Biodegradation of halogenated organic compounds.

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

Volatile halogenated organic compounds in European estuaries

Sources, sinks, and distribution patterns ofvolatile halogenated organic compounds (VHOC)in estuaries were investigated during 5 cruises within the BIOGEST programme. Due to their chemical and physical properties (e.g. toxicity, persistence, mobility) these compounds are of considerable environmental concern. A wide range of compounds has been identified and quantified generally ranging from 0.1 ng l^–1 to 350 ng l^–1. Insome samples extraordinarily high values up to4700 ng l^–1 were observed indicatingcontribution from anthropogenic sources.Generally, concentrations of halogenatedcompounds of anthropogenic origin dominatedthose of prevalent natural origin. Data ofselected VHOC are presented in relation tosalinity, particular organic carbon, and totalsuspended matter. Furthermore the observedconcentrations are compared with establishedwater quality regulations. Distributionpatterns of VHOC along the estuary indicatedcommon sources for specific halogenatedcompounds. Decreasing concentrations of mostVHOC along the estuary confirm that degassingto the atmosphere and dilution with sea waterare the dominating processes controlling thefate of these compounds in estuaries.     

Transformations of halogenated organic compounds under denitrification conditions

Trihalomethanes, carbon tetrachloride, 1,1,1-trichloroethane, 1,2-dibromoethane, chlorinated benzenes, ethylbenzene, and naphthalene at concentrations commonly found in surface and groundwater were incubated under anoxic conditions to study their transformability in the presence of denitrifying bacteria. None of the aromatic compounds showed significant utilization relative to sterile controls at initial concentrations from 41 to 114 micrograms/liter after 11 weeks of incubation. Of the halogenated aliphatic compounds studied, transformations of carbon tetrachloride and brominated trihalomethanes were observed after 8 weeks in batch denitrification cultures. Carbon from the decomposition of carbon tetrachloride was both assimilated into cell material and mineralized to carbon dioxide. How this was possible remains unexplained, since carbon tetrachloride is transformed to CO2 by hydrolysis and not by oxidation-reduction. Chloroform was detected in bacterial cultures with carbon tetrachloride initially present, indicating that reductive dechlorination had occurred in addition to hydrolysis. The data suggest that transformations of certain halogenated aliphatic compounds are likely to occur under denitrification conditions in the environment.     

Transformations of halogenated organic compounds under denitrification conditions.

Trihalomethanes, carbon tetrachloride, 1,1,1-trichloroethane, 1,2-dibromoethane, chlorinated benzenes, ethylbenzene, and naphthalene at concentrations commonly found in surface and groundwater were incubated under anoxic conditions to study their transformability in the presence of denitrifying bacteria. None of the aromatic compounds showed significant utilization relative to sterile controls at initial concentrations from 41 to 114 micrograms/liter after 11 weeks of incubation. Of the halogenated aliphatic compounds studied, transformations of carbon tetrachloride and brominated trihalomethanes were observed after 8 weeks in batch denitrification cultures. Carbon from the decomposition of carbon tetrachloride was both assimilated into cell material and mineralized to carbon dioxide. How this was possible remains unexplained, since carbon tetrachloride is transformed to CO2 by hydrolysis and not by oxidation-reduction. Chloroform was detected in bacterial cultures with carbon tetrachloride initially present, indicating that reductive dechlorination had occurred in addition to hydrolysis. The data suggest that transformations of certain halogenated aliphatic compounds are likely to occur under denitrification conditions in the environment.     

