Utilization and cooxidation of chlorinated phenols by Pseudomonas sp. B 13

Enhanced Bioremediation of 4-Chlorophenol by Electrically Neutral Reactive Species Generated from Nonthermal Atmospheric-Pressure Plasma

4-Chlorophenol (4-CP) is a chlorinated aromatic compound with broad industrial applications. It is released into the environment as an industrial byproduct and is highly resistant to biodegradation. Pseudomonas sp. in the environment and activated sludge are used for 4-CP bioremediation; however, the degradation of 4-CP takes a long time. Consequently, the toxicity of 4-CP is a major barrier to its bioremediation. In this study, we investigated the synergistic effect of electrically neutral reactive species on the bacterial bioremediation of 4-CP. Our results showed that the concentration of 4-CP decreased from 2.0 to 0.137 mM and that it was converted to 4-chlorocatechol (4-CC; 0.257 mM), 4-chlororesorcinol (0.157 mM), hydroquinone (0.155 mM), and trihydroxy chlorobenzene and their respective ring-cleaved products following irradiation of neutral reactive species. These compounds were less toxic than 4-CP, except for 4-CC, which reduced the toxicity of 4-CP to Pseudomonas putida. When the neutral reactive species-treated 4-CP fraction was added to P. putida cultured in a synthetic sewage medium for 48 h, the 4-CP concentration was reduced to 0.017 mM, whereas nontreated 4-CP (2.0 mM) was hardly degraded by P. putida. These results suggest that the biodegradation of 4-CP can be efficiently improved by combining irradiation of neutral reactive species with microbial treatment. The irradiation of neutral reactive species of environmental pollutants may additionally lead to further improvements in bioremediation processes.

Challenges and opportunities for the biodegradation of chlorophenols: Aerobic, anaerobic and bioelectrochemical processes

Chlorophenols (CPs) are highly toxic and refractory contaminants which widely exist in various environments and cause serious harm to human and environment health and safety. This review provides comprehensive information on typical CPs biodegradation technologies, the most green and benign ones for CPs removal. The known aerobic and anaerobic degradative bacteria, functional enzymes, and metabolic pathways of CPs as well as several improving methods and critical parameters affecting the overall degradation efficiency are systematically summarized and clarified. The challenges for CPs mineralization are also discussed, mainly including the dechlorination of polychlorophenols (poly-CPs) under aerobic condition and the ring-cleavage of monochlorophenols (MCPs) under anaerobic condition. The coupling of functional materials and degraders as well as the operation of sequential anaerobic-aerobic bioreactors and bioelectrochemical system (BES) are promising strategies to overcome some current limitations. Future perspective and research gaps in this field are also proposed, including the further understanding of microbial information and the specific role of materials in CPs biodegradation, the potential application of innovative biotechnologies and new operating modes to optimize and maximize the function of the system, and the scale-up of bioreactors towards the efficient biodegradation of CPs.

Identification of the upstream 4-chlorophenol biodegradation pathway using a recombinant monooxygenase from Arthrobacter chlorophenolicus A6

This study aimed to clarify the initial 4-chlorophenol (4-CP) biodegradation pathway promoted by a two-component flavin-diffusible monooxygenase (TC-FDM) consisting of CphC-I and CphB contained in Arthrobacter chlorophenolicus A6 and the decomposition function of CphC-I. The TC-FDM genes were cloned from A. chlorophenolicus A6, and the corresponding enzymes were overexpressed. Since CphB was expressed in an insoluble form, Fre, a flavin reductase obtained from Escherichia coli, was used. These enzymes were purified using Ni²⁺-NTA resin. It was confirmed that TC-FDM catalyzes the oxidation of 4-CP and the sequential conversion of 4-CP to benzoquinone (BQN)→hydroquinone (HQN)→HQL. This indicated that CphC-I exhibits substrate specificity for 4-CP, BQN, and HQN. The activity of CphC-I for 4-CP was 63.22U/mg-protein, and the Michaelis-Menten kinetic parameters were v_max=0.21mM/min, K_M=0.19mM, and k_cat/K_M=0.04mM^-1min^-1. These results would be useful for the development of a novel biochemical treatment technology for 4-CP and phenolic hydrocarbons.

