Anaerobic bacteria that dechlorinate perchloroethene

Engineered in situ biogeochemical transformation as a secondary treatment following ISCO - A field test

ISCO using activated sodium persulphate is a widely used technology for treating chlorinated solvent source zones. In sensitive areas, however, high groundwater sulphate concentrations following treatment may be a drawback. In situ biogeochemical transformation, a technology that degrades contaminants via reduced iron minerals formed by microbial activity, offers a potential solution for such sites, the bioreduction of sulphate and production of iron sulphides that abiotically degrade chlorinated ethenes acting as a secondary technology following ISCO. This study assesses this approach in the field using hydrochemical and molecular tools, solid phase analysis and geochemical modelling. Following a neutralisation and bioaugmentation, favourable conditions for iron- and sulphate-reducers were created, resulting in a remarkable increase in their relative abundance. The abundance of dechlorinating bacteria (Dehalococcoides mccartyi, Dehalobacter sp. and Desulfitobacterium spp.) remained low throughout this process. The activity of iron- and sulphate-reducers was further stimulated through application of magnetite plus starch and microiron plus starch, resulting in an increase in ferrous iron concentration (from <LOQ to 337 mg/l), a decrease in sulphate concentration by 74-95% and production of hydrogen sulphide (from <LOQ to 25.9 mg/l). At the same time, a gradual revival of dechlorinators and an increase in ethene concentration was also observed. Tetrachloroethene and trichloroethene concentrations decreased by 98.5-99.98% and 75.4-98.5%, respectively. A decline in chlorine number indicated that biological dechlorination contributed to CVOC removal. This study brings new insights into biogeochemical processes that, when properly engineered, could provide a viable solution for secondary treatment.

Diversity and Niche of Archaea in Bioremediation

Bioremediation is the use of microorganisms for the degradation or removal of contaminants. Most bioremediation research has focused on processes performed by the domain Bacteria; however, Archaea are known to play important roles in many situations. In extreme conditions, such as halophilic or acidophilic environments, Archaea are well suited for bioremediation. In other conditions, Archaea collaboratively work alongside Bacteria during biodegradation. In this review, the various roles that Archaea have in bioremediation is covered, including halophilic hydrocarbon degradation, acidophilic hydrocarbon degradation, hydrocarbon degradation in nonextreme environments such as soils and oceans, metal remediation, acid mine drainage, and dehalogenation. Research needs are addressed in these areas. Beyond bioremediation, these processes are important for wastewater treatment (particularly industrial wastewater treatment) and help in the understanding of the natural microbial ecology of several Archaea genera.

Identification of a reductive tetrachloroethene dehalogenase in Shewanella sediminis

The genome sequence of psychrophilic Shewanella sediminis revealed the presence of five putative reductive dehalogenases (Rdhs). We found that cell extracts of pyruvate/fumarate-grown S. sediminis cells catalysed reduced methyl viologen-dependent reductive dechlorination of tetrachloroethene (PCE) to trichloroethene (TCE) at a specific activity of approximately 1 nmol TCE min(-1) (mg protein)(-1). Dechlorination of PCE followed Michaelis-Menten kinetics with an apparent Km of 120 μM PCE. No PCE dechlorination was observed with heat-denatured extract or when cyanocobalamin was omitted from the growth medium; however, the presence of PCE in the growth medium increased PCE transformation rates. Analysis of mutants carrying in-frame deletions of all five Rdhs encoding genes showed that only deletion of Ssed_3769 resulted in the loss of PCE dechlorination activity suggesting that Ssed_3769 is a functional Rdh. This is the first study to show reductive dechlorination activity of PCE in a sediment-dwelling Shewanella species that may be important for linking the flux of organohalogens to organic carbon via reductive dehalogenation in marine sediments.

Involvement of a Collar Structure in Polar Growth and Cell Division of Strain DCB-1

Microscopic methods were used to investigate the unique collar structure of the gram-negative sulfate-reducing bacterium, strain DCB-1. Polar cell growth apparently occurred from the collar. When the daughter cell was approximately equal in length to the mother cell and the collar was thus centrally located, cell division occurred within the collar region. Division was by a novel mechanism which conserved the collar of the mother cell and gave rise to a new collar of the daughter cell. Cells of DCB-1 were also found to contain stacked internal membranes and glycogen bodies.

Characterization of Chloroethylene Dehalogenation by Cell Extracts of Desulfomonile tiedjei and Its Relationship to Chlorobenzoate Dehalogenation

We characterized the reductive dehalogenation of tetrachloroethylene in cell extracts of Desulfomonile tiedjei and compared it with this organism's 3-chlorobenzoate dehalogenation activity. Tetrachloroethylene was sequentially dehalogenated to trichloro- and dichloroethylene; there was no evidence for dichloroethylene dehalogenation. Like the previously characterized 3-chlorobenzoate dehalogenation activity, tetrachloroethylene dehalogenation was heat sensitive, not oxygen labile, and increased in proportion to the amount of protein in assay mixtures. In addition, both dehalogenation activities were dependent on hydrogen or formate as an electron donor and had an absolute requirement for either methyl viologen or triquat as an electron carrier in vitro. Both activities appear to be catalyzed by integral membrane proteins with similar solubilization characteristics. Dehalogenation of tetrachloroethylene was inhibited by 3-chlorobenzoate but not by the structural isomers 2- and 4-chlorobenzoate. The last two compounds are not substrates for D. tiedjei. These findings lead us to suggest that the dehalogenation of tetrachloroethylene in D. tiedjei is catalyzed by a dehalogenase previously thought to be specific for meta-halobenzoates.

