Anaerobic degradation of fluorinated aromatic compounds

Anaerobic enrichment cultures with sediment from an intertidal strait as inoculum were established under denitrifying, sulfate-reducing, iron-reducing and methanogenic conditions to examine the biodegradation of mono-fluorophenol and mono-fluorobenzoate isomers. Both phenol and benzoate were utilized within 2–6 weeks under all electron-accepting conditions. However, no degradation of the fluorophenols was observed within 1 year under any of the anaerobic conditions tested. Under denitrifying conditions, 2-fluorobenzoate and 4-fluorobenzoate were depleted within 84 days and 28 days, respectively. No loss of 3-fluorobenzoate was observed. All three fluorobenzoate isomers were recalcitrant under sulfate-reducing, iron-reducing, and methanogenic conditions. The degradation of the fluorobenzoate isomers under denitrifying conditions was examined in more detail using soils and sediments from different geographic regions around the world. Stable enrichment cultures were obtained on 2-fluorobenzoate or 4-fluorobenzoate with inoculum from most sites. Fluoride was released stoichiometrically, and nitrate reduction corresponded to the values predicted for oxidation of fluorobenzoate to CO₂ coupled to denitrification. The 2-fluorobenzoate-utilizing and 4-fluorobenzoate-utilizing cultures were specific for fluorobenzoates and did not utilize other halogenated (chloro-, bromo-, iodo-) benzoic acids. Two denitrifying strains were isolated that utilized 2-fluorobenzoate and 4-fluorobenzoate as growth substrates. Preliminary characterization indicated that the strains were closely related to Pseudomonas stutzeri. Received: 1 September 1999 / Accepted in revised form: 30 September 1999     

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     

Anaerobic biodegradation of aromatic compounds

Many aromatic compounds and their monomers are existing in nature. Besides they are introduced into the environment by human activity. The conversion of these aromatic compounds is mainly an aerobic process because of the involvement of molecular oxygen in ring fission and as an electron acceptor. Recent literatures indicated that ring fission of monomers and obligomers mainly occurs in anaerobic environments through anaerobic respiration with nitrate, sulphate, carbon dioxide or carbonate as electron acceptors. These anaerobic processes will help to work out the better situation for bioremediation of contaminated environments. While there are plenty of efforts to reduce the release of these chemicals to the environment, already contaminated sites need to be remediated not only to restore the sites but to prevent the leachates spreading to nearby environment. Basically microorganisms are better candidates for breakdown of these compounds because of their wider catalytic mechanisms and the ability to act even in the absence of oxygen. These microbes can be grouped based on their energy mechanisms. Normally, the aerobic counterparts employ the enzymes like mono-and-dioxygenases. The end product is basically catechol, which further may be metabolised to CO2 by means of quinones reductases cycles. In the absense of reductases compounds, the reduced catechols tend to become oxidised to form many quinone compounds. The quinone products are more recalcitrant and lead to other aesthetic problems like colour in water, unpleasant odour, etc. On the contrary, in the reducing environment this process is prevented and in a cascade of pathways, the cleaved products are converted to acetyl co-A to be integrated into other central metabolite paths. The central metabolite of anaerobic degradation is invariably co-A thio-esters of benzoic acid or hydroxy benzoic acid. The benzene ring undergoes various substitution and addition reactions to form chloro-, nitro-, methyl- compounds. For complete degradation the side chains must be removed first and then the benzene ring is activated by carboxylation or hydroxylation or co-A thioester formation. In the next step the activated ring is converted to a form that can be collected in the central pool of metabolism. The third step is the channeling reaction in which the products of the catalysis are directed into central metabolite pool. The enzymes involved in these mechanisms are mostly benzyl co-A ligase, benzyl alcohol dehydrogenase. Other enzymes involved in this path are yet to be purified though many of the reactions products that have been theoretically postulated have been identified. This is mainly due to the instability of intermediate compounds as well as the association of the enzyme substrate is femoral and experimental conditions need to be sophisticated further for isolation of these enzymes. The first structural genes of benzoate and hydroxy benzoate ligases were isolated from Rhodopseudomonas palustris. This gene cluster of 30 kb size found in Rhodopseudomonas palustris coded for the Bad A protein. Similarly, some of the bph A,B,C and D cluster of genes coding for the degradation of pentachlorobenzenes were located in Pseudomonas pseudoalgaligenesKF 707.     

Anaerobic degradation of non-substituted aromatic hydrocarbons

Aromatic hydrocarbons are among the most prevalent organic pollutants in the environment. Their removal from contaminated systems is of great concern because of the high toxicity effect on living organisms including humans. Aerobic degradation of aromatic hydrocarbons has been intensively studied and is well understood. However, many aromatics end up in habitats devoid of molecular oxygen. Nevertheless, anaerobic degradation using alternative electron acceptors is much less investigated. Here, we review the recent literature and very early progress in the elucidation of anaerobic degradation of non-substituted monocyclic (i.e. benzene) and polycyclic aromatic hydrocarbons (PAH such as naphthalene and phenanthrene). A focus will be on benzene and naphthalene as model compounds. This review concerns the microbes involved, the biochemistry of the initial activation and subsequent enzyme reactions involved in the pathway.     