Persistent Isolation of an Unusual Pseudomonas Species From a Phenolic Disinfectant System

A well-characterized unusual Pseudomonas species contaminated the piped disinfectant system of a newly-opened laminar air flow intensive care facility. This organism was frequently isolated (104-106 cfu/ml) from phenolic diluted 1:256 in the system, and could also be recovered (0.01-0.2 cfu/ml) from undiluted phenolic. During the 20-month period when this unusual Pseudomonas was present, none of the severely compromised, granulocytopenic oncology patients treated in the intensive care facility were either colonized or infected with this Pseudomonas sp. Eradication of the organism from the system proved difficult and was accomplished by removing a contaminated reservoir of diluted phenolic disinfectant followed by transient cleansing of the system with very high concentrations (84,000 ppm) of chlorine. This experience demonstrates that phenolics should be added to the list of disinfectants which can harbor Pseudomonas spp. in the clinical setting.

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.

"Rational" management of dichlorophenols biodegradation by the microalga Scenedesmus obliquus

The microalga Scenedesmus obliquus exhibited the ability to biodegrade dichlorophenols (dcps) under specific autotrophic and mixotrophic conditions. According to their biodegradability, the dichlorophenols used can be separated into three distinct groups. Group I (2,4-dcp and 2,6 dcp – no meta-substitution) consisted of quite easily degraded dichlorophenols, since both chloride substituents are in less energetically demanding positions. Group II (2,3-dcp, 2,5-dcp and 3,4-dcp – one meta-chloride) was less susceptible to biodegradation, since one of the two substituents, the meta one, required higher energy for C-Cl-bond cleavage. Group III (3,5-dcp – two meta-chlorides) could not be biodegraded, since both chlorides possessed the most energy demanding positions. In general, when the dcp-toxicity exceeded a certain threshold, the microalga increased the energy offered for biodegradation and decreased the energy invested for biomass production. As a result, the biodegradation per cell volume of group II (higher toxicity) was higher, than group I (lower toxicity) and the biodegradation of dichlorophenols (higher toxicity) was higher than the corresponding monochlorophenols (lower toxicity). The participation of the photosynthetic apparatus and the respiratory mechanism of microalga to biodegrade the group I and the group II, highlighted different bioenergetic strategies for optimal management of the balance between dcp-toxicity, dcp-biodegradability and culture growth. Additionally, we took into consideration the possibility that the intermediates of each dcp-biodegradation pathway could influence differently the whole biodegradation procedures. For this reason, we tested all possible combinations of phenolic intermediates to check cometabolic interactions. The present contribution bring out the possibility of microalgae to operate as “smart” bioenergetic “machines”, that have the ability to continuously “calculate” the energy reserves and “use” the most energetically advantageous dcp-biodegradation strategy. We tried to manipulate the above fact, changing the energy reserves and as a result the chosen strategy, in order to take advantage of their abilities in detoxifying the environment.

Bioenergetic strategy for the biodegradation of p-cresol by the unicellular green alga Scenedesmus obliquus

Cultures from the unicellular green alga Scenedesmus obliquus biodegrade the toxic p-cresol (4-methylphenol) and use it as alternative carbon/energy source. The biodegradation procedure of p-cresol seems to be a two-step process. HPLC analyses indicate that the split of the methyl group (first step) that is possibly converted to methanol (increased methanol concentration in the growth medium), leading, according to our previous work, to changes in the molecular structure and function of the photosynthetic apparatus and therefore to microalgal biomass increase. The second step is the fission of the intermediately produced phenol. A higher p-cresol concentration results in a higher p-cresol biodegradation rate and a lower total p-cresol biodegradability. The first biodegradation step seems to be the most decisive for the effectiveness of the process, because methanol offers energy for the further biodegradation reactions. The absence of LHCII from the Scenedesmus mutant wt-lhc stopped the methanol effect and significantly reduced the p-cresol biodegradation (only 9%). The present contribution deals with an energy distribution between microalgal growth and p-cresol biodegradation, activated by p-cresol concentration. The simultaneous biomass increase with the detoxification of a toxic phenolic compound (p-cresol) could be a significant biotechnological aspect for further applications.