Isolation and Characterization of a Facultatively Aerobic Bacterium That Reductively Dehalogenates Tetrachloroethene to cis-1,2-Dichloroethene

A rapidly-growing facultatively aerobic bacterium that transforms tetrachloroethene (PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (cis-1,2-DCE) at high rates in a defined medium was isolated from a contaminated site. Metabolic characterization, cellular fatty acid analysis, and partial sequence analysis of 16S rRNA showed that the new isolate, strain MS-1, has characteristics matching those of the members of the family Enterobacteriaceae. Strain MS-1 can oxidize about 58 substrates including many carbohydrates, short-chain fatty acids, amino acids, purines, and pyrimidines. It can transform up to 1 mM PCE (aqueous) at a rate of about 0.5 (mu)mol of PCE(middot) h(sup-1)(middot)mg (dry weight) of cell(sup-1). PCE transformation occurs following growth on or with the addition of single carbon sources such as glucose, pyruvate, formate, lactate, or acetate or with complex nutrient sources such as yeast extract or a mixture of amino acids. PCE dehalogenation requires the absence of oxygen, nitrate, and high concentrations of fermentable compounds such as glucose. Enterobacter agglomerans biogroup 5 (ATCC 27993), a known facultative bacterium that is closely related to strain MS-1, also reductively dehalogenated PCE to cis-1,2-DCE. To our knowledge, this is the first report on isolation of a facultative bacterium that can reductively transform PCE to cis-1,2-DCE under defined physiological conditions. Also, this is the first report of the ability of E. agglomerans to dehalogenate PCE.

In vitro studies on reductive vinyl chloride dehalogenation by an anaerobic mixed culture

Reductive dehalogenation of vinyl chloride (VC) was studied in an anaerobic mixed bacterial culture. In growth experiments, ethene formation from VC increased exponentially at a rate of about 0.019 h(sup-1). Reductive VC dehalogenation was measured in vitro by using cell extracts of the mixed culture. The apparent K(infm) for VC was determined to be about 76 (mu)M; the V(infmax) was about 28 nmol (middot) min(sup-1) (middot) mg of protein(sup-1). The VC-dehalogenating activity was membrane associated. Propyl iodide had an inhibitory effect on the VC-dehalogenating activity in the in vitro assay. However, this inhibition could not be reversed by illumination. Cell extracts also catalyzed the reductive dehalogenation of cis-1,2-dichloroethene (cis-DCE) and, at a lower rate, of trichloroethene (TCE). Tetrachloroethene (PCE) was not transformed. The results indicate that the reductive dehalogenation of VC and cis-DCE described here is different from previously reported reductive dehalogenation of PCE and TCE.

In situ study of tetrachloroethylene bioremediation with different microbial community shifting

In this study, we characterized the microbial community in groundwater contaminated with tetrachloroethylene (PCE) in order to evaluate the intrinsic and enhanced bioremediation of PCE. Variable behaviour of microbes was observed between natural attenuation and biostimulation, where the latter was mediated by the addition of nutrients. Results of denaturing gradient gel electrophoresis (DGGE) of amplified bacterial 16S rDNA in the case of biostimulation showed that the microbial community was dominated by species phylogenetically related to the beta-proteobacteria. With regards to natural attenuation, sequences were found belonging to multiple species of different phyla. Interestingly, we found sequences that matched the species belonging to the Firmicutes, which contains bacteria capable of reductive dehalogenation. These results suggest the possibility of the presence of some Clostridium-like PCE degraders within the microbial community when using bioremediation or biostimulation.

Biochemical and molecular characterization of a tetrachloroethene dechlorinating Desulfitobacterium sp. strain Y51: a review

A strict anaerobic bacterium, Desulfitobacterium sp. strain Y51, is capable of very efficiently dechlorinating tetrachloroethene (PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (cis-DCE) at concentrations as high as 960 microM and as low as 0.06 microM. Dechlorination was highly susceptible to air oxidation and to potential alternative electron acceptors, such as nitrite, nitrate or sulfite. The PCE reductive dehalogenase (encoded by the pceA gene and abbreviated as PceA dehalogenase) of strain Y51 was purified and characterized. The purified enzyme catalyzed the reductive dechlorination of PCE to cis-DCE at a specific activity of 113.6 nmol min(-1) mg protein(-1). The apparent K(m) values for PCE and TCE were 105.7 and 535.3 microM, respectively. In addition to PCE and TCE, the enzyme exhibited dechlorination activity for various chlorinated ethanes such as hexachloroethane, pentachloroethane, 1,1,1,2-tetrachloroethane and 1,1,2,2-tetrachloroethane. An 8.4-kb DNA fragment cloned from the Y51 genome revealed eight open reading frames, including the pceAB genes. Immunoblot analysis revealed that PceA dehalogenase is localized in the periplasm of Y51 cells. Production of PceA dehalogenase was induced upon addition of TCE. Significant growth inhibition of strain Y51 was observed in the presence of cis-DCE, More interestingly, the pce gene cluster was deleted with high frequency when the cells were grown with cis-DCE.

