Complete reductive dechlorination of 1,2-dichloropropane by anaerobic bacteria

Microbiological hydrogen (H2 ) thresholds in anaerobic continuous-flow systems: Effects of system characteristics

Hydrogen (H₂ ) concentrations that were associated with microbiological respiratory processes (RPs) such as sulfate reduction and methanogenesis were quantified in continuous-flow systems (CFSs) (e.g., bioreactors, sediments). Gibbs free energy yield (ΔǴ ~ 0) of the relevant RP has been proposed to control the observed H₂ concentrations, but most of the reported values do not align with the proposed energetic trends. Alternatively, we postulate that system characteristics of each experimental design influence all system components including H₂ concentrations. To analyze this proposal, a Monod-based mathematical model was developed and used to design a gas-liquid bioreactor for hydrogenotrophic methanogenesis with Methanobacterium bryantii M.o.H. Gas-to-liquid H₂ mass transfer, microbiological H₂ consumption, biomass growth, methane formation, and Gibbs free energy yields were evaluated systematically. Combining model predictions and experimental results revealed that an initially large biomass concentration created transients during which biomass consumed [H₂ ]_L rapidly to the thermodynamic H₂ -threshold (≤1 nM) that triggerred the microorganisms to stop H₂ oxidation. With no H₂ oxidation, continuous gas-to-liquid H₂ transfer increased [H₂ ]_L to a level that signaled the methanogens to resume H₂ oxidation. Thus, an oscillatory H₂ -concentration profile developed between the thermodynamic H₂ -threshold (≤1 nM) and a low [H₂ ]_L (~10 nM) that relied on the rate of gas-to-liquid H₂ -transfer. The transient [H₂ ]_L values were too low to support biomass synthesis that could balance biomass losses through endogenous oxidation and advection; thus, biomass declined continuously and disappeared. A stable [H₂ ]_L (1807 nM) emerged as a result of abiotic H₂ -balance between gas-to-liquid H₂ transfer and H₂ removal via advection of liquid-phase.

Long-term dechlorination of cis-DCE to ethene with co-immobilized Dehalococcoides mccartyi BAV1 and Clostridium butyricum in silica gel system

Chloroethenes are common groundwater pollutants, and have been classified as toxic and carcinogenic to humans. The metabolites of chloroethenes, cis-dichloroethene (cis-DCE) and vinyl chloride (VC) commonly accumulate in groundwater due to their recalcitrant reductive dechlorination under anaerobic conditions. Dehalococcoides mccartyi (Dhc) is the key anaerobic bacteria for complete dechlorination of chloroethene, and Clostridium butyricum (C. butyricum) can provide hydrogen for supporting the growth of Dhc. In this study, we co-immobilized Dhc strain BAV1 and C. butyricum in a silica gel to determine the ability of the complete dechlorination of cis-DCE. Our results showed that our immobilized system could protect BAV1 from a high concentration (8 mM) of cis-DCE to carry out complete dechlorination. After the long-term use of our immobilized system, the activity of complete dechlorination was maintained for more than 180 consecutive days. Furthermore, we applied the immobilized system to remediate contaminated groundwater and uncovered the complete dechlorination of cis-DCE into ethene, a non-toxic product, within 28 days. Therefore, this novel co-immobilized system could serve a solution for bioremediation at chloroethene-contaminated sites.

Perspectives of genetically engineered microbes for groundwater bioremediation

Biodegradation is the main process for the removal of organic compounds from the environment, but proceeds slowly for many synthetic chemicals of environmental concern. Research on microbial biodegradation pathways revealed that recalcitrance is - among other factors - caused by biochemical blockages resulting in dysfunctional catabolic routes. This has raised interest in the possibility to construct microorganisms with improved catabolic activities by genetic engineering. Although this goal has been pursued for decades, no full-scale applications have emerged. This perspective explores the lagging implementation of genetically engineered microorganisms in practical bioremediation. The major technical and scientific issues are illustrated by comparing two examples, that of 1,2-dichloroethane where successful full-scale application of pump-and-treat biotreatment processes has been achieved, and 1,2,3-trichloropropane, for which protein and genetic engineering yielded effective bacterial cultures that still await application.

1,2-Dichloropropane (1,2-DCP)-Induced Angiogenesis in Dermatitis

1,2-Dichloropropane (1,2-DCP) has been used as an industrial solvent and a chemical intermediate, as well as in soil fumigants. Human exposure may occur during its production and industrial use. The target organs of 1,2-DCP are the eyes, respiratory system, liver, kidneys, central nervous system, and skin. Repeated or prolonged contact may cause skin sensitization. In this study, 1,2-DCP was dissolved in corn oil at 0, 2.73, 5.75, and 8.75 mL/kg. The skin of mice treated with 1,2-DCP was investigated using western blotting, hematoxylin and eosin staining, and immunohistochemistry. 1,2-DCP was applied to the dorsal skin and both ears of C57BL/6J mice. The thickness of ears and the epidermis increased significantly following treatment, and the appearance of blood vessels was observed in the dorsal skin. Additionally, the expression of vascular endothelial growth factor, which is tightly associated with neovascularization, increased significantly. The levels of protein kinase-B (PKB), phosphorylated PKB, mammalian target of rapamycin (mTOR), and phosphorylated mTOR, all of which are key components of the phosphoinositide 3-kinase/PKB/mTOR signaling pathway, were also enhanced. Taken together, 1,2-DCP induced angiogenesis in dermatitis through the PI3K/PKB/mTOR pathway in the skin.

Biotic and Abiotic Dehalogenation of 1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113): Implications for Bacterial Detoxification of Chlorinated Ethenes

Chlorofluorocarbons including 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) often occur in groundwater plumes comingled with chlorinated solvents such as trichloroethene (TCE). We show that CFC-113 inhibits reductive dechlorination by Dehalococcoides mccartyi (Dhc) in a concentration-dependent manner, causing cis-1,2-dichloroethene (cis-DCE) stalls. Following a 17-day exposure of Dhc-containing consortium SDC-9 to 76 μM CFC-113, cis-DCE dechlorination activity did not recover after CFC-113 removal. River sediment microcosms demonstrated that CFC-113 was subject to microbial degradation under anoxic conditions, and chlorotrifluoroethene (CTFE) was observed as a transformation product. No degradation of CFC-113 was observed in killed controls and in incubations with reactive minerals including mackinawite, green rust, magnetite, and manganese dioxide. In vitro experiments with reduced corrinoid (i.e., vitamin B₁₂) mediated reductive dechlorination of CFC-113 to CTFE and trifluoroethene (TFE) followed by reductive defluorination of TFE to cis-1,2-difluoroethene (cis-DFE) as an end product. This biomimetic degradation of CFC-113 to cis-DFE was also demonstrated in vivo using the corrinoid-producing homoacetogen Sporomusa ovata, suggesting the cometabolic microbial reductive dechlorination and reductive defluorination of CFC-113 to cis-DFE is feasible under anoxic in situ conditions. The CFC-113 degradation intermediates CTFE, TFE, and cis-DFE did not inhibit TCE dechlorination by Dhc, indicating that the initial reductive transformation step can overcome cis-DCE stalls.