Biotransformation of halogenated compounds

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

Utilization of organic nitrogen compounds by Hydrogenomonas eutropha

The chemolithotroph, Hydrogenomonas eutropha, was tested for its ability to utilize a variety of single nitrogen sources during growth in an atmosphere of H₂? O₂? CO₂ The present data show that H. eutropha can utilize the nitrogen from many, but not all, amino acids, several sulfur-containing amino acids, glucosamine, and two aliphatic amides. The nitrogen concentration that supported maximum growth for NH₄Cl, L -glutamate, L -glutamine, urea, and glycine was in the 0.010–0.019M range. H. eutropha failed to remove the nitrogen from primary and secondary amines, eycloleucine, tert-DL -leucine, DL -p-fluorophenylalanine, DL -5-methyltryptophan, creatine, and creatine. This microorganism was able to partially degrade at least six substituted indoles and/or tryptophan catabolites and six substituted imidazoles and/or histidine catabolites. All of a series of 17 dipeptides were able to serve as a nitrogen source for growth in the absence of NH₄Cl. Extracts of H. eutropha were able to catalyze the hydrolysis of 16 α-dipeptides, 2 tripetides, a tetrapeptide, a polypeptide, a β-aspartyl peptide, 2 γ-glutamyl peptides, a N-acetyl amino acid, and 4 amino acid amides. These results emphasize the effectiveness of H. eutropha in utilizing a wide diversity of organic nitrogenous compounds containing amino and amide groups, heterocyclic rings, and peptide bonds.     

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.Abbreviations CBz chlorobenzene - DCBz dichlorobenzene - TrCBz trichlorobenzene - TCBz tetrachlorobenzene - PCBz pentachlorobenzene - HCBz hexachlorobenzene - CBA chlorobenzoic acid - BBA bromobenzoic acid - FBA fluorobenzoic acid - IBA iodobenzoic acid - CP chlorophenol - CA chloroaniline - PCBs polychlorinated biphenyls - CB chlorobiphenyl - 2,4-D 2,4-dichlorophenoxyacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid     

Transformations of 1- and 2-carbon halogenated aliphatic organic compounds under methanogenic conditions

Several 1- and 2-carbon halogenated aliphatic organic compounds present at low concentrations (less than 100 micrograms/liter) were degraded under methanogenic conditions in batch bacterial cultures and in a continuous-flow methanogenic fixed-film laboratory-scale column. Greater than 90% degradation was observed within a 2-day detention time under continuous-flow methanogenic conditions with acetate as a primary substrate. Carbon-14 measurements indicated that chloroform, carbon tetrachloride, and 1,2-dichloroethane were almost completely oxidized to carbon dioxide, confirming removal by biooxidation. The initial step in the transformations of tetrachloroethylene and 1,1,2,2-tetrachloroethane to nonchlorinated end products appeared to be reductive dechlorination to trichloroethylene and 1,1,2-trichloroethane, respectively. Transformations of the brominated aliphatic compounds appear to be the result of both biological and chemical processes. The data suggest that transformations of halogenated aliphatic compounds can occur under methanogenic conditions in the environment.     

Utilization of organic nitrogen compounds by yeasts of the genusSaccharomyces

Yeasts belonging to 27 species ofSaccharomyces were tested for their ability to used-amino acids,l-lysine and various amines and amides as nitrogen source. Most yeasts capable of growing onl-lysine or amines could utilized-amino acids.Saccharomyces (sensu strictu) have a very limited ability to grow on the organic nitrogen compounds tested. However, there is no obvious relationship between the utilization of these compounds and the proposed divisions of the genusSaccharomyces.Issued as N.R.C. No. 9845.     

Microbiological methods for the cleanup of soil and ground water contaminated with halogenated organic compounds

Abstract There is growing interest in the enhancement of microbial degradative activities as a means of bringing about the in situ cleanup of contaminated soils and ground water. The halogenated organic compounds are likely to be prime targets for such biotechnological processes because of their widespread utilisation and the biodegradability of many of the most commonly used compounds. The aim of this review is to consider the potential for microbiological cleanup of haloorganic-contaminated sites. The technologies available involve the provision of suitable environmental conditions to facilitate maximum biodegradation rates either in the subsurface or in on-site bioreactors. Methodologies include the supply of inorganic nutrients, the supply of oxygen gas, the addition of degradative microbial inocula and the introduction of co-metabolic substrates. The potential efficiencies and limitations of the methods are critically discussed from a microbiological viewpoint with respect to substrate degradability and population responses to supplementation.     