The characteristics and enzyme activities of 4-chlorophenol biodegradation by Fusarium sp

The effects of pH, temperature and sucrose addition on biodegradative capacity of Fusarium sp. HJ01 for 4-chlorophenol (4-CP) were examined, the property of dioxygenases produced by Fusarium sp. HJ01 during 4-CP degradation was investigated. The results show that Fusarium sp. HJ01 has a high capacity on degrading 4-CP in solution. The optimum values of pH, sucrose concentration and temperature are pH 7,1 g/L and 30°C, respectively. The strain can produce chlorocatechol 1,2-dioxygenase (CC12O) and chlorocatechol 2,3-dioxygenase (CC23O), which show the highest activities when 4-CP is used as the sole carbon source and energy, and the optimal values of pH and temperature are pH 7 and 50°C for CC12O as well as pH 8 and 60°C for CC23O. The kinetics of enzyme-catalyzed reactions accord with the Michaelis-Menten equation. To our knowledge, this is the first study on biodegradation of 4-CP by Fusarium sp. HJ01.

Bacterial methylation of chlorinated phenols and guaiacols: formation of veratroles from guaiacols and high-molecular-weight chlorinated lignin

Two strains of bacteria, provisionally assigned to the genus Arthrobacter, were shown to metabolize mono-, di-, tri-, and tetrachloroguaiacols and pentachlorophenol to the corresponding O-methyl compounds. Hydroxylated intermediates were formed only transiently, except for the synthesis by one strain of 3,4,5-trichlorosyringol from 3,4,5-trichloroguaiacol. Two isomeric trichloroveratroles and tetrachloroveratrole were formed by three of the strains from a high-molecular-weight chlorinated lignin isolated from kraft pulp mill bleach plant. The concentrations of methylated metabolites varied widely and did not appear to be correlated with degradation. The possible environmental consequences resulting from synthesis of these highly lipophilic substances are discussed briefly.

Reduction of pollutants in pulp paper mill effluent treated by PCP-degrading bacterial strains

Two PCP-degrading bacterial strains, Bacillus cereus (ITRC-S6) and Serratia marcescens (ITRC-S7) were used for the treatment of pulp and paper mill effluent at conditions; 1.0% glucose and 0.5% peptone at 30 +/- 1 degrees C at 120 rpm for 168 h of incubation. These two bacterial strains effectively reduced colour (45-52%), lignin (30-42%), BOD (40-70%), COD (50-60%), total phenol (32-40%) and PCP (85-90%) within 168 h of incubation. However, the highest reduction in colour (62%), lignin (54%), BOD (70%), COD (90%), total phenol (90%) and PCP (100%) was recorded by mixed culture treatment. The bacterial mechanism for the degradation of pulp and paper mill effluent may be explained by an increase in the cells biomass using added co-substrates resulting liberation of significant amount of chloride due to bacterial dechlorination of chlorolignins and chlorophenols this showed reduction in colour, lignin and toxicity in the effluent. Further, GC-MS analysis of ethyl acetate-extractable compounds from treated pulp paper mill effluent reinforces the bacterium capability for the degradation of lignin and pentachlorophenol, as many aromatic compounds such as 2-chlorophenol, 2, 4, 6-trichlorophenol and tetrachlorohydroquinone, 6-chlorohydroxyquinol and tetrachlorohydroquinone detected which were not present in the untreated effluent.

Colour removal of anaerobically treated pulp and paper mill effluent by microorganisms in two steps bioreactor

In the present study sequential anaerobic and aerobic treatment in two steps bioreactor was performed for removal of colour in the pulp and paper mill effluent. In anaerobic treatment, colour (70%), lignin (25%), COD (42%), AOX (15%) and phenol (39%) were reduced in 15 days. The anaerobically treated effluent was separately applied in bioreactor in presence of fungal strain, Paecilomyces sp., and bacterial strain, Microbrevis luteum. Data of study indicated reduction in colour (95%), AOX (67%), lignin (86%), COD (88%) and phenol (63%) by Paecilomyces sp. where as M. luteum showed removal in colour (76%), lignin (69%), COD (75%) AOX (82%) and phenol (93%) by day third when 7 days anaerobically treated effluent was further treated by aerobic microorganisms. Change in pH of the effluent, and increase in biomass of microorganisms substantiated results of the study, which was concomitant to the treatment method.