Remediation of Chlorinated Alkanes by Vitamin B12 and Zero-Valent Iron

Chlorinated alkanes were heavily used in a wide range of industrial applications including as degreasers, paint strippers, chemical intermediates, and soil fumigants. These compounds are an environmental concern due to the adverse health effects associated with them and have been detected in environmental matrices including soils and groundwater. Chlorinated alkanes are recalcitrant, and current remediation methods that employ zero-valent iron (ZVI) are unable to directly dehalogenate these compounds, limiting the available approaches for in situ remediation of these widely utilized chemicals. This study employed a novel approach for the remediation of 1,2,3-trichloropropane (TCP), 1,2-dichloropropane (1,2-DCP), 1,3-dichloropropane (1,3-DCP), 1-chloropropane (1-CP), and 1,2-dichloroethane (1,2-DCA) in the presence of ZVI and vitamin B12, a naturally occurring electron mediator. Batch reactions were performed in order to determine a kinetic model for the associated degradation mechanisms. Dechlorination byproducts were confirmed through gas chromatography-mass spectrometry (GC-MS) coupled to a purge and trap. Free chloride was quantified by ion chromatography (IC) utilizing suppressed conductivity detection. In the absence of vitamin B12, reductive dechlorination of chlorinated alkanes was observed to not occur when exposed to only reactive ZVI particles (<5 μm). However, in the presence of ZVI combined with vitamin B12, complete reductive dechlorination was observed and followed a pseudo-first-order reaction.

Draft Genome Sequences of the 1,2-Dichloropropane-Respiring Dehalococcoides mccartyi Strains RC and KS

Dehalococcoides mccartyi strains RC and KS respire toxic 1,2-dichloropropane to environmentally benign propene. Their genomes were sequenced with Ion Torrent technology, assembled, and annotated. The draft genomes of strains RC and KS were 1.50 and 1.49 Mb in size and carried 1,653 and 1,671 genes, respectively.

Enantioselectivity of haloalkane dehalogenase LinB on the degradation of 1,2-dichloropropane: A QM/MM study

The hydrolysis dechlorination mechanism of a chiral organochlorinepollutant, 1,2-dichloropropane (DCP), catalyzed by haloalkane dehalogenase LinB has been investigated by using a combined quantum mechanics/molecular mechanics method. LinB was confirmed to be enantioselective towards the catabolism of the racemic mixture. Based on the S_N2 nucleophilic substitution mechanism, the dechlorination process was identified as the rate-determining step in LinB-catalyzed degradation of 1,2-dichloropropane, the Boltzmann-weighted average potential barrier of which is 18.8kcal/mol for the (R)-isomer and 24.0kcal/mol for the (S)-isomer. A particular water molecule near (S)-DCP in the reaction system can strongly disturb the dechlorination process, which can account for the enantioselectivity of LinB. Further electrostatic influence analysis indicates that proper mutation of Gly37 may improve the catalytic efficiency of LinB towards DCP.

Stable Carbon Isotope Fractionation During 1,2-Dichloropropane-to-Propene Transformation by an Enrichment Culture Containing Dehalogenimonas Strains and a dcpA Gene

A stable enrichment culture derived from Besòs river estuary sediments stoichiometrically dechlorinated 1,2-dichloropropane (1,2-DCP) to propene. Sequential transfers in defined anaerobic medium with the inhibitor bromoethanesulfonate produced a sediment-free culture dechlorinating 1,2-DCP in the absence of methanogenesis. Application of previously published genus-specific primers targeting 16S rRNA gene sequences revealed the presence of a Dehalogenimonas strain, and no amplification was obtained with Dehalococcoides-specific primers. The partial sequence of the 16S rRNA amplicon was 100% identical with Dehalogenimonas alkenigignens strain IP3-3. Also, dcpA, a gene described to encode a corrinoid-containing 1,2-DCP reductive dehalogenase was detected. Resistance of the dehalogenating activity to vancomycin, exclusive conversion of vicinally chlorinated alkanes, and tolerance to short-term oxygen exposure is consistent with the hypothesis that a Dehalogenimonas strain is responsible for 1,2-DCP conversion in the culture. Quantitative PCR showed a positive correlation between the number of Dehalogenimonas 16S rRNA genes copies in the culture and consumption of 1,2-DCP. Compound specific isotope analysis revealed that the Dehalogenimonas-catalyzed carbon isotopic fractionation (εC(bulk)) of the 1,2-DCP-to-propene reaction was -15.0 ± 0.7‰ under both methanogenic and nonmethanogenic conditions. This study demonstrates that carbon isotope fractionation is a valuable approach for monitoring in situ 1,2-DCP reductive dechlorination by Dehalogenimonas strains.

Cathodic degradation of antibiotics: characterization and pathway analysis

Antibiotics in wastewaters must be degraded to eliminate their antibacterial activity before discharging into the environment. A cathode can provide continuous electrons for the degradation of refractory pollutants, however the cathodic degradation feasibility, efficiency and pathway for different kinds of antibiotics is poorly understood. Here, we investigated the degradation of four antibiotics, namely nitrofurazone (NFZ), metronidazole (MNZ), chloramphenicol (CAP), and florfenicol (FLO) by a poised cathode in a dual chamber electrochemical reactor. The cyclic voltammetry preliminarily proved the feasibility of the cathodic degradation of these antibiotics. The cathodic reducibility of these antibiotics followed the order of NFZ > MNZ > CAP > FLO. A decreased phosphate buffered solution (PBS) concentration as low as 2 mM or utilization of NaCl buffer solution as catholyte had significant influence on antibiotics degradation rate and efficiency for CAP and FLO but not for NFZ and MNZ. PBS could be replaced by Na2CO3-NaHCO3 buffer solution as catholyte for the degradation of these antibiotics. Reductive dechlorination of CAP proceeded only after the reduction of the nitro group to aromatic amine. The composition of the degradation products depended on the cathode potential except for MNZ. The cathodic degradation process could eliminate the antibacterial activity of these antibiotics. The current study suggests that the electrochemical reduction could serve as a potential pretreatment or advanced treatment unit for the treatment of antibiotics containing wastewaters.

Bio-beads with immobilized anaerobic bacteria, zero-valent iron, and active carbon for the removal of trichloroethane from groundwater

Chlorinated hydrocarbons are the most common organic pollutants in groundwater systems worldwide. In this study, we developed bio-beads with immobilized anaerobic bacteria, zero-valent iron (ZVI), and activated carbon (AC) powder and evaluated their efficacy in removing 1,1,1-trichloroethane (TCA) from groundwater. Bio-beads were produced by polyvinyl alcohol, alginate, and AC powder. We found that the concentration of AC powder used significantly affected the mechanical properties of immobilized bio-beads and that 1.0 % (w/v) was the optimal concentration. The bio-beads effectively degraded TCA (160 mg L(-1)) in the anaerobic medium and could be reused up to six times. The TCA degradation rate of bio-beads was 1.5 and 2.3 times greater, respectively, than ZVI + AC treatment or microbes + AC treatment. Measuring FeS produced by microbial reactions indicated that TCA removal occurred via FeS-catalyzed dechlorination. Analysis of clonal libraries derived from bio-beads demonstrated that the dominant species in the community were Betaproteobacteria and Gammaproteobacteria, which may contribute to the long-term stability of ZVI reactivity during TCA dechlorination. This study shows that the combined use of immobilized anaerobic bacteria, ZVI, and AC in bio-beads is effective and practical for TCA dechlorination and suggests they may be applicable towards developing a groundwater treatment system for the removal of TCA.