Microbiological methods for the cleanup of soil and ground water contaminated with halogenated organic compounds

There is growing interest in the enhancement of microbial degradative activities as a means of bringing about the in situ cleanup of contaminated soils and ground water. The halogenated organic compounds are likely to be prime targets for such biotechnological processes because of their widespread utilisation and the biodegradability of many of the most commonly used compounds. The aim of this review is to consider the potential for microbiological cleanup of haloorganic-contaminated sites. The technologies available involve the provision of suitable environmental conditions to facilitate maximum biodegradation rates either in the subsurface or in on-site bioreactors. Methodologies include the supply of inorganic nutrients, the supply of oxygen gas, the addition of degradative microbial inocula and the introduction of co-metabolic substrates. The potential efficiencies and limitations of the methods are critically discussed from a microbiological viewpoint with respect to substrate degradability and population responses to supplementation.     

Anaerobic degradation of halogenated aromatic compounds

Recent microbiological findings show how compounds, regarded hitherto as unusual substrates for anaerobic bacteria, are degraded under anaerobic conditions. The complete conversion of halobenzoic acids and halophenolic compounds to methane by lake sediment and sewage sludge microorganisms has been demonstrated. Since haloaromatic compounds are widely used and may be found in such effluents as those from the forest industry, these studies could stimulate a broader interest in anaerobic treatment of industrial waste waters which contain unusual organic compounds.     

Production of halogenated compounds byBjerkandera adusta

The white-rot fungusBjerkandera adusta produces volatile chlorinated phenyl compounds. The main compounds identified were 3-chloro-4-methoxybenzaldehyde (3-chloro-p-anisaldehyde), 3-chloro-4-methoxybenzyl alcohol (3-chloro-p-anisyl alcohol), 3,5-dichloro-4-methoxybenzaldehyde (3,5-dichloro-p-anisaldehyde), and 3,5-dichloro, 4-methoxybenzyl alcohol (3,5-dichloro-p-anisyl alcohol).p-Anisaldehyde, veratraldehyde and the corresponding alcohols,p-anisyl alcohol and veratryl alcohol were produced simultaneously. Even with a very low concentration of chloride in the medium (< 10^–5 m), chlorinated aromatic compounds were still observed. Addition of bromide to the culture medium led to the production of brominated compounds: 3-bromo-4-methoxybenzaldehyde, 3-bromo-4-methoxybenzyl alcohol, 3,5-dibromo-4-methoxybenzaldehyde and 3-bromo-5-chloro-4-methoxybenzaldehyde. These brominated compounds have not previously been reported as natural products. Although iodo-aromatic compounds were not produced by supplementation of the medium with iodide, isovanillin was found in the culture broth under these conditions. This compound may be formed by substitution of the iodine intermediate by a hydroxyl group on the third carbon of the ring. Diiodomethane or chloroiodomethane were also found. It is the first time that the production of halomethane has been related to the production of halogenated aromatic compounds. All the strains tested have these capabilities.     

Transformations of 1- and 2-carbon halogenated aliphatic organic compounds under methanogenic conditions.

Several 1- and 2-carbon halogenated aliphatic organic compounds present at low concentrations (less than 100 micrograms/liter) were degraded under methanogenic conditions in batch bacterial cultures and in a continuous-flow methanogenic fixed-film laboratory-scale column. Greater than 90% degradation was observed within a 2-day detention time under continuous-flow methanogenic conditions with acetate as a primary substrate. Carbon-14 measurements indicated that chloroform, carbon tetrachloride, and 1,2-dichloroethane were almost completely oxidized to carbon dioxide, confirming removal by biooxidation. The initial step in the transformations of tetrachloroethylene and 1,1,2,2-tetrachloroethane to nonchlorinated end products appeared to be reductive dechlorination to trichloroethylene and 1,1,2-trichloroethane, respectively. Transformations of the brominated aliphatic compounds appear to be the result of both biological and chemical processes. The data suggest that transformations of halogenated aliphatic compounds can occur under methanogenic conditions in the environment.