Chlorinated aliphatic hydrocarbon-induced degradation of trichloroethylene in Wautersia numadzuensis sp. nov

Two strains of trichloroethylene (TCE)-degrading bacteria were isolated from soils at polluted and unpolluted sites. The isolates, strains TE26(T) and K6, showed co-substrate-independent TCE-degrading activity. TCE degradation was accelerated by preincubation with tetrachloroethylene, cis-dichloroethylene (DCE) and 1,1-DCE. TCE-degrading activities of strains TE26(T) and K6 were 0.23, 0.24 micromol min(-1) g(-1) dry cells, respectively. 16S rDNA sequences of strains TE26(T) and K6 were almost identical (99.7% similarity), and most closely related to Ralstonia basilensis (ATCC17697(T)) (98.5% similarity). From the results of DNA-DNA hybridizations, strain TE26(T) was genetically coherent to strain K6 (94 and 88% hybridization), and exhibited lower relatedness to R. basilensis (DSM11853(T)) (44% and 15%). In addition, because of the differences in chemotaxonomic properties, strain TE26(T) and strain K6 appear to be distinct from all established species of the Ralstonia group. Based on these results and the proposal of transferring R. basilensis and related species to Wautersia gen. nov., we propose that these strains should be assigned to the genus Wautersia as Wautersia numadzuensis sp. nov.

Reductive dechlorination of tetrachloroethene by a mixed bacterial culture growing on ethyl lactate

Chloroethenes like tetrachloroethene (PCE) are the most prevalent groundwater contaminants in the USA. Their presence as nonaqueous phase liquids (NAPLs) makes remediation difficult. Among options for NAPL cleanup, co-solvent injection has demonstrated success. However, the process has the potential to leave considerable residue of the co-solvent as well as residual chloroethene. Our rationale in this study was to examine whether this residual solvent could be a potential electron donor for the remediation of the residual chloroethene. We hypothesized that ethyl lactate, a "green" solvent, could serve both as a NAPL extraction solvent and an electron donor for reductive dechlorination of residual chloroethene. We examined whether a mixed culture known to degrade PCE with lactate could also grow on ethyl lactate and whether it could stimulate PCE dechlorination. Biomass growth and PCE dechlorination were observed by protein and chloride production, respectively, in the culture; with a specific dechlorination rate of 50 150 microg (mg cell d)(-1). Ethyl lactate abiotically breaks down to ethanol and lactate, the latter being a rich source of hydrogen fo reductive dechlorination. The results demonstrate that ethyl lactate may be promising for in situ bioremediation following NAPL extraction.

Description of Sulfurospirillum halorespirans sp. nov., an anaerobic, tetrachloroethene-respiring bacterium, and transfer of Dehalospirillum multivorans to the genus Sulfurospirillum as Sulfurospirillum multivorans comb. nov

An anaerobic, halorespiring bacterium (strain PCE-M2(T) = DSM 13726(T) = ATCC BAA-583(T)) able to reduce tetrachloroethene to cis-dichloroethene was isolated from an anaerobic soil polluted with chlorinated aliphatic compounds. The isolate is assigned to the genus Sulfurospirillum as a novel species, Sulfurospirillum halorespirans sp. nov. Furthermore, on the basis of all available data, a related organism, Dehalospirillum multivorans DSM 12446(T), is reclassified to the genus Sulfurospirillum as Sulfurospirillum multivorans comb. nov.

Kinetic and phylogenetic characterization of an anaerobic dechlorinating microbial community

The reductive dechlorination (RD) of tetrachloroethene (PCE) to vinyl chloride (VC) and, to a lesser extent, to ethene (ETH) by an anaerobic microbial community has been investigated by studying the processes and kinetics of the main physiological components of the consortium. Molecular hydrogen, produced by methanol-utilizing acetogens, was the electron donor for the PCE RD to VC and ETH without forming any appreciable amount of other chlorinated intermediates and in the near absence of methanogenic activity. The microbial community structure of the consortium was investigated by preparing a 16S rDNA clone library and by fluorescence in situ hybridization (FISH). The PCR primers used in the clone library allowed the harvest of 16S rDNA from both bacterial and archaeal members in the community. A total of 616 clones were screened by RFLP analysis of the clone inserts followed by the sequencing of RFLP group representatives and phylogenetic analysis. The clone library contained sequences mostly from hitherto undescribed bacteria. No sequences similar to those of the known RD bacteria like 'Dehalococcoides ethenogenes' or Dehalobacter restrictus were found in the clone library, and none of these bacteria was present in the RD consortium according to FISH. Almost all clones fell into six previously described phyla of the bacterial domain, with the majority (56.6 %) being deep-branching members of the Spirochaetes phylum. Other clones were in the Firmicutes phylum (18.5 %), the Chloroflexi phylum (16.4 %), the Bacteroidetes phylum (6.3 %), the Synergistes genus (1.1 %) and a lineage that could not be affiliated with existing phyla (1.1 %). No archaeal clones were found in the clone library. Owing to the phylogenetic novelty of the microbial community with regard to previously cultured micro-organisms, no specific microbial component(s) could be hypothetically affiliated with the RD phenotype. The predominance of Spirochaetes in the microbial consortium, the main group revealed by clone library analysis, was confirmed by FISH using a purposely developed probe.

