent solution showed that in addition to catalysis of reductive dehalogenation the new material was able to remove very effectively the organic residua, with neither any PCB nor any breakdown products identifiable by GC/MS.

Genomic insights into gene regulation of Desulfovibrio vulgaris Hildenborough

Traditional laboratory studies of the sulfate-reducing bacteria have focused primarily on the biochemistry of the organisms. As genomic sequences of sulfate-reducing species have become available, insights have been gained into the metabolic and regulatory networks of these organisms. A computational analysis is reported of the transcriptional regulatory networks of Desulfovibrio vulgaris Hildenborough, the first mesophilic gram-negative sulfate-reducing bacterium for which a genome sequence is available. A set of conserved DNA motifs were derived from libraries of potential promoter regions of putative D. vulgaris regulons with the AlignACE program suite. Although one motif showed similarity to the Escherichia coli GlpR binding site, most of the motifs returned were apparently unique. A number of expected orthologs for regulatory proteins have not yet been recognized in D. vulgaris.

Degradation of 2-chlorophenol via a hydrogenotrophic biofilm under different reductive conditions

This research studies the 2-chlorophenol (2-CP) degradation by the hydrogenotrophic biofilm cultivated in three silicone-tube membrane bioreactors under the conditions of denitrification (DN), sulfate-reduction (DS) and dechlorination (DC). Experimental results showed that after acclimation for more than four months with 2-CP, the respective 2-CP removal efficiency was 95% in DN, 94% in DS and 95% in DC reactors, under the condition of influent 2-CP 25 mg/l with hydraulic retention time (HRT) of 15 h. The metabolic pathway of 2-CP was different in different reactors. The 2-CP was thought to be utilized as carbon and energy source in DN and DS reactors, while the dechlorination occurred in the DC reactor in lack of nitrate and sulfate. The pH dramatically affected the 2-CP degradation in all reactors. Experimental results showed that the optimal pH range was around 6+/-0.2 in DN, 7+/-0.2 in DS, and 5.8-7.2 in DC reactors. Both nitrate and sulfate inhibited the 2-CP dechlorination, but the inhibition levels were different. Nitrate completely inhibited the dechlorination at once, while sulfate took a longer time to reach complete inhibition, only after the bacteria were adapted to the sulfate-reducing condition. Both inhibitions were accomplished by taking the place of 2-CP as electron acceptors. H2 served as an electron donor for dechlorination of 2-CP. The dechlorination was apparently stopped when lacking H2 and another pathway was responsible for the 2-CP degradation.

Detection and characterization of a dehalogenating microorganism by terminal restriction fragment length polymorphism fingerprinting of 16S rRNA in a sulfidogenic, 2-bromophenol-utilizing enrichment

Terminal restriction fragment length polymorphism analysis of reverse-transcribed 16S rRNA during periods of community flux was used as a tool to delineate the roles of the members of a 2-bromophenol-degrading, sulfate-reducing consortium. Starved, washed cultures were amended with 2-bromophenol plus sulfate, 2-bromophenol plus hydrogen, phenol plus sulfate, or phenol with no electron acceptor and were monitored for substrate use. In the presence of sulfate, 2-bromophenol and phenol were completely degraded. In the absence of sulfate, 2-bromophenol was dehalogenated and phenol accumulated. Direct terminal restriction fragment length polymorphism fingerprinting of the 16S rRNA in the various subcultures indicated that phylotype 2BP-48 (a Desulfovibrio-like sequence) was responsible for the dehalogenation of 2-bromophenol. A stable coculture was established which contained predominantly 2BP-48 and a second Desulfovibrio-like bacterium (designated BP212 based on terminal restriction fragment length polymorphism fingerprinting) that was capable of dehalogenating 2-bromophenol to phenol. Strain 2BP-48 in the coculture could couple reductive dehalogenation to growth with 2-bromophenol, 2,6-dibromophenol, or 2-iodophenol and lactate or formate as the electron donor. In addition to halophenols, strain 2BP-48 appears to use sulfate, sulfite, and thiosulfate as electron acceptors and is capable of simultaneous sulfidogenesis and reductive dehalogenation in the presence of sulfate.