Herbaspirillum chlorophenolicum sp. nov., a 4-chlorophenol-degrading bacterium

A 4-chlorophenol-degrading bacterial strain, formerly designated as a strain of Comamonas testosteroni, was reclassified as a member of the genus Herbaspirillum based on its phenotypic and chemotaxonomic characteristics, as well as phylogenetic analysis using 16S rDNA sequences. Phylogenetic inference based on 16S rDNA sequences showed that strain CPW301T clusters in a phylogenetic branch that contains Herbaspirillum species. 16S rDNA sequence similarity of strain CPW301T to species of the genus Herbaspirillum with validly published names is in the range 98·7–98·9 %. Despite the considerably high 16S rDNA sequence similarity, strain CPW301T could be distinguished clearly from type strains of Herbaspirillum species with validly published names by DNA–DNA relatedness values, which were <15·7 %. The genomic DNA G+C content of strain CPW301T is 61·3 mol%. The predominant ubiquinone is Q-8 and the major cellular fatty acids are C16 : 0 and cyclo-C17 : 0. The strain does not fix nitrogen and is not plant-associated. It is an aerobic rod with one unipolar flagellum. On the basis of these characteristics, a novel Herbaspirillum species, Herbaspirillum chlorophenolicum sp. nov., is proposed. The type strain of the novel species is strain CPW301T (=KCTC 12096T=IAM 15024T).

Kinetics of phenol and chlorophenol utilization by Acinetobacter species

Although microbial transformations via cometabolism have been widely observed, the few available kinetic models of cometabolism have not adequately addressed the case of inhibition from both the growth and nongrowth substrates. The present study investigated the degradation kinetics of self-inhibitory growth (phenol) and nongrowth (4-chlorophenol, 4-CP) substrates, present individually and in combination. Specifically, batch experiments were performed using an Acinetobacter isolate growing on phenol alone and with 4-CP present. In addition, batch experiments were also performed to evaluate the transformation of 4-CP by resting, phenol-induced Acinetobacter cultures. The Haldane kinetic model adequately predicted the biodegradation of phenol alone, although a slight discrepancy was noted in cases of higher initial phenol concentrations. Similarly, a Haldane model for substrate utilization was also able to describe the trends in 4-CP transformation by the resting cell cultures. The 4-CP transformation by the Acinetobacter species growing on phenol was modeled using a competitive kinetic model of cometabolism, which included growth and nongrowth substrate inhibition and cross-inhibition terms. Excellent agreement was obtained between the model predictions using experimentally estimated parameter values and the experimental data for the synchronous disappearance of phenol and 4-CP.

Preferential oxidative dehalogenation upon conversion of 2-halophenols by Rhodococcus opacus 1G

The regiospecificity of hydroxylation of C2-halogenated phenols by Rhodococcus opacus 1G was investigated. Oxidative defluorination at the C2 position ortho with respect to the hydroxyl moiety was preferred over hydroxylation at the non-fluorinated C6 position for all 2-fluorophenol compounds studied. Initial hydroxylation of 2,3, 5-trichlorophenol resulted in the exclusive formation of 3, 5-dichlorocatechol. These results indicate that, in contrast to all other phenol ortho-hydroxylases studied so far, phenol hydroxylase from R. opacus 1G is capable of catalyzing preferential oxidative defluorination but also oxidative dechlorination.

Degradation of 4-Chlorophenol via the meta Cleavage Pathway by Comamonas testosteroni JH5

Comamonas testosteroni JH5 used 4-chlorophenol (4-CP) as its sole source of energy and carbon up to a concentration of 1.8 mM, accompanied by the stoichiometric release of chloride. The degradation of 4-CP mixed with the isomeric 2-CP by resting cells led to the accumulation of 3-chlorocatechol (3-CC), which inactivated the catechol 2,3-dioxygenase. As a result, further 4-CP breakdown was inhibited and 4-CC accumulated as a metabolite. In the crude extract of 4-CP-grown cells, catechol 1,2-dioxygenase and muconate cycloisomerase activities were not detected, whereas the activities of catechol 2,3-dioxygenase, 2-hydroxymuconic semialdehyde dehydrogenase, 2-hydroxymuconic semialdehyde hydrolase, and 2-oxopent-4-enoate hydratase were detected. These enzymes of the meta cleavage pathway showed activity with 4-CC and with 5-chloro-2-hydroxymuconic semialdehyde. The activities of the dioxygenase and semialdehyde dehydrogenase were constitutive. Two key metabolites of the meta cleavage pathway, the meta cleavage product (5-chloro-2-hydroxymuconic semialdehyde) and 5-chloro-2-hydroxymuconic acid, were detected. Thus, our previous postulation that C. testosteroni JH5 uses the meta cleavage pathway for the complete mineralization of 4-CP was confirmed.