Tetrachloroethylene (TCE) removal during anaerobic granulation in an upflow anaerobic sludge blanket (UASB) reactor

The anaerobic biotransformation of TCE was investigated during the granulation process by reducing the HRT and increasing the chemical oxygen demand (COD) and TCE loadings in a 21 laboratory-scale upflow anaerobic sludge blanket (UASB) reactor. Anaerobic unacclimated sludge and glucose were used as seed and primary substrate, respectively. Initial granules were developed after 1.5 months of start-up, which grew at an accelerated pace for 7 months and then became fully grown. The effect of operational parameters such as influent TCE concentrations, COD and TCE loading, food to microorganism (F/M) ratio and specific methanogenic activity (SMA) were also considered during granulation. The granular sludge cultivated had a maximum diameter of 2.5 mm and SMA of 1.32 gCOD (gTSS day)(-1) while 94% COD and 90% TCE removal efficiencies were achieved when the reactor was operating at loading rates as high as 160 mg TCE (1 day)(-1) and 14 gCOD (1 day)(-1), respectively after 230 days of continuos operation. This corresponds to HRT of 0.28 day and F/M ratio of 0.60 gCOD (gTSS day)(-1). Kinetic coefficients of k (maximum specific substrate utilization rate), Ks (half velocity coefficient), Y (growth yield coefficient) and b (decay coefficient) were determined to be 2.38 mgCOD (mgTSS day)(-1), 108 mgCOD 1(-1), 0.17 mgTSS (mgCOD)(-1) and 0.015 day(-1), respectively during TCE biotransformation based on glucose-COD during granulation.

Purification, cloning, and sequencing of an enzyme mediating the reductive dechlorination of tetrachloroethylene (PCE) fromClostridium bifermentansDPH-1

An enzyme mediating the reductive dechlorination of tetrachloroethylene (PCE) from cell-free extracts of Clostridium bifermentans DPH-1 was purified, cloned, and sequenced. The enzyme catalyzed the reductive dechlorination of PCE to cis-1,2-dichloroethylene via trichloroethylene, at a Vmaxand Kmof 73 nmol/mg protein and 12 µM, respectively. Maximal activity was recorded at 35°C and pH 7.5. Enzymatic activity was independent of metal ions but was oxygen sensitive. A mixture of propyl iodide and titanium citrate caused a light-reversible inhibition of enzymatic activity suggesting the involvement of a corrinoid cofactor. The molecular mass of the native enzyme was estimated to be approximately 70 kDa. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and matrix-assisted laser desorption ionization-time of flight/mass spectrometry (MALDI–TOF/MS) revealed molecular masses of approximately 35 kDa and 35.7 kDa, respectively. A broad spectrum of chlorinated aliphatic compounds (PCE, trichloroethylene, cis-1,2-dichloroethylene, trans-1,2-dichloroethylene, 1,1-dichloroethylene, 1,2-dichloropropane, and 1,1,2-trichloroethane) was degraded. With degenerate primers designed from the N-terminal sequence (27 amino acid residues), a partial sequence (81 bp) of the encoding gene was amplified by polymerase chain reaction (PCR) and sequenced. Southern analysis of C. bifermentans genomic DNA using the PCR product as a probe revealed restriction fragment bands. A 5.0 kb ClaI fragment, harboring the relevant gene (designated pceC) was cloned (pDEHAL5) and the complete nucleotide sequence of pceC was determined. The gene showed homology mainly with microbial membrane proteins and no homology with any known dehalogenase, suggesting a distinct PCE dehalogenase.Key words: tetrachloroethylene, Clostridium bifermentans DPH-1, PCE dehalogenase, gene cloning.

Constitutive dechlorination of chlorinated ethenes by a methanol degrading methanogenic consortium

The ability of granular methanogenic sludge to dechlorinate chloroethenes was investigated with unadapted sludge from an upflow anaerobic sludge blanket (UASB) reactor fed with methanol. The sludge degraded chlorinated ethenes, but the degradation rates were low. The addition of primary substrate was necessary to sustain dechlorination. The dechlorinating activity seemed to be constitutively present in the anaerobic bacteria. Usually, one chlorine atom was removed via reductive hydrogenolysis. Only trichloroethene (TCE) was converted to substantial amounts of vinylchloride (VC). 1,1-Dichloroethene (1,1DCE) was observed to be an important intermediate in the dechlorination by unadapted granular sludge, although previously this compound had not been commonly observed. Furthermore, the dechlorination of 1,1DCE was faster than the dechlorination of the other chloroethenes.