A Pseudomonas putida strain genetically engineered for 1,2,3-trichloropropane bioremediation

1,2,3-Trichloropropane (TCP) is a toxic compound that is recalcitrant to biodegradation in the environment. Attempts to isolate TCP-degrading organisms using enrichment cultivation have failed. A potential biodegradation pathway starts with hydrolytic dehalogenation to 2,3-dichloro-1-propanol (DCP), followed by oxidative metabolism. To obtain a practically applicable TCP-degrading organism, we introduced an engineered haloalkane dehalogenase with improved TCP degradation activity into the DCP-degrading bacterium Pseudomonas putida MC4. For this purpose, the dehalogenase gene (dhaA31) was cloned behind the constitutive dhlA promoter and was introduced into the genome of strain MC4 using a transposon delivery system. The transposon-located antibiotic resistance marker was subsequently removed using a resolvase step. Growth of the resulting engineered bacterium, P. putida MC4-5222, on TCP was indeed observed, and all organic chlorine was released as chloride. A packed-bed reactor with immobilized cells of strain MC4-5222 degraded >95% of influent TCP (0.33 mM) under continuous-flow conditions, with stoichiometric release of inorganic chloride. The results demonstrate the successful use of a laboratory-evolved dehalogenase and genetic engineering to produce an effective, plasmid-free, and stable whole-cell biocatalyst for the aerobic bioremediation of a recalcitrant chlorinated hydrocarbon.

Identification and environmental distribution of dcpA, which encodes the reductive dehalogenase catalyzing the dichloroelimination of 1,2-dichloropropane to propene in organohalide-respiring chloroflexi

Dehalococcoides mccartyi strains KS and RC grow with 1,2-dichloropropane (1,2-D) as an electron acceptor in enrichment cultures derived from hydrocarbon-contaminated and pristine river sediments, respectively. Transcription, expression, enzymatic, and PCR analyses implicated the reductive dehalogenase gene dcpA in 1,2-D dichloroelimination to propene and inorganic chloride. Quantitative real-time PCR (qPCR) analyses demonstrated a D. mccartyi cell increase during growth with 1,2-D and suggested that both D. mccartyi strains carried a single dcpA gene copy per genome. D. mccartyi strain RC and strain KS produced 1.8 × 10(7) ± 0.1 × 10(7) and 1.4 × 10(7) ± 0.5 × 10(7) cells per μmol of propene formed, respectively. The dcpA gene was identified in 1,2-D-to-propene-dechlorinating microcosms established with sediment samples collected from different geographical locations in Europe and North and South America. Clone library analysis revealed two distinct dcpA phylogenetic clusters, both of which were captured by the dcpA gene-targeted qPCR assay, suggesting that the qPCR assay is useful for site assessment and bioremediation monitoring at 1,2-D-contaminated sites.

Dechlorination of commercial PCBs and other multiple halogenated compounds by a sediment-free culture containing Dehalococcoides and Dehalobacter

At the contaminated sites, polychlorinated biphenyls (PCBs) frequently coexist with other halogenated compounds, such as polybrominated diphenyl ethers (PBDEs), chloroethanes, and chloroethenes. The presence of multiple halogenated compounds usually poses toxicity to dehalogenating microbes, because few cultures are capable of detoxifying a broad spectrum of halogenated compounds. In this study, a sediment-free culture, designed as AD14, is able to sequentially remove halogens from PCBs and other cocontaminants. Culture AD14 dechlorinated the commercial PCB mixture-Aroclor 1260-mainly by removing flanked para- and doubly flanked meta-chlorines. It also dehalogenated octa-brominated diphenyl ether mixture predominantly to tetra-BDEs, 2,4,6-trichlorophenol (2,4,6-TCP) to 4-CP, and tetrachloroethene (PCE)/1,2-dichloroethane (1,2-DCA) completely to ethene. When applied to a mixture of the above-mentioned compounds, culture AD14 stepwise removed halogens from 2,4,6-TCP, 1,2-DCA, PCE, PBDEs, and PCBs. Illumina sequencing analysis of 16S rRNA genes showed that only two known dechlorinating genera, Dehalococcoides and Dehalobacter, were present in culture AD14. Quantitative real-time PCR analysis showed that the 16S rRNA gene copies of Dehalococcoides and Dehalobacter increased from 1.14 × 10(5) to 7.04 × 10(6) copies mL(-1) and from 1.15 × 10(5) to 8.20 × 10(6) copies mL(-1) after removing 41.13 μM of total chlorine from PCBs. The above results suggest that both Dehalobacter and Dehalococcoides could be responsible for PCB dechlorination. Although two Dehalococoides mccartyi strains with identical 16S rRNA genes were isolated from the PCBs-dechlorinating mixed culture using trichloroethene (TCE) and vinyl chloride (VC) as alternatives to PCBs, the two isolates are incapable of dechlorinating PCBs. In all, culture AD14 is promising for bioremediation applications at sites cocontaminated with PCBs and other halogenated compounds.

Phylogenetically distinct bacteria involve extensive dechlorination of aroclor 1260 in sediment-free cultures

Microbial reductive dechlorination of the persistent polychlorinated biphenyls (PCBs) is attracting much attention in cleanup of the contaminated environment. Nevertheless, most PCB dechlorinating cultures require presence of sediment or sediment substitutes to maintain their dechlorination activities which hinders subsequent bacterial enrichment and isolation processes. The information on enriching sediment-free PCB dechlorinating cultures is still limited. In this study, 18 microcosms established with soils and sediments were screened for their dechlorination activities on a PCB mixture – Aroclor 1260. After one year of incubation, 10 out of 18 microcosms showed significant PCB dechlorination with distinct dechlorination patterns (e.g., Process H, N and T classified based on profiles of PCB congeners loss and new congeners formation). Through serial transfers in defined medium, six sediment-free PCB dechlorinating cultures (i.e., CW-4, CG-1, CG-3, CG-4, CG-5 and SG-1) were obtained without amending any sediment or sediment-substitutes. PCB dechlorination Process H was the most frequently observed dechlorination pattern, which was found in four sediment-free cultures (CW-4, CG-3, CG-4 and SG-1). Sediment-free culture CG-5 showed the most extensive PCB dechlorination among the six cultures, which was mediated by Process N, resulting in the accumulation of penta- (e.g., 236-24-CB) and tetra-chlorobiphenyls (tetra-CBs) (e.g., 24-24-CB, 24-25-CB, 24-26-CB and 25-26-CB) via dechlorinating 30.44% hepta-CBs and 59.12% hexa-CBs after three months of incubation. For culture CG-1, dechlorinators mainly attacked double flanked meta-chlorines and partially ortho-chlorines, which might represent a novel dechlorination pattern. Phylogenetic analysis showed distinct affiliation of PCB dechlorinators in the microcosms, including Dehalogenimonas and Dehalococcoides species. This study broadens our knowledge in microbial reductive dechlorination of PCBs, and provides essential information for culturing and stimulating PCB dechlorinators for in situ bioremediation applications.