Bioprotection of microbial communities from toxic phenol mixtures by a genetically designed pseudomonad

Pseudomonas sp. B13 SN45RE is a genetically engineered microorganism (GEM) that is able to simultaneously degrade mixtures of chloro- and methylaromatics ordinarily toxic for microbial communities via a designed novel ortho-cleavage pathway. The utility of the GEM was investigated in a laboratory scale sewage plant fed with mixtures of either 4-chlorophenol and 4-methyphenol or 3-chlorophenol and 4-methylphenol. In the model system the GEM significantly increased the rate and extent of degradation of the phenol mixtures. In the absence of the GEM, shock loads of the phenol mixtures (1 mM of each compound) reduced the numbers of culturable bacteria by three orders of magnitude, completely eliminated protozoa and metazoa, and caused a drastic decrease in oxygen consumption, whereas the presence of the GEM protected the indigenous microbial community and assured continued functioning of the sewage plant.

Biodegradation of 4-chlorophenol via a hydroquinone pathway by Arthrobacter ureafaciens CPR706

A newly isolated Arthrobacter ureafaciens, strain CPR706, could degrade 4-chlorophenol via a new pathway, in which the chloro-substituent was eliminated in the first step and hydroquinone was produced as a transient intermediate. Strain CPR706 exhibited much higher substrate tolerance and degradation rate than other strains that degraded 4-chlorophenol by the hydroxylation at the second carbon position to form chlorocatechol. Strain CPR706 could also degrade other para-substituted phenols (4-nitro-, 4-bromo-, 4-iodo-, and 4-fluoro-phenol) via the hydroquinone pathway.

Biodegradation of the mixtures of 4-chlorophenol and phenol by Comamonas testosteroni CPW301

A 4-chlorophenol (4-CP)-degrading bacterium, strain CPW301, was isolated from soil and identified as Comamonas testosteroni. This strain dechlorinated and degraded 4-CP via a meta-cleavage pathway. CPW301 could also utilize phenol as a carbon and energy source without the accumulation of any metabolites via the same meta-cleavage pathway. When phenol was added as an additional substrate, CPW301 could degrade 4-CP and phenol simultaneously. The addition of phenol greatly accelerated the degradation of 4-CP due to the increased cell mass. The simultaneous degradation of the 4-CP and phenol is useful not only for enhanced cell growth but also for the bioremediation of both compounds, which are normally present in hazardous waste sites as a mixture.

Formation of Dimethylmuconolactones from Dimethylphenols by Alcaligenes eutrophus JMP 134

2,3-, 2,4-, 2,5-, 3,4-, and 3,5-dimethylphenols were cometabolized by 2,4-dichlorophenoxyacetate-grown Alcaligenes eutrophus JMP 134 or the constitutive derivative JMP 134-1 via the ortho pathway into dimethylmuconolactones as dead-end products. Formation of two distinct lactones from 3,4-dimethylphenol is indicative of 2- as well as 6-hydroxylation. Induction of the meta-cleavage pathway by 2,3- and 3,4-dimethylphenols resulted in growth and no accumulation of products. In contrast, 3,5-dimethylphenol is not metabolized by the meta-cleavage pathway.

Purification and characterization of 6-chlorohydroxyquinol 1,2-dioxygenase from Streptomyces rochei 303: comparison with an analogous enzyme from Azotobacter sp. strain GP1