Dehalococcoides ethenogenes strain 195 reductively dechlorinates diverse chlorinated aromatic pollutants

Dehalococcoides ethenogenes strain 195 dechlorinates tetrachloroethene to vinyl chloride and ethene, and its genome has been found to contain up to 17 putative dehalogenase gene homologues, suggesting diverse dehalogenation ability. We amended pure or mixed cultures containing D. ethenogenes strain 195 with 1,2,3,4-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3-dichlorodibenzo-p-dioxin, 1,2,3,4-tetrachlorodibenzofuran, 2,3,4,5,6-pentachlorobiphenyl, 1,2,3,4-tetrachloronaphthalene, various chlorobenzenes, or a mixture of 2-, 3-, and 4-chlorophenol to determine the dehalogenation ability. D. ethenogenes strain 195 dechlorinated 1,2,3,4-tetrachlorodibenzo-p-dioxin to a mixture of 1,2,4-trichlorodibenzo-p-dioxin and 1,3-dichlorodibenzo-p-dioxin. 2,3,4,5,6- Pentachlorobiphenyl was dechlorinated to 2,3,4,6- and/or 2,3,5,6-tetrachlorobiphenyl and 2,4,6-trichlorobiphenyl. 1,2,3,4-Tetrachloronaphthalene was dechlorinated primarily to an unidentified dichloronaphthalene congener. The predominant end products from hexachlorobenzene dechlorination were 1,2,3,5-tetrachlorobenzene and 1,3,5-trichlorobenzene. We did not detect dechlorination daughter products from monochlorophenols, 2,3-dichlorodibenzo-p-dioxin or 2,3,7,8- tetrachlorodibenzo-p-dioxin. D. ethenogenes strain 195 has the ability to dechlorinate many different types of chlorinated aromatic compounds, in addition to its known chloroethene respiratory electron acceptors. Remediation of sediments contaminated with aromatic halogenated organic pollutants such as polychlorinated biphenyls and polychlorinated dibenzo-p-dioxins could require billions of dollars in the coming years. Harnessing microorganisms such as Dehalococcoides spp. that detoxify these compounds via removal of halogens may lead to cost-effective biotechnological approaches for remediation.

Reductive dehalogenation of brominated phenolic compounds by microorganisms associated with the marine sponge Aplysina aerophoba

Marine sponges are natural sources of brominated organic compounds, including bromoindoles, bromophenols, and bromopyrroles, that may comprise up to 12% of the sponge dry weight. Aplysina aerophoba sponges harbor large numbers of bacteria that can amount to 40% of the biomass of the animal. We postulated that there might be mechanisms for microbially mediated degradation of these halogenated chemicals within the sponges. The capability of anaerobic microorganisms associated with the marine sponge to transform haloaromatic compounds was tested under different electron-accepting conditions (i.e., denitrifying, sulfidogenic, and methanogenic). We observed dehalogenation activity of sponge-associated microorganisms with various haloaromatics. 2-Bromo-, 3-bromo-, 4-bromo-, 2,6-dibromo-, and 2,4,6-tribromophenol, and 3,5-dibromo-4-hydroxybenzoate were reductively debrominated under methanogenic and sulfidogenic conditions with no activity observed in the presence of nitrate. Monochlorinated phenols were not transformed over a period of 1 year. Debromination of 2,4,6-tribromophenol, and 2,6-dibromophenol to 2-bromophenol was more rapid than the debromination of the monobrominated phenols. Ampicillin and chloramphenicol inhibited activity, suggesting that dehalogenation was mediated by bacteria. Characterization of the debrominating methanogenic consortia by using terminal restriction fragment length polymorphism (TRFLP) and denaturing gradient gel electrophoresis analysis indicated that different 16S ribosomal DNA (rDNA) phylotypes were enriched on the different halogenated substrates. Sponge-associated microorganisms enriched on organobromine compounds had distinct 16S rDNA TRFLP patterns and were most closely related to the delta subgroup of the proteobacteria. The presence of homologous reductive dehalogenase gene motifs in the sponge-associated microorganisms suggested that reductive dehalogenation might be coupled to dehalorespiration.