Tetrachloroethene-dehalogenating bacteria

Tetrachloroethene is a frequent groundwater contaminant often persisting in the subsurface environments. It is recalcitrant under aerobic conditions because it is in a highly oxidized state and is not readily susceptible to oxidation. Nevertheless, at least 15 organisms from different metabolic groups, viz. halorespirators (9), acetogens (2), methanogens (3) and facultative anaerobes (2), that are able to metabolize tetrachloroethene have been isolated as axenic cultures to-date. Some of these organisms couple dehalo-genation to energy conservation and utilize tetrachloroethene as the only source of energy while others dehalogenate tetrachloroethene fortuitously. Halorespiring organisms (halorespirators) utilize halogenated organic compounds as electron acceptors in an anaerobic respiratory process. Different organisms exhibit differences in the final products of tetrachloroethene dehalogenation, some strains convert tetrachloroethene to trichloroethene only, while others also carry out consecutive dehalogenation to dichloroethenes and vinyl chloride. Thus far, only a single organism, 'Dehalococcoides ethenogenes' strain 195, has been isolated which dechlorinates tetrachloroethene all the way down to ethylene. The majority of tetrachloroethene-dehalogenating organisms have been isolated only in the past few years and several of them, i.e., Dehalobacter restrictus, Desulfitobacterium dehalogenans, 'Dehalococcoides ethenogenes', 'Dehalospirillum multivorans', Desulfuromonas chloroethenica, and Desulfomonile tiedjei, are representatives of new taxonomic groups. This contribution summarizes the available information regarding the axenic cultures of the tetrachloroethene-dehalogenating bacteria. The present knowledge about the isolation of these organisms, their physiological characteristics, morphology, taxonomy and their ability to dechlorinate tetrachloroethene is presented to facilitate a comprehensive comparison.

Isolation and characterization of a tetrachloroethylene dechlorinating bacterium, Clostridium bifermentans DPH-1

A tetrachloroethylene (PCE)-degrading gram-positive, endospore forming, anaerobic bacterium, strain DPH-1, was isolated from a contaminated site. The organism was identified as Clostridium bifermentans by 16S rRNA gene sequence analysis and based on its physiological characteristics. Strain DPH-1 could dechlorinate high concentrations of PCE (0.9 mM), via trichloroethylene (TCE) to cis-1,2-dichloroethylene (cDCE) at a rate of 0.43 micromol/h.mg protein, as well as a number of other halogenated aliphatic compounds.

Adhesion of biodegradative anaerobic bacteria to solid surfaces

In order to exploit the ability of anaerobic bacteria to degrade certain contaminants for bioremediation of polluted subsurface environments, we need to understand the mechanisms by which such bacteria partition between aqueous and solid phases, as well as the environmental conditions that influence partitioning. We studied four strictly anaerobic bacteria, Desulfomonile tiedjei, Syntrophomonas wolfei, Syntrophobacter wolinii, and Desulfovibrio sp. strain G11, which theoretically together can constitute a tetrachloroethylene- and trichloroethylene-dechlorinating consortium. Adhesion of these organisms was evaluated by microscopic determination of the numbers of cells that attached to glass coverslips exposed to cell suspensions under anaerobic conditions. We studied the effects of the growth phase of the organisms on adhesion, as well as the influence of electrostatic and hydrophobic properties of the substratum. Results indicate that S. wolfei adheres in considerably higher numbers to glass surfaces than the other three organisms. Starvation greatly decreases adhesion of S. wolfei and Desulfovibrio sp. strain G11 but seems to have less of an effect on the adhesion of the other bacteria. The presence of Fe(3+) on the substratum, which would be electropositive, significantly increased the adhesion of S. wolfei, whereas the presence of silicon hydrophobic groups decreased the numbers of attached cells of all species. Measurements of transport of cells through hydrophobic-interaction and electrostatic-interaction columns indicated that all four species had negatively charged cell surfaces and that D. tiedjei and Desulfovibrio sp. strain G11 possessed some hydrophobic cell surface properties. These findings are an early step toward understanding the dynamic attachment of anaerobic bacteria in anoxic environments.

Reductive dechlorination of tetrachloroethene to cis-1, 2-dichloroethene by a thermophilic anaerobic enrichment culture

Thermophilic anaerobic biodegradation of tetrachloroethene (PCE) was investigated with various inocula from geothermal and nongeothermal areas. Only polluted harbor sediment resulted in a stable enrichment culture that converted PCE via trichloroethene to cis-1, 2-dichloroethene at the optimum temperature of 60 to 65 degrees C. After several transfers, methanogens were eliminated from the culture. Dechlorination was supported by lactate, pyruvate, fructose, fumarate, and malate as electron donor but not by H2, formate, or acetate. Fumarate and L-malate led to the highest dechlorination rate. In the absence of PCE, fumarate was fermented to acetate, H2, CO2, and succinate. With PCE, less H2 was formed, suggesting that PCE competed for the reducing equivalents leading to H2. PCE dechlorination, apparently, was not outcompeted by fumarate as electron acceptor. At the optimum dissolved PCE concentration of approximately 60 microM, a high dechlorination rate of 1.1 micromol h-1 mg-1 (dry weight) was found, which indicates that the dechlorination is not a cometabolic activity. Microscopic analysis of the fumarate-grown culture showed the dominance of a long thin rod. Molecular analysis, however, indicated the presence of two dominant species, both belonging to the low-G+C gram positives. The highest similarity was found with the genus Dehalobacter (90%), represented by the halorespiring organism Dehalobacter restrictus, and with the genus Desulfotomaculum (86%).