The enzyme which cleaves the benzene ring of 6-chlorohydroxyquinol was purified to apparent homogeneity from an extract of 2,4,6-trichlorophenol-grown cells of Streptomyces rochei 303. Like the analogous enzyme from Azotobacter sp. strain GP1, it exhibited a highly restricted substrate specificity and was able to cleave only 6-chlorohydroxyquinol and hydroxyquinol and not catechol, chlorinated catechols, or pyrogallol. No extradiol-cleaving activity was observed. In contrast to 6-chlorohydroxyquinol 1,2-dioxygenase from Azotobacter sp. strain GP1, the S. rochei enzyme had a distinct preference for 6-chlorohydroxyquinol over hydroxyquinol (kcat/Km = 1.2 and 0.57 s-1.microM-1, respectively). The enzyme from S. rochei appears to be a dimer of two identical 31-kDa subunits. It is a colored protein and was found to contain 1 mol of iron per mol of enzyme. The NH2-terminal amino acid sequences of 6-chlorohydroxyquinol 1,2-dioxygenase from S. rochei 303 and from Azotobacter sp. strain GP1 showed a high degree of similarity.

Evolution of chlorocatechol catabolic pathways. Conclusions to be drawn from comparisons of lactone hydrolases

The aerobic bacterial degradation of chloroaromatic compounds often involves chlorosubstituted catechols as central intermediates. They are converted to 3-oxoadipate in a series of reactions similar to that for catechol catabolism and therefore designated as modified ortho-cleavage pathway. Among the enzymes of this catabolic route, the chlorocatechol 1,2-dioxygenases are known to have a relaxed substrate specificity. In contrast, several chloromuconate cycloisomerases are more specific, and the dienelactone hydrolases of chlorocatechol catabolic pathways do not even convert the corresponding intermediate of catechol degradation, 3-oxoadipate enol-lactone. While the sequences of chlorocatechol 1,2-dioxygenases and chloromuconate cycloisomerases are very similar to those of catechol 1,2-dioxygenases and muconate cycloisomerases, respectively, the relationship between dienelactone hydrolases and 3-oxoadipate enol-lactone hydrolases is more distant. They seem to share an alpha/beta hydrolase fold, but the sequences comprising the fold are quite dissimilar. Therefore, for chlorocatechol catabolism, dienelactone hydrolases might have been recruited from some other, preexisting pathway. Their relationship to dienelactone (hydrolases identified in 4-fluorobenzoate utilizing strains of Alcaligenes and Burkholderia (Pseudomonas) cepacia is investigated). Sequence evidence suggests that the chlorocatechol catabolic operons of the plasmids pJP4, pAC27, and pP51 have been derived from a common precursor. The latter seems to have evolved for the purpose of halocatechol catabolism, and may be considerably older than the chemical industry.

Regulation of chloro- and methylphenol degradation in Comamonas testosteroni JH5

Comamonas testosteroni JH5 was isolated from a mixed bacterial culture enriched on different chloro- and methylphenols. The strain completely mineralized a mixture consisting of 4-chlorophenol (4-CP) and 4-methylphenol (4-MP). During degradation of the mixture, 4-hydroxybenzyl alcohol, 4-hydroxybenzaldehyde, 4-hydroxybenzoic acid, and 4-chlorocatechol were detected as short-lived intermediates. Mineralization of 4-CP and that of 4-MP occurred successively and were accompanied by diauxic growth, whereas 4-CP and 2-methylphenol were mineralized simultaneously. It was ascertained that neither a reversible enzyme inhibition nor potential toxic intermediates caused the observed diauxie. Some facts support the hypothesis that the successive degradation of 4-CP and 4-MP is regulated on the level of transcription. C. testosteroni JH5 contained a meta-cleaving enzyme when pregrown on 4-CP and the isomeric monomethylphenols. Inactivation of this enzyme in the presence of 3-chlorocatechol was observed.

Unspecific degradation of halogenated phenols by the soil fungus Penicillium frequentans Bi 7/2

Resting phenol-grown mycelia of the fungus Penicillium frequentans strain Bi 7/2 were shown to be capable of metabolizing various monohalogenated phenols as well as 3,4-dichlorophenol. 2,4.dichlorophenol could be metabolized in the presence of phenol as cosubstrate. In the first degradation step the halogenated phenols were oxidized to the corresponding halocatechols. Halocatechols substituted in para-position (4-halocatechols) were further degraded under formation of 4-carboxymethylenbut-2-en-4-olide. A partial dehalogenation took place splitting the ring system. 3-Halocatechols were cleaved to 2-halomuconic acids as dead end metabolites without a dehalogenation step. Dichlorophenols were only transformed to the corresponding catechols. In addition 3,5-dichloro-catechol was O-methylated to give two isomers of dichloroguiacol. The halogenated catechols with the exception of 4-fluorocatechol partly polymerized oxidatively in the culture fluid to form insoluble dark-brown products. The degradation of halophenols are due to the action of unspecific intracellular enzymes responsible for phenol catabolism (phenol hydroxylase, catechol-1,2-dioxygenase, muconate cycloisomerase I).