Potential for anaerobic conversion of xenobiotics

This review covers the latest research on the anaerobic biodegradation of aromatic xenobiotic compounds, with emphasis on surfactants, polycyclic aromatic hydrocarbons, phthalate esters, polychlorinated biphenyls, halogenated phenols, and pesticides. The versatility of anaerobic reactor systems regarding the treatment of xenobiotics is shown with the focus on the UASB reactor, but the applicability of other reactor designs for treatment of hazardous waste is also included. Bioaugmentation has proved to be a viable technique to enhance a specific activity in anaerobic reactors and recent research on reactor and in situ bioaugmentation is reported.

Anaerobic Dehalogenation of Organohalide Contaminants in the Marine Environment

Microbially mediated dehalogenation processes contribute to the global cycling of both biogenic and anthropogenic halogenated organic compounds. Detailed information on biodegradation mechanisms for a variety of organohalides and on the microorganisms mediating these processes has greatly increased our understanding of the cycling and fate of these unique and widespread compounds in our environment. The marine environment appears to be a particularly rich source of dehalogenating microorganisms. It is well established by laboratory and field studies that anaerobic dehalogenation of sediment contaminants, such as PCBs, pesticides, and dioxins, occurs intrinsically and can be enhanced via various methods. Specific dehalogenating bacterial populations can be enriched on various organohalides. Biodehalogenation processes are likely to be significantly affected by the prevailing terminal electron-accepting condition, and thus, biotransformation of organohalide contaminants in marine and estuarine environments will vary as a function of the redox conditions within the sediment profile. Fundamental knowledge of the activities and interactions of dehalogenating microorganisms is providing a strong basis for development of new bioremediation technologies for removal of harmful halogenated compounds from our environment.

Anaerobic biotransformation of tetrabromobisphenol A, tetrachlorobisphenol A, and bisphenol A in estuarine sediments

Biotransformation of the flame retardants tetrabromobisphenol A and tetrachlorobisphenol A, and their ultimate biodehalogenation product, bisphenol A, was examined in anoxic estuarine sediments. Dehalogenation of tetrabromobisphenol A and tetrachlorobisphenol A was examined under conditions promoting either methanogenesis or sulfate reduction as the primary terminal electron-accepting process. Complete dehalogenation of tetrabromobisphenol A to bisphenol A with no further degradation of bisphenol A, was observed under both methanogenic and sulfate-reducing conditions. Dehalogenation of tetrachlorobisphenol A under both methanogenic and sulfate-reducing conditions resulted in the accumulation of a persistent dichlorinated bisphenol A isomer, while no bisphenol A was formed. Co-amendment of sediment enrichments with either 2,6-dibromo- or 2,6-dichlorophenol did not affect the extent of dehalogenation as compared to sediments that were amended only with the flame retardants. Sediment cultures pre-acclimated on 2-bromophenol dehalogenated the flame retardants in a manner similar to that of fresh sediments. No loss of bisphenol A was observed in separate incubations within 162 days under conditions promoting either methanogenesis, sulfate-reduction, iron(III)-reduction, or nitrate-reduction. Furthermore, identical enrichments that readily degraded 4-hydroxybenzoate, a structural analogue of bisphenol A, did not exhibit bisphenol A degradation. The dehalogenation of tetrabromo- and tetrachlorobisphenol A and the potential for accumulation of bisphenol A in anoxic sediments is significant given the widespread use of these chemicals.