Isolation from estuarine sediments of a Desulfovibrio strain which can grow on lactate coupled to the reductive dehalogenation of 2,4, 6-tribromophenol

Strain TBP-1, an anaerobic bacterium capable of reductively dehalogenating 2,4,6-tribromophenol to phenol, was isolated from estuarine sediments of the Arthur Kill in the New York/New Jersey harbor. It is a gram-negative, motile, vibrio-shaped, obligate anaerobe which grows on lactate, pyruvate, hydrogen, and fumarate when provided sulfate as an electron acceptor. The organism accumulates acetate when grown on lactate and sulfate, contains desulfoviridin, and will not grow in the absence of NaCl. It will not utilize acetate, succinate, propionate, or butyrate for growth via sulfate reduction. When supplied with lactate as an electron donor, strain TBP-1 will utilize sulfate, sulfite, sulfur, and thiosulfate for growth but not nitrate, fumarate, or acrylate. This organism debrominates 2-, 4-, 2,4-, 2,6-, and 2,4,6-bromophenol but not 3- or 2,3-bromophenol or monobrominated benzoates. It will not dehalogenate monochlorinated, fluorinated, or iodinated phenols or chlorinated benzoates. Together with its physiological characteristics, its 16S rRNA gene sequence places it in the genus Desulfovibrio. The average growth yield of strain TBP-1 grown on a defined medium supplemented with lactate and 2,4,6-bromophenol is 3.71 mg of protein/mmol of phenol produced, and the yield was 1.42 mg of protein/mmol of phenol produced when 4-bromophenol was the electron acceptor. Average growth yields (milligrams of protein per millimole of electrons utilized) for Desulfovibrio sp. strain TBP-1 grown with 2,4,6-bromophenol, 4-bromophenol, or sulfate are 0.62, 0.71, and 1.07, respectively. Growth did not occur when either lactate or 2,4,6-bromophenol was omitted from the growth medium. These results indicate that Desulfovibrio sp. strain TBP-1 is capable of growth via halorespiration.

A freshwater anaerobe coupling acetate oxidation to tetrachloroethylene dehalogenation

Strain TT4B has been isolated from anaerobic sediments known to be contaminated with a variety of organic solvents. It is a gram-negative, rod-shaped bacterium and grew anaerobically with acetate as the electron donor and tetrachloroethylene as the electron acceptor in a mineral medium. cis-Dichloroethylene was the halogenated product. This strain did not grow fermentatively and used only acetate or pyruvate as electron donors. Tetrachloroethylene and trichloroethylene were used as electron acceptors, as were ferric nitriloacetate and fumarate. Nitrogen and sulfur oxyanions were not able to substitute as the electron acceptor for this organism. Modest growth occurred in a two-phase system with 1 ml of hexadecane containing 50 to 200 mM tetrachloroethylene (aqueous concentrations, 25 to 100 microM) and 10 ml of anaerobic mineral solution with Na2S as the reducing agent. Growth was completely inhibited at tetrachloroethylene levels above 100 microM.

Characterization of an H2-utilizing enrichment culture that reductively dechlorinates tetrachloroethene to vinyl chloride and ethene in the absence of methanogenesis and acetogenesis

We have been studying an anaerobic enrichment culture which, by using methanol as an electron donor, dechlorinates tetrachloroethene (PCE) to vinyl chloride and ethene. Our previous results indicated that H2 was the direct electron donor for rductive dechlorination of PCE by the methanol-PCE culture. Most-probable-number counts performed on this culture indicated low numbers (< or equal to 10(4)/ml)) of methanogens and PCE dechlorinators using methanol and high numbers (> or equal to 10(6)/ml)) of sulfidogens, methanol-utilizing acetogens, fermentative heterotrophs, and PCE dechlorinators using H2. An anaerobic H2-PCE enrichment culture was derived from a 10(-6) dilution of the methanol-PCE culture. This H2-PCE culture used PCE at increasing rates over time when transferred to fresh medium and could be transferred indefinitely with H2 as the electron donor for the PCE dechlorination, indicating that H2-PCE can serve as an electron donor-acceptor pair for energy conservation and growth. Sustained PCE dechlorination by this culture was supported by supplementation with 0.05 mg of vitamin B12 per liter, 25% (vol/vol) anaerobic digestor sludge supernatant, and 2 mM acetate, which presumably served as a carbon source. Neither methanol nor acetate could serve as an electron donor for dechlorination by the H2-PCE culture, and it did not produce CH4 or acetate from H2-CO2 or methanol, indicating the absence of methanogenic and acetogenic bacteria. Microscopic observatios of the pruified H2-PCE culture showed only two major morphotypes: irregular cocci and small rods.