Dehalogenimonas spp. can Reductively Dehalogenate High Concentrations of 1,2-Dichloroethane, 1,2-Dichloropropane, and 1,1,2-Trichloroethane

The contaminant concentrations over which type strains of the species Dehalogenimonas alkenigignens and Dehalogenimonas lykanthroporepellens were able to reductively dechlorinate 1,2-dichloroethane (1,2-DCA), 1,2-dichloropropane (1,2-DCP), and 1,1,2-trichloroethane (1,1,2-TCA) were evaluated. Although initially isolated from an environment with much lower halogenated solvent concentrations, D. alkenigignens IP3-3T was found to reductively dehalogenate chlorinated alkanes at concentrations comparable to D. lykanthroporepellens BL-DC-9T. Both species dechlorinated 1,2-DCA, 1,2-DCP, and 1,1,2-TCA present at initial concentrations at least as high as 8.7, 4.0, and 3.5 mM, respectively. The ability of Dehalogenimonas spp. to carry out anaerobic reductive dechlorination even in the presence of high concentrations of chlorinated aliphatic alkanes has important implications for remediation of contaminated soil and groundwater.

Transformation and biodegradation of 1,2,3-trichloropropane (TCP)

PURPOSE

1,2,3-Trichloropropane (TCP) is a persistent groundwater pollutant and a suspected human carcinogen. It is also is an industrial chemical waste that has been formed in large amounts during epichlorohydrin manufacture. In view of the spread of TCP via groundwater and its toxicity, there is a need for cheap and efficient technologies for the cleanup of TCP-contaminated sites. In situ or on-site bioremediation of TCP is an option if biodegradation can be achieved and stimulated. This paper presents an overview of methods for the remediation of TCP-contaminated water with an emphasis on the possibilities of biodegradation.

CONCLUSIONS

Although TCP is a xenobiotic chlorinated compound of high chemical stability, a number of abiotic and biotic conversions have been demonstrated, including abiotic oxidative conversion in the presence of a strong oxidant and reductive conversion by zero-valent zinc. Biotransformations that have been observed include reductive dechlorination, monooxygenase-mediated cometabolism, and enzymatic hydrolysis. No natural organisms are known that can use TCP as a carbon source for growth under aerobic conditions, but anaerobically TCP may serve as electron acceptor. The application of biodegradation is hindered by low degradation rates and incomplete mineralization. Protein engineering and genetic modification can be used to obtain microorganisms with enhanced TCP degradation potential.

Two-step denaturing gradient gel electrophoresis (2S-DGGE), a gel-based strategy to capture full-length 16S rRNA gene sequences

Obtaining full-length 16S rRNA gene sequences is important for generating accurate taxonomy assignments of bacteria, which normally is realized via clone library construction. However, the application of clone library has been hindered due to its limitations in sample throughput and in capturing minor populations (<1 % of total microorganisms). To overcome these limitations, a new strategy, two-step denaturing gradient gel electrophoresis (2S-DGGE), is developed to obtain full-length 16S rRNA gene sequences. 2S-DGGE can compare microbial communities based on its first-round DGGE profiles and generate partial 16S rRNA gene sequences (8-534 bp, Escherichia coli numbering). Then, strain-specific primers can be designed based on sequence information of bacteria of interest to PCR amplify their remaining 16S rRNA gene sequences (515-1541 bps, E. coli numbering). The second-round DGGE can confirm DNA sequence purity of these PCR products. Finally, the full-length 16S rRNA gene sequences can be obtained through combining the two partial DNA sequences. By employing 2S-DGGE, taxonomies of a group of dehalogenating bacteria have been assigned based on their full-length 16S rRNA gene sequences, several of which existed in dehalogenating microcosms as minor populations. In all, 2S-DGGE can be utilized as a medium throughput method for taxonomic identification of interested/minor populations from single or multiple microbial consortia.

Evaluation of substrate removal kinetics for UASB reactors treating chlorinated ethanes

PURPOSE

Lack of focus on the treatment of wastewaters bearing potentially hazardous pollutants like 1,1,2 trichloroethane and 1,1,2,2 tetrachloroethane in anaerobic reactors has provided an impetus to undertake this study. The objective of this exercise was to quantify the behavior of upflow anaerobic sludge blanket reactors and predict their performance based on the overall organic substrate removal.

METHODS

The reactors (wastewater-bearing TCA (R2), and wastewater-bearing TeCA (R3)) were operated at different hydraulic retention times (HRTs), i.e., 36, 30, 24, 18, and 12 h corresponding to food-to-mass ratios varying in the range of 0.2–0.7 mg chemical oxygen demand (COD) mg−1 volatile suspended solids day−1. The process kinetics of substrate utilization was evaluated on the basis of experimental results, by applying three mathematical models namely first order, Grau second order, and Michaelis-Menten type kinetics.

RESULTS

The results showed that the lowering of HRT below 24 h resulted in reduced COD removal efficiencies and higher effluent pollutant concentrations in the reactors. The Grau second-order model was successfully applied to obtain the substrate utilization kinetics with high value of R 2 (>0.95). The Grau second-order substrate removal constant (K 2) was calculated as 1.12 and 7.53 day−1 for reactors R2 and R3, respectively.

CONCLUSION

This study demonstrated the suitability of Grau second-order kinetic model over other models, for predicting the performance of reactors R2 and R3, in treating wastewaters containing chlorinated ethanes under different organic and hydraulic loading conditions.

Effects of 1,1,1-trichloroethane on enzymatic activity and bacterial community in anaerobic microcosm form sequencing batch reactors

1,1,1-Trichloroethane (TCA), a major organic and groundwater contaminant, has very strong toxic effects on humans, plants and microorganisms. Effects of TCA on enzymatic activity and microbial diversity were investigated in the anaerobic sequencing batch reactor (ASBR) under methanogenic, nitrate-reducing, sulfate-reducing and benzene/toluene degrading conditions. The activities of three enzymes (lactate dehydrogenase, phosphatase and protease) were significantly decreased in the presence of 5 mg/L TCA. Within these three affected enzymes, phosphatase activity may serve as a noteworthy marker of bacterial toxicity. The activity of phosphatase was 0.2 U/L in methanogenic conditions with 5 mg/L TCA, which was 99% lower than the controls, and the enzyme activity was 18.6 U/L in methanogenic conditions with 1 mg/L TCA, which was 7% lower than the controls. DGGE profiles showed that TCA altered the bacterial community distribution and diversity obviously during the 21 day of TCA exposure. The enzyme activities decreased second lowest but TCA degrading strains Clostridium sp. DhR-2/LM-G01, Bacterial clone DCE25 and Bacterial clone DPHB06 were enriched in the methanogenic ASBR amended TCA.

Genomic determinants of organohalide-respiration in Geobacter lovleyi, an unusual member of the Geobacteraceae

BACKGROUND

Geobacter lovleyi is a unique member of the Geobacteraceae because strains of this species share the ability to couple tetrachloroethene (PCE) reductive dechlorination to cis-1,2-dichloroethene (cis-DCE) with energy conservation and growth (i.e., organohalide respiration). Strain SZ also reduces U(VI) to U(IV) and contributes to uranium immobilization, making G. lovleyi relevant for bioremediation at sites impacted with chlorinated ethenes and radionuclides. G. lovleyi is the only fully sequenced representative of this distinct Geobacter clade, and comparative genome analyses identified genetic elements associated with organohalide respiration and elucidated genome features that distinguish strain SZ from other members of the Geobacteraceae.