Biodegradation of diphenyl ether and its monohalogenated derivatives by Sphingomonas sp. strain SS3

The bacterium Sphingomonas sp. strain SS3, which utilizes diphenyl ether and its 4-fluoro, 4-chloro, and (to a considerably lesser extent) 4-bromo derivatives as sole sources of carbon and energy, was enriched from soil samples of an industrial waste deposit. The bacterium showed cometabolic activities toward all other isomeric monohalogenated diphenyl ethers. During diphenyl ether degradation in batch culture experiments, phenol and catechol were produced as intermediates which were then channeled into the 3-oxoadipate pathway. The initial step in the degradation follows the recently discovered mechanism of 1,2-dioxygenation, which yields unstable phenolic hemiacetals from diphenyl ether structures. Oxidation of the structure-related dibenzo-p-dioxin yielded 2-(2-hydroxyphenoxy)-muconate upon ortho cleavage of the intermediate 2,2',3-trihydroxydiphenyl ether. Formation of phenol, catechol, halophenol, and halocatechol from the conversion of monohalogenated diphenyl ethers gives evidence for a nonspecific attack of the dioxygenating enzyme system.

Microbial breakdown of halogenated aromatic pesticides and related compounds

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

Degradation of halogenated aromatic compounds

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

Degradation and dehalogenation of monochlorophenols by the phenol-assimilating yeast Candida maltosa

The phenol-assimilating yeast Candida maltosa is able to degrade monochlorophenols but cannot grow on these substrates. 3- and 4-chlorophenol were broken down very rapidly by phenol-grown cells under the formation of 4-chlorocatechol, 5-chloropyrogallol and 4-carboxymethylenebut-2-en-4-olide with concomitant release of chloride. 2-Chlorophenol was partially converted into cis,cis-2-chloromuconic acid via 3-chlorocatechol which was also obtained from 3-chlorophenol in low amounts. No further metabolites containing chloride were found. The dehalogenation step in the chlorophenol degradation is the cycloisomerization of the cis,cis-chloromuconic acid to 4-carboxymethylenebut-2-en-4-olide in the ortho fission pathway.

Degradation of polychlorinated phenols by Streptomyces rochei 303

The strain Streptomyces rochei 303 (VKM Ac-1284D) is capable of utilizing 2-chloro-, 2,4-, 2,6-dichloro- and 2,4,6-trichlorophenols as the sole source of carbon. Its resting cells completely dechlorinated and degraded 2-, 3-chloro-; 2,4-, 2,6-, 2,3-, 2,5-, 3,4-, 3,5-dichloro-; 2,4-, 2,6-dibromo-; 2,4,6-, 2,4,5-, 2,3,4-, 2,3,5-, 2,3,6-trichlorophenols; 2,3,5,6-tetrachloro- and pentachlorophenol. During chlorophenol degradation, a stoichiometric amount of chloride ions was released and chlorohydroquinols were formed as intermediates. In cell-free extracts of S. rochei, the activity of hydroxyquinol 1,2-dioxygenase was found. The enzyme was induced with chlorophenols. Of all so far described strains degrading polychlorophenols, S. rochei 303 utilized a wider range of chlorinated phenols as the sole sourse of carbon and energy.