Complete degradation of tetrachloroethene by combining anaerobic dechlorinating and aerobic methanotrophic enrichment cultures

Degradation of tetrachloroethene (perchloroethylene, PCE) was investigated by combining the metabolic abilities of anaerobic bacteria, capable of reductive dechlorination of PCE, with those of aerobic methanotrophic bacteria, capable of co-metabolic degradation of the less-chlorinated ethenes formed by reductive dechlorination of PCE. Anaerobic communities reductively dechlorinating PCE, trichloroethene (TCE) and dichloroethenes were enriched from various sources. The maximum rates of dechlorination observed for various chloroethenes in these batch enrichments were: PCE to TCE (341 mumol l-1 day-1), TCE to cis-dichloroethene (159 mumol l-1 day-1), cis-dichloroethene to chloroethene (99 mumol l-1 day-1) and trans-dichloroethene to chloroethene (22 mumol l-1 day-1). A mixture of these enrichments was inoculated into an anoxic fixed-bed upflow column. In this column PCE was converted mainly into cis-1,2-dichloroethene, small amounts of TCE and chloroethene, and chloride. Enrichments of aerobic methanotrophic bacteria were grown in an oxic fixed-bed downflow column. Less-chlorinated ethenes, formed in the anoxic column, were further metabolized in this oxic methanotrophic column. On the basis of analysis of chloride production and the disappearance of chlorinated ethenes it was demonstrated that complete degradation of PCE was possible by combining these two columns. Operation of the two-column system under various process conditions indicated that the sensitivity of the methanotrophic bacteria to chlorinated intermediates represented the bottle-neck in the sequential anoxic/oxic degradation process of PCE.

Tetrachloroethene and 3-chlorobenzoate dechlorination activities are co-induced in Desulfomonile tiedjei DCB-1

Desulfomonile tiedjei, a strict anaerobe capable of reductively dechlorinating 3-chlorobenzoate, also dechlorinates tetrachloroethene and trichloroethene. It is not known, however, if the aryl and aliphatic dechlorination activities are catalyzed by the same enzymatic system. Cultures induced for 3-chlorobenzoate activity dechlorinated tetrachloroethene and trichloroethene to lower chlorinated products while uninduced parallel cultures did not dechlorinate either substrate. The observed rate of PCE dechlorination in induced cultures was 22 mumol h-1 g protein-1, which is considerably faster than previous rates obtained with defined cultures of this organism. These results show that both dechlorination activities are co-induced and therefore, that the dechlorination mechanisms may share at least some components.

Bacterial dehalogenases: biochemistry, genetics, and biotechnological applications

This review is a survey of bacterial dehalogenases that catalyze the cleavage of halogen substituents from haloaromatics, haloalkanes, haloalcohols, and haloalkanoic acids. Concerning the enzymatic cleavage of the carbon-halogen bond, seven mechanisms of dehalogenation are known, namely, reductive, oxygenolytic, hydrolytic, and thiolytic dehalogenation; intramolecular nucleophilic displacement; dehydrohalogenation; and hydration. Spontaneous dehalogenation reactions may occur as a result of chemical decomposition of unstable primary products of an unassociated enzyme reaction, and fortuitous dehalogenation can result from the action of broad-specificity enzymes converting halogenated analogs of their natural substrate. Reductive dehalogenation either is catalyzed by a specific dehalogenase or may be mediated by free or enzyme-bound transition metal cofactors (porphyrins, corrins). Desulfomonile tiedjei DCB-1 couples energy conservation to a reductive dechlorination reaction. The biochemistry and genetics of oxygenolytic and hydrolytic haloaromatic dehalogenases are discussed. Concerning the haloalkanes, oxygenases, glutathione S-transferases, halidohydrolases, and dehydrohalogenases are involved in the dehalogenation of different haloalkane compounds. The epoxide-forming halohydrin hydrogen halide lyases form a distinct class of dehalogenases. The dehalogenation of alpha-halosubstituted alkanoic acids is catalyzed by halidohydrolases, which, according to their substrate and inhibitor specificity and mode of product formation, are placed into distinct mechanistic groups. beta-Halosubstituted alkanoic acids are dehalogenated by halidohydrolases acting on the coenzyme A ester of the beta-haloalkanoic acid. Microbial systems offer a versatile potential for biotechnological applications. Because of their enantiomer selectivity, some dehalogenases are used as industrial biocatalysts for the synthesis of chiral compounds. The application of dehalogenases or bacterial strains in environmental protection technologies is discussed in detail.

Physiological meaning and potential for application of reductive dechlorination by anaerobic bacteria

The physiological meaning of reductive dechlorination reactions catalyzed by anaerobic bacteria can be explained as a co-metabolic activity or as a novel type of respiration. Co-metabolic activities have been found mainly with alkyl halides. They are non-specific reactions catalyzed by various enzyme systems of facultative as well as obligate anaerobic bacteria. In contrast, the reductive dechlorinations involved in metabolic respiration processes are very specific reactions. Only a limited number of alkyl and aryl chlorinated compounds is presently known to function as a terminal electron acceptor in a few, recently isolated bacteria. Metabolic dechlorination rates are in general several orders of magnitude higher than co-metabolic ones. Both reaction types are suitable for the anaerobic treatment of waste streams.