RESULTS

Sequencing the G. lovleyi strain SZ genome revealed a 3.9 Mbp chromosome with 54.7% GC content (i.e., the percent of the total guanines (Gs) and cytosines (Cs) among the four bases within the genome), and average amino acid identities of 53-56% compared to other sequenced Geobacter spp. Sequencing also revealed the presence of a 77 kbp plasmid, pSZ77 (53.0% GC), with nearly half of its encoded genes corresponding to chromosomal homologs in other Geobacteraceae genomes. Among these chromosome-derived features, pSZ77 encodes 15 out of the 24 genes required for de novo cobalamin biosynthesis, a required cofactor for organohalide respiration. A plasmid with 99% sequence identity to pSZ77 was subsequently detected in the PCE-dechlorinating G. lovleyi strain KB-1 present in the PCE-to-ethene-dechlorinating consortium KB-1. Additional PCE-to-cis-DCE-dechlorinating G. lovleyi strains obtained from the PCE-contaminated Fort Lewis, WA, site did not carry a plasmid indicating that pSZ77 is not a requirement (marker) for PCE respiration within this species. Chromosomal genomic islands found within the G. lovleyi strain SZ genome encode two reductive dehalogenase (RDase) homologs and a putative conjugative pilus system. Despite the loss of many c-type cytochrome and oxidative-stress-responsive genes, strain SZ retained the majority of Geobacter core metabolic capabilities, including U(VI) respiration.

CONCLUSIONS

Gene acquisitions have expanded strain SZ's respiratory capabilities to include PCE and TCE as electron acceptors. Respiratory processes core to the Geobacter genus, such as metal reduction, were retained despite a substantially reduced number of c-type cytochrome genes. pSZ77 is stably maintained within its host strains SZ and KB-1, likely because the replicon carries essential genes including genes involved in cobalamin biosynthesis and possibly corrinoid transport. Lateral acquisition of the plasmid replicon and the RDase genomic island represent unique genome features of the PCE-respiring G. lovleyi strains SZ and KB-1, and at least the latter signifies adaptation to PCE contamination.

Fnr (EtrA) acts as a fine-tuning regulator of anaerobic metabolism in Shewanella oneidensis MR-1

Background

EtrA in Shewanella oneidensis MR-1, a model organism for study of adaptation to varied redox niches, shares 73.6% and 50.8% amino acid sequence identity with the oxygen-sensing regulators Fnr in E. coli and Anr in Pseudomonas aeruginosa, respectively; however, its regulatory role of anaerobic metabolism in Shewanella spp. is complex and not well understood.

Results

The expression of the nap genes, nrfA, cymA and hcp was significantly reduced in etrA deletion mutant EtrA7-1; however, limited anaerobic growth and nitrate reduction occurred, suggesting that multiple regulators control nitrate reduction in this strain. Dimethyl sulfoxide (DMSO) and fumarate reductase gene expression was down-regulated at least 2-fold in the mutant, which, showed lower or no reduction of these electron acceptors when compared to the wild type, suggesting both respiratory pathways are under EtrA control. Transcript analysis further suggested a role of EtrA in prophage activation and down-regulation of genes implicated in aerobic metabolism.

Conclusion

In contrast to previous studies that attributed a minor regulatory role to EtrA in Shewanella spp., this study demonstrates that EtrA acts as a global transcriptional regulator and, in conjunction with other regulators, fine-tunes the expression of genes involved in anaerobic metabolism in S. oneidensis strain MR-1. Transcriptomic and sequence analyses of the genes differentially expressed showed that those mostly affected by the mutation belonged to the "Energy metabolism" category, while stress-related genes were indirectly regulated in the mutant possibly as a result of a secondary perturbation (e.g. oxidative stress, starvation). We also conclude based on sequence, physiological and expression analyses that this regulator is more appropriately termed Fnr and recommend this descriptor be used in future publications.

Dechlorination of chloroethenes is inhibited by 2-bromoethanesulfonate in the absence of methanogens

2-Bromoethanesulfonate (BES) inhibited the reductive dechlorination of chloroethenes in several sediment-free enrichment cultures in the absence of methanogenic archaea. Archaeon-specific PCR primers confirmed the absence of methanogens in the enrichment cultures. BES should not be used to attribute dechlorination activities to methanogens.

Microbial structure of an anaerobic bioreactor population that continuously dechlorinates 1,2-dichloropropane

The bacterial diversity of an anaerobic 1,2-dichloropropane (DCP) dechlorinating bioreactor consortium derived from river sediment has been investigated by a combined molecular approach. By using rDNA clone libraries, denaturing gradient gel electrophoresis and quantitative real-time PCR, both Dehalococcoides ethenogenes- and Dehalobacter restrictus-like 16S rDNA sequences were found within the community. Both species are known for reductive dechlorination of tetrachloroethene. Furthermore, numerous yet-uncultured members of the Green non-sulfur bacteria occurred within the consortium. The community analyses over a period of 14 months revealed a clear population shift. D. restrictus 16S rDNA was enriched significantly and became the most abundant rDNA sequence type, suggesting that Dehalobacter spp. play a key role within the reductive dechlorination of DCP in this consortium. We propose the use of this species as an indicator to monitor the transformation process within the bioreactor.

Stable carbon isotope fractionation of 1,2-dichloropropane during dichloroelimination by Dehalococcoides populations

The isotope fractionation of 1,2-dichloropropane (1,2-D) during dichloroelimination to propene by Dehalococcoides populations was explored in laboratory experiments in order to provide data for the characterization of the fate of 1,2-D in heterogeneous subsurface systems. Compound specific stable carbon isotope analysis (CSIA) was used to determine the bulk enrichment factors (epsilonbulk), reactive position specific enrichment factors (epsilonreactive), and apparent kinetic isotope effect (AKIE) values for 1,2-D dichloroelimination in two distinct Dehalococcoides-containing cultures. The epsilonbulk factors calculated in the two cultures were statistically identical, -10.8 +/- 0.9 and -11.3 +/- 0.8 per thousand, even though the cultures were derived from geographically distinct locations. AKIE values for 1,2-D dichloroelimination assuming stepwise and concerted reaction mechanisms were approximately 1.033 and 1.017, respectively. These values are within the range of previously reported values for dichloroelimination reactions and were equivalent to values reported for biotic 1,2-dichloroethane and abiotic 1,1,2,2,-tetrachloroethane and pentachloroethane dichloroelimination reactions.

A Dehalococcoides-containing co-culture that dechlorinates tetrachloroethene to trans-1,2-dichloroethene

In the microbial reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE), dechlorinators usually produce cis-1,2-dichloroethene (cis-DCE) as the predominant product or an intermediate. This study shows that dechlorination of PCE and TCE can also lead to the generation of trans-1,2-dichloroethene (trans-DCE) by a co-culture MB. During its enrichment process, the ratio of trans- to cis-DCE increased from 1.4 (+/-0.1):1-3.7 (+/-0.4):1, whereas the TCE reductive dechlorination rate went up from approximately 26.2 to approximately 68.8 micromol l(-1) day(-1). PCR-denaturing gradient gel electrophoresis (PCR-DGGE) revealed that the increased ratio of trans- /cis-DCE was well correlated with the increased proportions of Dehalococcoides and the disappearance of Desulfuromonas during the enrichment process. As shown by PCR-DGGE, similar Dehalococcoides species were consistently present in another three sediment-free cultures with various trans- /cis-DCE ratios. The 16S rRNA gene sequence of this Dehalococcoides sp. in co-culture MB is 100% identical (over 1489 bp) to that of Dehalococcoides ethenogenes strain 195 (CP000027), which belongs to the Cornell subgroup of the Dehalococcoides cluster. The other bacterium in this co-culture MB was a Sedimentibacter species, which showed no PCE or TCE dechlorination activity. Results from this study show that microbial dechlorination of chloroethenes by this particular subgroup of Dehalococcoides could result in significant accumulation of trans-DCE in the environment if no trans-DCE dechlorinators coexist in the contaminated sites.