Evidence for degradation of 2-chlorophenol by enrichment cultures under denitrifying conditions

Although chlorophenol (CP) degradation has been studied, no bacterium responsible for degradation of CP under denitrifying conditions has been isolated. Moreover, little substantial evidence for anaerobic degradation of CPs coupled with denitrification is available even for mixed cultures. Degradation of CP [2-CP, 3-CP, 4-CP, 2,4-dichlorophenol (DCP) or 2,6-DCP] under denitrifying conditions was examined in anaerobic batch culture inoculated with activated sludge. Although 3-CP, 4-CP, 2,4-DCP and 2,6-DCP were not stably degraded, 2-CP was degraded and its degradation capability was sustained in a subculture. However, the rate of 2-CP degradation was not significantly enhanced by subculturing. In 2-CP-degrading cultures, nitrate was consumed stoichiometrically and concomitantly during 2-CP degradation, and a dechlorination intermediate was not detected, suggesting that 2-CP degradation was coupled with nitrate reduction. A 2-CP-degrading enrichment culture degraded 2-CP in the presence of nitrate, but did not in the absence of nitrate or the presence of sulfate. This suggests that the enrichment culture strictly requires nitrate for degradation of 2-CP. The apparent specific growth rate of the 2-CP degrading species was 0.0139 d(-1). Thus the apparent doubling time of the 2-CP-degrading population in the enrichment culture was greater than 50 d, which may explain difficulty in enrichment and isolation of micro-organisms responsible for CP degradation under denitrifying conditions.

Anaerobic-aerobic process for microbial degradation of tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is a flame retardant that is used as an additive during manufacturing of plastic polymers and electronic circuit boards. Little is known about the fate of this compound in the environment. In the current study we investigated biodegradation of TBBPA, as well as 2,4,6-tribromophenol (TBP), in slurry of anaerobic sediment from a wet ephemeral desert stream bed contaminated with chemical industry waste. Anaerobic incubation of the sediment with TBBPA and peptone-tryptone-glucose-yeast extract medium resulted in a 80% decrease in the TBBPA concentration and accumulation of a single metabolite. This metabolite was identified by gas chromatography-mass spectrometry (GC-MS) as nonbrominated bisphenol A (BPA). On the other hand, TBP was reductively dehalogenated to phenol, which was further metabolized under anaerobic conditions. BPA persisted in the anaerobic slurry but was degraded aerobically. A gram-negative bacterium (strain WH1) was isolated from the contaminated soil, and under aerobic conditions this organism could use BPA as a sole carbon and energy source. During degradation of BPA two metabolites were detected in the culture medium, and these metabolites were identified by GC-MS and high-performance liquid chromatography as 4-hydroxybenzoic acid and 4-hydroxyacetophenone. Both of those compounds were utilized by WH1 as carbon and energy sources. Our findings demonstrate that it may be possible to use a sequential anaerobic-aerobic process to completely degrade TBBPA in contaminated soils.

Anaerobic decomposition of halogenated aromatic compounds

Halogenated compounds constitute one of the largest groups of environmental pollutants, partly as a result of their widespread use as biocides, solvents and other industrial chemicals. A critical step in degradation of organohalides is the cleavage of the carbon?halogen bond. Reductive dehalogenation is generally the initial step in metabolism under methanogenic conditions, which requires a source of reducing equivalents, with the halogenated compound serving as an electron acceptor. Dehalogenation is greatly influenced by alternate electron acceptors; e.g. sulfate frequently inhibits reductive dehalogenation. On the other hand, a number of halogenated aromatic compounds can be degraded under different electron-accepting conditions and their complete oxidation to CO(2) can be coupled to processes such as denitrification, iron(III)-reduction, sulfate reduction and methanogenesis. Reductive dehalogenation was the initial step in degradation not only under methanogenic, but also under sulfate- and iron(III)-reducing conditions. Dehalogenation rates were in general slower under sulfidogenic and iron-reducing conditions, suggesting that dehalogenation was affected by the electron acceptor. The capacity for dehalogenation appears to be widely distributed in anoxic environments; however, the different substrate specificities and activities observed for the halogenated aromatic compounds suggest that distinct dehalogenating microbial populations are enriched under the different reducing conditions. Characterization of the microbial community structure using a combination of biomolecular techniques, such as cellular fatty acid profiling, and 16 S rRNA fingerprinting/sequence analysis, was used to discern the distinct populations enriched with each substrate and under each electron-accepting condition. These combined techniques will aid in identifying the organisms responsible for dehalogenation and degradation of halogenated aromatic compounds.