Anaerobic degradation of halogenated benzoic acids by photoheterotrophic bacteria

Abstract From light-exposed enrichment cultures containing benzoate and a mixture of chlorobenzoates, a pure culture was obtained able to grow with 3-chlorobenzoate (3-CBA) or 3-bromobenzoate (3-BrBA) as the sole growth substrate anaerobically in the light. The thus isolated organism is a photoheterotroph, designated isolate DCP3. It is preliminarily identified as a Rhodopseudomonas palustris strain. It differs from Rhodopseudomonas palustris WS17, the only other known photoheterotroph capable of using 3-CBA for growth, in its independence of benzoate for growth with 3-CBA and in its wider substrate range: if grown on 3-CBA, it can also use 2-CBA, 4-CBA or 3,5-CBA.     

Anaerobic degradation of halogenated phenols by sulfate-reducing consortia.

Sulfidogenic consortia enriched from an estuarine sediment were maintained on either 2-, 3-, or 4-chlorophenol as the only source of carbon and energy for over 5 years. The enrichment culture on 4-chlorophenol was the most active and this consortium was selected for further characterization. Utilization of chlorophenol resulted in sulfate depletion corresponding to the values expected for complete mineralization to CO2. Degradation of 4-chlorophenol was coupled to sulfate reduction, since substrate utilization was dependent on sulfidogenesis and chlorophenol loss did not proceed in the absence of sulfate. Other sulfur oxyanions, sulfite or thiosulfate, also served as electron acceptors for chlorophenol utilization, while carbonate, nitrate, and fumarate did not. The sulfidogenic consortium utilized phenol, 4-bromophenol, and 4-iodophenol in addition to 4-chlorophenol. 4-Fluorophenol, however, did not serve as a substrate. 4-Bromo- and 4-iodophenol were degraded with stoichiometric release of halide, and 4-[14C]bromophenol was mineralized, with 90% of the radiolabel recovered as CO2.     

Aerobic degradation of aromatic compounds

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Highlights► Metabolic and regulatory networks are finely tuned for biodegradation of aromatics. ► New pathways and widespread bacterial biodegradation capabilities revealed by omics. ► Full characterization of hybrid pathways expands the scope of aromatic biodegradation. ► The metabolism of aromatics plays a pivotal role in cell to cell communication. ► Computational and synthetic biology approaches design novel biodegradation pathways.     

Bacterial degradation of aromatic compounds

The bacterial degradation of catechol, 3-methylcatechol, 2,3-dihydroxy-β-phenylpropionic acid, and protocatechuic acid has been studied in detail. From the results obtained a general sequence has been proposed for the microbial oxidation of dihydroxy aromatic compounds.     

Anaerobic oxidation of aromatic compounds and hydrocarbons

Aromatic compounds and hydrocarbons have in common a great stability due to resonance energy and inertness of CbondH and CbondC bonds. It has been taken for granted that the metabolism of these compounds obligatorily depends on molecular oxygen. Oxygen is required first to introduce hydroxyl groups into the substrate and then to cleave the aromatic ring. However, newly discovered bacterial enzymes and reactions involved in oxidation of aromatic and hydrocarbon compounds to CO(2) in the complete absence of molecular oxygen have been discovered. Of special interest are two reactions: the reduction of the aromatic ring of benzoyl-coenzyme A and the addition of fumarate to hydrocarbons. These reactions transform aromatic rings and hydrocarbons into products that can be oxidized via more conventional beta-oxidation pathways.     

Microbial breakdown of halogenated aromatic pesticides and related compounds

Abstract 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.     

Anaerobic degradation of aromatic compounds coupled to Fe(III) reduction by Ferroglobus placidus

Aromatic compounds are an important component of the organic matter in some of the anaerobic environments that hyperthermophilic microorganisms inhabit, but the potential for hyperthermophilic microorganisms to metabolize aromatic compounds has not been described previously. In this study, aromatic metabolism was investigated in the hyperthermophile Ferroglobus placidus . F. placidus grew at 85°C in anaerobic medium with a variety of aromatic compounds as the sole electron donor and poorly crystalline Fe(III) oxide as the electron acceptor. Growth coincided with Fe(III) reduction. Aromatic compounds supporting growth included benzoate, phenol, 4-hydroxybenzoate, benzaldehyde, p -hydroxybenzaldehyde and t -cinnamic acid (3-phenyl-2-propenoic acid). These aromatic compounds did not support growth when nitrate was provided as the electron acceptor, even though nitrate supports the growth of this organism with Fe(II) or H₂ as the electron donor. The stoichiometry of benzoate and phenol uptake and Fe(III) reduction indicated that F. placidus completely oxidized these aromatic compounds to carbon dioxide, with Fe(III) serving as the sole electron acceptor. This is the first example of an Archaea that can anaerobically oxidize an aromatic compound. These results also demonstrate for the first time that hyperthermophilic microorganisms can anaerobically oxidize aromatic compounds and suggest that hyperthermophiles may metabolize aromatic compounds in hot environments such as the deep hot subsurface and in marine and terrestrial hydrothermal zones in which Fe(III) is available as an electron acceptor.     

Cometabolic degradation of chlorinated aromatic compounds

The degradation of chlorobenzene was investigated with the specially chosen strain Methylocystis sp. GB 14 DSM 12955, using 23 ml headspace vials and in a soil column filled with quaternary aquifer material from a depth of 20 m. A long-term experiment was carried out in this column, situated in a mobile test unit at a contaminated location in Bitterfeld (Germany). Groundwater polluted by chlorobenzene was continuously fed through the column, through which a mixture comprising 4% CH(4) and 96% air was bubbled. Chlorobenzene was oxidized by up to 80% under pure culture conditions in the model experiments and was completely degraded under the mixed culture conditions of the column experiments. Over a period of 4 months, the stability of the biological system was monitored regularly by analyzing the sMMO activity as well as by classical microbiological and molecular biological methods.     

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.