Isolation and characterization of "Dehalococcoides" sp. strain MB, which dechlorinates tetrachloroethene to trans-1,2-dichloroethene

In an attempt to understand the microorganisms involved in the generation of trans-1,2-dichloroethene (trans-DCE), pure-culture "Dehalococcoides" sp. strain MB was isolated from environmental sediments. In contrast to currently known tetrachloroethene (PCE)- or trichloroethene (TCE)-dechlorinating pure cultures, which generate cis-DCE as the predominant product, Dehalococcoides sp. strain MB reductively dechlorinates PCE to trans-DCE and cis-DCE at a ratio of 7.3 (+/-0.4):1. It utilizes H2 as the sole electron donor and PCE or TCE as the electron acceptor during anaerobic respiration. Strain MB is a disc-shaped, nonmotile bacterium. Under an atomic force microscope, the cells appear singly or in pairs and are 1.0 microm in diameter and approximately 150 nm in depth. The purity was confirmed by culture-based approaches and 16S rRNA gene-based analysis and was corroborated further by putative reductive dehalogenase (RDase) gene-based, quantitative real-time PCR. Although strain MB shares 100% 16S rRNA gene sequence identity with Dehalococcoides ethenogenes strain 195, these two strains possess different dechlorinating pathways. Microarray analysis revealed that 10 putative RDase genes present in strain 195 were also detected in strain MB. Successful cultivation of strain MB indicates that the biotic process could contribute significantly to the generation of trans-DCE in chloroethene-contaminated sites. It also enhances our understanding of the evolution of this unusual microbial group, Dehalococcoides species.

Spatial and temporal distributions of Geobacter lovleyi and Dehalococcoides spp. during bioenhanced PCE-NAPL dissolution

The spatial and temporal distributions of multiple reductively dechlorinating bacteria were simultaneously assessed in a one-dimensional sand column containing a tetrachloroethene (PCE) nonaqueous phase liquid (NAPL) source and associated plume zones. The column was uniformly inoculated with a PCE-to-ethene dechlorinating microbial consortium that contained Dehalococcoides spp., Dehalobacter spp., and Geobacter lovleyi strain SZ. Geobacter and Dehalococcoides populations grew and colonized the column material, including the mixed-NAPL (0.25 mol/mol PCE in hexadecane) source zone. In contrast, Dehalobacter cells did not colonize the porous column material, and planktonic Dehalobacter cell titers remained below the detection limit of ca. 2.6 x 10(2) cells/mL throughout the experiment. Significant PCE dechlorination was observed and resulted in bioenhanced NAPL dissolution up to 21-fold (maximum) and 5.2-fold (cumulative) relative to abiotic dissolution. cis-1,2-Dichloroethene (cis-DCE) wasthe primary dechlorination product although vinyl chloride (VC) was also formed throughout the experiment. Ethene production occurred after significant depletion of PCE from the NAPL and when cis-DCE concentrations dropped below 6 microM. Data obtained after increasing the column residence time from 1.1 to 2.8 days and introducing a VC pulse to the column indicated that both the residence time and cis-DCE inhibition limited significant VC and ethene production. Although both Geobacter and Dehalococcoides cells were present and active in the mixed-NAPL source zone and plume region, Geobacter cell numbers were typically more than 1 order of magnitude higher than Dehalococcoides cell numbers, which is consistent with the production of predominantly cis-DCE. Analysis of both liquid- and solid-phase samples indicated that Geobacter cells grew and remained attached to the porous medium within the source zone but were largely planktonic in the plume region. In contrast Dehalococcoides cell were attached throughoutthecolumn,and Dehalococcoides cell titers increased by 1 to 2 orders of magnitude over the length of the column, correlating to increases in VC concentrations. The results from this study highlight that bioenhanced dissolution is governed by a complex interplay between resident dechlorinators, contaminant concentrations, and other aquifer-specific characteristics (e.g., hydrology).

Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene

Dehalobacter and "Dehalococcoides" spp. were previously shown to be involved in the biotransformation of 1,1,2-trichloroethane (1,1,2-TCA) and 1,2-dichloroethane (1,2-DCA) to ethene in a mixed anaerobic enrichment culture. Here we report the further enrichment and characterization of a Dehalobacter sp. from this mixed culture in coculture with an Acetobacterium sp. Through a series of serial transfers and dilutions with acetate, H(2), and 1,2-DCA, a stable coculture of Acetobacterium and Dehalobacter spp. was obtained, where Dehalobacter grew during dechlorination. The isolated Acetobacterium strain did not dechlorinate 1,2-DCA. Quantitative PCR with specific primers showed that Dehalobacter cells did not grow in the absence of a chlorinated electron acceptor and that the growth yield with 1,2-DCA was 6.9 (+/-0.7) x 10(7) 16S rRNA gene copies/mumol 1,2-DCA degraded. PCR with degenerate primers targeting reductive dehalogenase genes detected three distinct Dehalobacter/Desulfitobacterium-type sequences in the mixed-parent culture, but only one of these was present in the 1,2-DCA-H(2) coculture. Reverse transcriptase PCR revealed the transcription of this dehalogenase gene specifically during the dechlorination of 1,2-DCA. The 1,2-DCA-H(2) coculture could dechlorinate 1,2-DCA but not 1,1,2-TCA, nor could it dechlorinate chlorinated ethenes. As a collective, the genus Dehalobacter has been show to dechlorinate many diverse compounds, but individual species seem to each have a narrow substrate range.

Development of microbial genome-probing microarrays using digital multiple displacement amplification of uncultivated microbial single cells

A crucial problem in the use of previously developed genome-probing microarrays (GPM) has been the inability to use uncultivated bacterial genomes to take advantage of the high sensitivity and specificity of GPM in microbial detection and monitoring. We show here a method, digital multiple displacement amplification (MDA), to amplify and analyze various genomes obtained from single uncultivated bacterial cells. We used 15 genomes from key microbes involved in dichloromethane (DCM)-dechlorinating enrichment as microarray probes to uncover the bacterial population dynamics of samples without PCR amplification. Genomic DNA amplified from single cells originating from uncultured bacteria with 80.3-99.4% similarity to 16S rRNA genes of cultivated bacteria. The digital MDA-GPM method successfully monitored the dynamics of DCM-dechlorinating communities from different phases of enrichment status. Without a priori knowledge of microbial diversity, the digital MDA-GPM method could be designed to monitor most microbial populations in a given environmental sample.

Microbial activity and distribution during enhanced contaminant dissolution from a NAPL source zone

Laboratory experiments were conducted to assess microbial reductive dechlorination in one-dimensional sand columns containing a 10 cm long source zone of uniformly distributed residual tetrachloroethene (PCE) nonaqueous phase liquid (NAPL), a 10 cm long transition zone directly down-gradient of the source zone containing some nonuniformly distributed NAPL ganglia, and a 40 cm long plume region down-gradient of the transition zone. The activity and distribution of Sulfurospirillum multivorans, a PCE-to-1,2-cis-dichloroethene (cis-DCE) dechlorinating bacterium, was evaluated in columns containing either a mixed-NAPL (0.25 mol/mol PCE in hexadecane) or pure PCE-NAPL. Significant dechlorination of PCE to cis-DCE was observed in the mixed-NAPL column, resulting in 53% PCE-NAPL mass recovery in the effluent with PCE-NAPL dissolution enhanced by up to 13.6-fold (maximum) and 4.6-fold (cumulative) relative to abiotic dissolution. Quantitative real-time PCR targeting pceA, the PCE reductive dehalogenase gene of S. multivorans, revealed that S. multivorans cells were present in the NAPL source zone, and increased in numbers (i.e., grew) throughout the source and transition zones. In contrast, minimal reductive dechlorination and microbial growth were observed in the column containing pure PCE-NAPL, where aqueous-phase PCE concentrations reached saturation. These results demonstrate that microbial growth within NAPL source zones is possible, provided that contaminant concentrations remain below levels toxic to the dechlorinating organisms, and that microbial growth can result in significant bioenhanced NAPL dissolution.