Effect of Specific Inhibitors on the Anaerobic Reductive Dechlorination of 2,4,6-Trichlorophenol by a Stable Methanogenic Consortium

The transformation of 2,4,6-trichlorophenol (TCP) into 4-chlorophenol (4CP) was studied using a stable methanogenic enrichment culture derived from an anaerobic fixed bed reactor. Using acetate as a growth substrate, different inhibitors of methanogenesis exhibited distinct effects on TCP dechlorination. Whereas reductive dechlorination activity was not affected by 2% ethylene in the gas phase, 25 mM bromoethanesulfonic acid (BESA) had a direct inhibitory effect on this process. The choice of BESA as a specific inhibitor for identifying the subpopulations involved in reductive dechlorination of chloroaromatics is thus questionable. Inhibitors of sulfate reduction such as molybdate (20 mM) and selenate (20 mM) had a direct inhibitory effect on reductive dechlorination independently of the presence of sulfate in the medium supplemented with acetate as growth substrate. Consequently much more care must also be taken with these inhibitors to prove that reductive chlorination is coupled to sulfate reduction.

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

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

Dehalogenation and biodegradation of brominated phenols and benzoic acids under iron-reducing, sulfidogenic, and methanogenic conditions

The anaerobic biodegradation of monobrominated phenols and benzoic acids by microorganisms enriched from marine and estuarine sediments was determined in the presence of different electron acceptors [i.e., Fe(III), SO4(2-), or HCO3-]. Under all conditions tested, the bromophenol isomers were utilized without a lengthy lag period whereas the bromobenzoate isomers were utilized only after a lag period of 23 to 64 days. 2-Bromophenol was debrominated to phenol, with the subsequent utilization of phenol under all three reducing conditions. Debromination of 3-bromophenol and 4-bromophenol was also observed under sulfidogenic and methanogenic conditions but not under iron-reducing conditions. In the bromobenzoate-degrading cultures, no intermediates were observed under any of the conditions tested. Debromination rates were higher under methanogenic conditions than under sulfate-reducing or iron-reducing conditions. The stoichiometric reduction of sulfate or Fe(III) and the utilization of bromophenols and phenol indicated that biodegradation was coupled to sulfate or iron reduction, respectively. The production of phenol as a transient intermediate demonstrates that reductive dehalogenation is the initial step in the biodegradation of bromophenols under iron- and sulfate-reducing conditions.

Degradation of Monochlorinated and Nonchlorinated Aromatic Compounds under Iron-Reducing Conditions

The capacity for Fe(sup3+) to serve as an electron acceptor in the microbial degradation of monochlorinated and nonchlorinated aromatic compounds was investigated in anoxic sediment enrichments. The substrates tested included phenol, benzoate, aniline, their respective monochlorinated isomers, o-, m-, and p-cresol, and all six dimethylphenol isomers. Phenol and 2-, 3-, and 4-chlorophenol were utilized by anaerobic microorganisms, with the concomitant reduction of Fe(sup3+) to Fe(sup2+). The amount of Fe(sup2+) produced in the enrichments was 89 to 138% of that expected for the stoichiometric degradation of these substrates to CO(inf2), suggesting complete mineralization at the expense of Fe reduction. Under Fe-reducing conditions, there was initial loss of benzoate and 3-chlorobenzoate but not of 2- or 4-chlorobenzoate. In addition, there was initial microbial utilization of aniline but not of the chloroaniline isomers. There was also initial loss of o-, m-, and p-cresol in our enrichments. None of the dimethylphenol isomers, however, was degraded within 300 days. Furthermore, we tested the capacity of an Fe-reducing, benzoate-grown culture of Geobacter metallireducens GS-15 to utilize monochlorinated benzoates and phenols. G. metallireducens was able to degrade benzoate and phenol but none of their chlorinated isomers, suggesting that the degradation of chlorophenols in our sediment enrichments may be due to novel Fe-reducing organisms that have yet to be isolated.