Degradation of 2,4,6-trichlorophenol by Azotobacter sp. strain GP1

A bacterium which utilizes 2,4,6-trichlorophenol (TCP) as a sole source of carbon and energy was isolated from soil. The bacterium, designated strain GP1, was identified as an Azotobacter sp. TCP was the only chlorinated phenol which supported the growth of the bacterium. Resting cells transformed monochlorophenols, 2,6-dichlorophenol, and 2,3,6-trichlorophenol. Phenol and a number of phenolic compounds, including 4-methylphenol, all of the monohydroxybenzoates, and several dihydroxybenzoates, were very good carbon sources for Azotobacter sp. strain GP1. The organism utilized up to 800 mg of TCP per liter; the lag phase and time for degradation, however, were severely prolonged at TCP concentrations above 500 mg/liter. Repeated additions of 200 mg of TCP per liter led to accelerated degradation, with an optimum value of 100 mg of TCP per liter per h. TCP degradation was significantly faster in shaken than in nonshaken cultures. The optimum temperature for degradation was 25 to 30 degrees C. Induction studies, including treatment of the cells with chloramphenicol prior to TCP or phenol addition, revealed that TCP induced TCP degradation but not phenol degradation and that phenol induced only its own utilization. Per mol of TCP, 3 mol of Cl- was released. 2,6-Dichloro-p-benzoquinone was detected in the resting-cell medium of Azotobacter sp. strain GP1. By chemical mutagenesis, mutants blocked in either TCP degradation or phenol degradation were obtained. No mutant defective in the degradation of both phenols was found, indicating separate pathways for the dissimilation of the compounds. In some of the phenol-deficient mutants, pyrocatechol was found to accumulate, and in some of the TCP-deficient mutants, 2,6-dichlorohydroquinone was found to accumulate.

Dissimilation of 2,4-dichlorophenoxyacetic acid by Azotobacter chroococcum

1. A strain of Azotobacter chroococcum which could use 2,4-dichlorophenoxyacetic acid (2,4-D) as sole carbon source was isolated. 2. The strain metabolized 2,4-D via p-chlorophenoxyacetic acid, p-chlorophenol and 4-chlorocatechol; the last metabolite was cleaved by catechol 1,2-dioxygenase. 3. The enzyme exhibited broad substrate specificity.

Mechanisms of bacterial degradation and transformation of chlorinated monoaromatic compounds

Chloroaromatics are xenobiotic compounds of environmental concern. They can be removed from the environment by (bio)degradation or by (bio)transformation. Recognition of the mechanisms and requirements of their biodegradation is of cardinal importance for understanding the fate of these chemicals in the environment, and for developing methods for biological treatment of wastes containing compounds of this type. Cleavage of the carbon-halogen bond is the critical step in degradation of chloroaromatics. As exemplified with chlorophenols, chlorobenzoates and chlorobenzenes in this review, two distinct strategies are employed by bacteria for degradation of chlorinated aromatic compounds: the particular chlorine substituents are removed either directly from the aromatic ring (as an initial step in degradation) or after oxygenative ring cleavage (from chlorinated aliphatic intermediates). Direct elimination of chlorine substituents from the aromatic ring occurs by displacement with either hydroxyl groups (hydrolytically or oxygenolytically) or hydrogen atoms (reductive dechlorination). Dechlorinations of the latter type require reducing power and are significant in anaerobic environments, but have also been observed with strictly aerobic bacteria. Various biotransformation reactions, with only minor alteration of the parent compound, are an alternative to biogradation. Two environmentally significant transformation reactions discussed here are O-methylation and O-demethylation. The capability to O-methylate chlorinated hydroxybenzenes seems to be widespread in bacteria. O-Methylation is an environmentally important transformation reaction, since methylation increases the lipophilicity of the compound and thus the potential for bioaccumulation. Bacterial O-demethylation of chlorinated methoxylated compounds has been observed under both aerobic and anaerobic conditions.

Monohydroxylation of phenol and 2,5-dichlorophenol by toluene dioxygenase in Pseudomonas putida F1

Pseudomonas putida F1 contains a multicomponent enzyme system, toluene dioxygenase, that converts toluene and a variety of substituted benzenes to cis-dihydrodiols by the addition of one molecule of molecular oxygen. Toluene-grown cells of P. putida F1 also catalyze the monohydroxylation of phenols to the corresponding catechols by an unknown mechanism. Respirometric studies with washed cells revealed similar enzyme induction patterns in cells grown on toluene or phenol. Induction of toluene dioxygenase and subsequent enzymes for catechol oxidation allowed growth on phenol. Tests with specific mutants of P. putida F1 indicated that the ability to hydroxylate phenols was only expressed in cells that contained an active toluene dioxygenase enzyme system. 18O2 experiments indicated that the overall reaction involved the incorporation of only one atom of oxygen in the catechol, which suggests either a monooxygenase mechanism or a dioxygenase reaction with subsequent specific elimination of water.