Reductive dehalogenation as a respiratory process

Anaerobic bacteria can reductively dehalogenate aliphatic and aromatic halogenated compounds in a respiratory process. Only a few of these bacteria have been isolated in pure cultures. However, long acclimation periods, substrate specificity, high dehalogenation rates, and the possibility to enrich for the dehalogenation activity by subcultivation in media containing an electron donor indicate that many of the reductive dehalogenations in the environment are catalyzed by specific bacteria. Molecular hydrogen or formate appear to be good electron donors for the enrichment of such organisms. Furthermore, systems have to be employed which supply the cultures with the halogenated compounds beyond their toxicity level. All bacteria that are presently available in pure culture and grow with a halogenated compound as electron acceptor are members of new genera. Based on experimental results with the membrane-impermeable electron mediator methyl viologen, a model of the respiration system of Dehalobacter restrictus, a tetrachloroethene-dechlorinating bacterium, is presented. Further studies of the biochemistry and energetics of respiratory-dehalogenating strains will help to understand the mechanisms involved and perhaps reveal the evolutionary origin of the dehalogenating enzyme systems.

Effects of Polychlorinated Biphenyl Congener Concentration and Sediment Supplementation on Rates of Methanogenesis and 2,3,6-Trichlorobiphenyl Dechlorination in an Anaerobic Enrichment

We have employed a method of enrichment that allows us to significantly increase the rate of reductive polychlorinated biphenyl (PCB) dechlorination. This method shortens the time required to investigate the effects that culture conditions have on dechlorination and provides an estimate of the potential activity of the PCB-dechlorinating anaerobes. The periodic supplementation of sterile sediment and PCB produced an enhanced, measurable, and sustained rate of dechlorination. We observed volumetric rates of the dechlorination of 2,3,6-trichlorobiphenyl (2,3,6-CB) to 2,6-dichlorobiphenyl (2,6-CB) of more than 300 μmol liter -1 day -1 when the cultures were supplemented daily. A calculation of this activity that is based on an estimate of the number of dechlorinating anaerobes present indicates that 1.13 pmol of 2,3,6-CB was dechlorinated to 2,6-CB day -1 bacterial cell -1 . This rate is similar to that of the reductive dechlorination of 3-chlorobenzoate by Desulfomonile tiedjei. Methanogenesis declined from 585.3 to 125.9 μmol of CH 4 liter -1 day -1 , while dechlorination increased from 8.2 to 346.0 μmol of 2,3,6-CB dechlorinated to 2,6-CB liter -1 day -1 .

Biodegradation of chlorinated aliphatic hydrocarbon mixtures in a single-pass packed-bed reactor

Aliphatic chlorinated compounds, such as trichloroethylene (TCE) and tetrachloroethylene (PCE), are major contaminants of ground water. A single-pass packed-bed bioreactor was utilized to study the biodegradation of organic waste mixtures consisting of PCE, TCE, and other short-chain chlorinated organics. The bioreactor consisted of two 1960-mL glass columns joined in a series. One column was packed with sand containing a microbial consortia enriched from a contaminated site. The other column provided a reservoir for oxygen and a carbon source of methane/propane that was recirculated through the reactor. Sampling was accomplished by both direct headspace and liquid effluent concentration analyses. The reactor was operated in a single-pass mode. Greater than 99% degradation of trichloroethylene, approaching drinking water standards, was observed when the bioreactor residence time ranged from 1.9 to 3.2 d. Typically, when the reactor was pulse-fed with methane, propane, and air, 1 mol of TCE was degraded/110 mol of substrate utilized. Perturbation studies were performed to characterize reactor behavior. The system's degradation behavior was affected by providing different carbon sources, a pulse feeding regime, supplementing microbial biomass, and by altering flow rates.

A highly purified enrichment culture couples the reductive dechlorination of tetrachloroethene to growth

A microscopically pure enrichment culture of a gram-negative anaerobic bacterium, in the present article referred to as PER-K23, was isolated from an anaerobic packed-bed column in which tetrachloroethene (PCE) was reductively transformed to ethane via trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE), chloroethene, and ethene. PER-K23 catalyzes the dechlorination of PCE via TCE to cis-1,2-DCE and couples this reductive dechlorination to growth. H2 and formate were the only electron donors that supported growth with PCE or TCE as an electron acceptor. The culture did not grow in the absence of PCE or TCE. Neither O2, NO3-, NO2-, SO4(2-), SO3(2-), S2O3(2-), S, nor CO2 could replace PCE or TCE as an electron acceptor with H2 as an electron donor. Also, organic electron acceptors such as acetoin, acetol, dimethyl sulfoxide, fumarate, and trimethylamine N-oxide and chlorinated ethanes, DCEs, and chloroethene were not utilized. PER-K23 was not able to grow fermentatively on any of the organic compounds tested. Transferring the culture to a rich medium revealed that a contaminant was still present. Dechlorination was optimal between pH 6.8 and 7.6 and a temperature of 25 to 35 degrees C. H2 consumption was paralleled by chloride production, PCE degradation, cis-1,2-DCE formation, and growth of PER-K23. Electron balances showed that all electrons derived from H2 or formate consumption were recovered in dechlorination products and biomass. Exponential growth could be achieved only in gently shaken cultures.(ABSTRACT TRUNCATED AT 250 WORDS)