Thermodynamic and kinetic analysis of the H2 threshold for Methanobacterium bryantii M.o.H

H2 thresholds, concentrations below which H2 consumption by a microbial group stops, have been associated with microbial respiratory processes such as dechlorination, denitrification, sulfate reduction, and methanogenesis. Researchers have proposed that observed H2 thresholds occur when the available Gibbs free energy is minimal (DeltaG approximately 0) for a specific respiratory reaction. Others suggest that microbial kinetics also may play a role in controlling the thresholds. Here, we comprehensively evaluate H2 thresholds in light of microbial thermodynamic and kinetic principles. We show that a thermodynamic H2 threshold for Methanobacterium bryantii M.o.H. is not controlled by DeltaG for methane production from H2 + HCO3-. We repeatedly attain a H2 threshold near 0.4 nM, with a range of 0.2-1 nM, and DeltaG for methanogenesis from H2 + HCO3- is positive, +5 to +7 kJ/mol-H2, at the threshold in most cases. We postulate that the H2 threshold is controlled by a separate reaction other than methane production. The electrons from H2 oxidation are transferred to an electron sink that is a solid-phase component of the cells. We also show that a kinetic threshold (S(min)) occurs at a theoretically computed H2 concentration of about 2400 nM at which biomass growth shifts from positive to negative.

Enrichment, cultivation, and detection of reductively dechlorinating bacteria

Strategies and procedures for enriching, isolating, and cultivating reductively dechlorinating bacteria that use chloroorganic compounds as metabolic electron acceptors from environmental samples are described. Further, nucleic acid-based approaches used to detect and quantify dechlorinator (i.e., Dehalococcoides)-specific genes are presented.

Populations implicated in anaerobic reductive dechlorination of 1,2-dichloropropane in highly enriched bacterial communities

1,2-Dichloropropane (1,2-D), a widespread groundwater contaminant, can be reductively dechlorinated to propene by anaerobic bacteria. To shed light on the populations involved in the detoxification process, a comprehensive 16S rRNA gene-based bacterial community analysis of two enrichment cultures derived from geographically distinct locations was performed. Analysis of terminal restriction fragments, amplicons obtained with dechlorinator-specific PCR primers, and enumeration with quantitative real-time PCR as well as screening clone libraries all implied that Dehalococcoides populations were involved in 1,2-D dechlorination in both enrichment cultures. Physiological traits (e.g., dechlorination in the presence of ampicillin and a requirement for hydrogen as the electron donor) supported the involvement of Dehalococcoides populations in the dechlorination process. These findings expand the spectrum of chloroorganic compounds used by Dehalococcoides species as growth-supporting electron acceptors. The combined molecular approach allowed a comparison between different 16S rRNA gene-based approaches for the detection of Dehalococcoides populations.

The generation of high biomass from chlororespiring bacteria using a continuous fed-batch bioreactor

A continuous fed-batch reactor system was developed to rapidly obtain dense chlororespiring cultures of Anaeromyxobacter dehalogenans strain 2CP-C. A syringe pump continuously delivered concentrated 2,6-dichlorophenol (50-150 mM) to an anaerobic reactor vessel at a rate that sustained linear growth but prevented the substrate toxicity of chlorophenol. Dechlorination was not significantly inhibited by end product phenol up to 8 mM. A cell density of 76.8 mg protein l(-1) was obtained in 24 h. Specific growth rates averaged 0.033 h(-1)at 50% substrate limitation, which was in agreement with the maximum specific growth rate of 0.068 h(-1). This reactor system provides an efficient, cost-effective, and convenient method to rapidly obtain dense dechlorinating biomass and is promising to accelerate investigations of enzymes involved in chlororespiration.

Degradation of 1,1,2,2-tetrachloroethane and accumulation of vinyl chloride in wetland sediment microcosms and in situ porewater: biogeochemical controls and associations with microbial communities

The biodegradation pathways of 1,1,2,2-tetrachloroethane (TeCA) and 1,1,2-trichloroethane (112TCA) and the associated microbial communities in anaerobic wetland sediments were evaluated using concurrent geochemical and genetic analyses over time in laboratory microcosm experiments. Experimental results were compared to in situ porewater data in the wetland to better understand the factors controlling daughter product distributions in a chlorinated solvent plume discharging to a freshwater tidal wetland at Aberdeen Proving Ground, Maryland. Microcosms constructed with wetland sediment from two sites showed little difference in the initial degradation steps of TeCA, which included simultaneous hydrogenolysis to 112TCA and dichloroelimination to 1,2-dichloroethene (12DCE). The microcosms from the two sites showed a substantial difference, however, in the relative dominance of subsequent dichloroelimination of 112TCA. A greater dominance of 112TCA dichloroelimination in microcosms constructed with sediment that was initially iron-reducing and subsequently simultaneously iron-reducing and methanogenic caused approximately twice as much vinyl chloride (VC) production as microcosms constructed with sediment that was methanogenic only throughout the incubation. The microcosms with higher VC production also showed substantially more rapid VC degradation. Field measurements of redox-sensitive constituents, TeCA, and its anaerobic degradation products along flowpaths in the wetland porewater also showed greater production and degradation of VC with concurrent methanogenesis and iron reduction. Molecular fingerprinting indicated that bacterial species [represented by a peak at a fragment size of 198 base pairs (bp) by MnlI digest] are associated with VC production from 112TCA dichloroelimination, whereas methanogens (190 and 307 bp) from the Methanococcales or Methanobacteriales family are associated with VC production from 12DCE hydrogenolysis. Acetate-utilizing methanogens (acetotrophs) appear to be involved in the biodegradation of VC. The relative abundance of Methanosarcinaceae, the only methanogen group with acetotrophic members, doubled in microcosms in which degradation of VC was observed. In addition, molecular analyses using primers specific for known dehalorespiring bacteria in the Dehalococcoides and Desulfuromonas groups showed the presence of these bacteria in microcosm slurry from the site that showed the highest VC production and degradation. Determination of biogeochemical controls and microbial consortia involved in TeCA degradation is leading to a better understanding of the heterogeneity in biodegradation rates and daughter product distribution in the wetland, improving capabilities for developing remediation and monitoring plans.

Stereoselective microbial dehalorespiration with vicinal dichlorinated alkanes

The suspected carcinogen 1,2-dichloroethane (1,2-DCA) is the most abundant chlorinated C(2) groundwater pollutant on earth. However, a reductive in situ detoxification technology for this compound does not exist. Although anaerobic dehalorespiring bacteria are known to catalyze several dechlorination steps in the reductive-degradation pathway of chlorinated ethenes and ethanes, no appropriate isolates that selectively and metabolically convert them into completely dechlorinated end products in defined growth media have been reported. Here we report on the isolation of Desulfitobacterium dichloroeliminans strain DCA1, a nutritionally defined anaerobic dehalorespiring bacterium that selectively converts 1,2-dichloroethane and all possible vicinal dichloropropanes and -butanes into completely dechlorinated end products. Menaquinone was identified as an essential cofactor for growth of strain DCA1 in pure culture. Strain DCA1 converts chiral chlorosubstrates, revealing the presence of a stereoselective dehalogenase that exclusively catalyzes an energy-conserving anti mechanistic dichloroelimination. Unlike any known dehalorespiring isolate, strain DCA1 does not carry out reductive hydrogenolysis reactions but rather exclusively dichloroeliminates its substrates. This unique dehalorespiratory biochemistry has shown promising application possibilities for bioremediation purposes and fine-chemical synthesis.

Characterization of two tetrachloroethene-reducing, acetate-oxidizing anaerobic bacteria and their description as Desulfuromonas michiganensis sp. nov

Two tetrachlorethene (PCE)-dechlorinating populations, designated strains BB1 and BRS1, were isolated from pristine river sediment and chloroethene-contaminated aquifer material, respectively. PCE-to-cis-1,2-dichloroethene-dechlorinating activity could be transferred in defined basal salts medium with acetate as the electron donor and PCE as the electron acceptor. Taxonomic analysis based on 16S rRNA gene sequencing placed both isolates within the Desulfuromonas cluster in the delta subdivision of the Proteobacteria. PCE was dechlorinated at rates of at least 139 nmol min(-1) mg of protein(-1) at pH values between 7.0 and 7.5 and temperatures between 25 and 30 degrees C. Dechlorination also occurred at 10 degrees C. The electron donors that supported dechlorination included acetate, lactate, pyruvate, succinate, malate, and fumarate but not hydrogen, formate, ethanol, propionate, or sulfide. Growth occurred with malate or fumarate alone, whereas oxidation of the other electron donors depended strictly on the presence of fumarate, malate, ferric iron, sulfur, PCE, or TCE as an electron acceptor. Nitrate, sulfate, sulfite, thiosulfate, and other chlorinated compounds were not used as electron acceptors. Sulfite had a strong inhibitory effect on growth and dechlorination. Alternate electron acceptors (e.g., fumarate or ferric iron) did not inhibit PCE dechlorination and were consumed concomitantly. The putative fumarate, PCE, and ferric iron reductases were induced by their respective substrates and were not constitutively present. Sulfide was required for growth. Both strains tolerated high concentrations of PCE, and dechlorination occurred in the presence of free-phase PCE (dense non-aqueous-phase liquids). Repeated growth with acetate and fumarate as substrates yielded a BB1 variant that had lost the ability to dechlorinate PCE. Due to the 16S rRNA gene sequence differences with the closest relatives and the unique phenotypic characteristics, we propose that the new isolates are members of a new species, Desulfuromonas michiganensis, within the Desulfuromonas cluster of the Geobacteraceae.

Characterization of two tetrachloroethene-reducing, acetate-oxidizing anaerobic bacteria and their description as Desulfuromonas michiganensis sp. nov

Two tetrachlorethene (PCE)-dechlorinating populations, designated strains BB1 and BRS1, were isolated from pristine river sediment and chloroethene-contaminated aquifer material, respectively. PCE-to-cis-1,2-dichloroethene-dechlorinating activity could be transferred in defined basal salts medium with acetate as the electron donor and PCE as the electron acceptor. Taxonomic analysis based on 16S rRNA gene sequencing placed both isolates within the Desulfuromonas cluster in the delta subdivision of the Proteobacteria. PCE was dechlorinated at rates of at least 139 nmol min(-1) mg of protein(-1) at pH values between 7.0 and 7.5 and temperatures between 25 and 30 degrees C. Dechlorination also occurred at 10 degrees C. The electron donors that supported dechlorination included acetate, lactate, pyruvate, succinate, malate, and fumarate but not hydrogen, formate, ethanol, propionate, or sulfide. Growth occurred with malate or fumarate alone, whereas oxidation of the other electron donors depended strictly on the presence of fumarate, malate, ferric iron, sulfur, PCE, or TCE as an electron acceptor. Nitrate, sulfate, sulfite, thiosulfate, and other chlorinated compounds were not used as electron acceptors. Sulfite had a strong inhibitory effect on growth and dechlorination. Alternate electron acceptors (e.g., fumarate or ferric iron) did not inhibit PCE dechlorination and were consumed concomitantly. The putative fumarate, PCE, and ferric iron reductases were induced by their respective substrates and were not constitutively present. Sulfide was required for growth. Both strains tolerated high concentrations of PCE, and dechlorination occurred in the presence of free-phase PCE (dense non-aqueous-phase liquids). Repeated growth with acetate and fumarate as substrates yielded a BB1 variant that had lost the ability to dechlorinate PCE. Due to the 16S rRNA gene sequence differences with the closest relatives and the unique phenotypic characteristics, we propose that the new isolates are members of a new species, Desulfuromonas michiganensis, within the Desulfuromonas cluster of the Geobacteraceae.

A case study for demonstrating the application of U.S. EPA's monitored natural attenuation screening protocol at a hazardous waste site

Natural attenuation assessment data, collected at a Superfund site located in Louisiana, USA, are presented. The study site is contaminated with large quantities of DNAPL waste products. Source characterization data indicated that chlorinated ethene and ethane compounds are the major contaminants of concern. This case study illustrates the steps involved in implementing the U.S. EPA's [U.S. EPA, 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water, by Wiedmeier, T.H., Swnason, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Hass, P.E., Miller, R.N., Hansen, J. E., Chapelle, F.H., Office of Research and Development, EPA/600/R-98/128] monitored natural attenuation (MNA) screening protocol at this chlorinated solvent site. In the first stage of the MNA assessment process, the field data collected from four monitoring wells located in different parts of the plume were used to complete a biodegradation scoring analysis recommended by the protocol. The analysis indicates that the site has the potential for natural attenuation. In the second stage, a detailed conceptual model was developed to identify various contaminant transport pathways and exposure points. The U.S. EPA model and BIOCHLOR was used to assess whether the contaminants are attenuating at a reasonable rate along these transport paths so that MNA can be considered as a feasible remedial option for the site. The site data along with the modeling results indicate that the chlorinated ethene and chlorinated ethane plumes are degrading and will attenuate within 1000 ft down gradient from the source, well before reaching the identified exposure point Therefore, MNA can be considered as one of the feasible remediation options for the site.

Reductive dehalogenation of trichloroacetic acid by Trichlorobacter thiogenes gen. nov., sp. nov

A bacterium able to grow via reductive dechlorination of trichloroacetate was isolated from anaerobic soil enrichments. The isolate, designated strain K1, is a member of the delta proteobacteria and is related to other known sulfur and ferric iron reducers. In anaerobic mineral media supplemented with acetate and trichloroacetate, its doubling time was 6 h. Alternative electron donor and acceptors were acetoin and sulfur or fumarate, respectively. Trichloroacetate dehalogenation activity was constitutively present, and the dechlorination product was dichloroacetate and chloride. Trichloroacetate conversion seemed to be coupled to a novel sulfur-sulfide redox cycle, which shuttled electrons from acetate oxidation to trichloroacetate reduction. In view of its unique physiological characteristics, the name Trichlorobacter thiogenes is suggested for strain K1.