Riboflavin- and cobalamin-mediated biodegradation of chloroform in a methanogenic consortium

Soluble metal porphyrins - Zero-valent zinc system for effective reductive defluorination of branched per and polyfluoroalkyl substances (PFASs)

Nano zero-valent metals (nZVMs) have been extensively utilized for decades in the reductive remediation of groundwater contaminated with chlorinated organic compounds, owing to their robust reducing capabilities, simple application, and cost-effectiveness. Nevertheless, there remains a dearth of information regarding the efficient reductive defluorination of linear or branched per- and polyfluoroalkyl substances (PFASs) using nZVMs as reductants, largely due to the absence of appropriate catalysts. In this work, various soluble porphyrin ligands [[meso‑tetra(4-carboxyphenyl)porphyrinato]cobalt(III)]Cl·7H2O (CoTCPP), [[meso‑tetra(4-sulfonatophenyl) porphyrinato]cobalt(III)]·9H2O (CoTPPS), and [[meso‑tetra(4-N-methylpyridyl) porphyrinato]cobalt(II)](I)4·4H2O (CoTMpyP) have been explored for defluorination of PFASs in the presence of the nZn0 as reductant. Among these, the cationic CoTMpyP showed best defluorination efficiencies for br-perfluorooctane sulfonate (PFOS) (94%), br-perfluorooctanoic acid (PFOA) (89%), and 3,7-Perfluorodecanoic acid (PFDA) (60%) after 1 day at 70 °C. The defluorination rate constant of this system (CoTMpyP-nZn0) is 88-164 times higher than the VB12-nZn0 system for the investigated br-PFASs. The CoTMpyP-nZn0 also performed effectively at room temperature (55% for br-PFOS, 55% for br-PFOA and 25% for 3,7-PFDA after 1day), demonstrating the great potential of in-situ application. The effect of various solubilizing substituents, electron transfer flow and corresponding PFASs defluorination pathways in the CoTMpyP-nZn0 system were investigated by both experiments and density functional theory (DFT) calculations. SYNOPSIS: Due to the unavailability of active catalysts, available information on reductive remediation of PFAS by zero-valent metals (ZVMs) is still inadequate. This study explores the effective defluorination of various branched PFASs using soluble porphyrin-ZVM systems and offers a systematic approach for designing the next generation of catalysts for PFAS remediation.

A microbial consortium led by a novel Pseudomonas species enables degradation of carbon tetrachloride under aerobic conditions

Carbon tetrachloride (CT) is a recalcitrant and high priority pollutant known for its toxicity, environmental prevalence, and inhibitory activities. Although much is known about anaerobic CT biodegradation, microbial degradation of CT under aerobic conditions has not yet been reported. This study reports for the first time the enrichment of a stable aerobic CT-degrading bacterial consortium, from a CT-contaminated groundwater sample, capable of co-metabolically degrading 30 μM of CT within a week. A Pseudomonas strain (designated as Stari2) that is the predominant bacterium in this consortium was isolated, and further characterization showed that this bacterium can tolerate and co-metabolically degrade up to 5 mM of CT under aerobic conditions in the presence of different carbon/energy sources. The CT biodegradation profiles of strain Stari2 and the consortium were found to be identical, while no significant positive correlation between strain Stari2 and other bacteria was observed in the consortium during the period of higher CT biodegradation. These results confirmed that the isolated Pseudomonas strain Stari2 is the key player in the consortium catalyzing the biodegradation of CT. No chloroform (CF) or other chlorinated compound was detected during the cometabolism of CT. The whole genome sequencing of strain Stari2 showed that it is a novel Pseudomonas species. The findings demonstrated that biodegradation of CT under aerobic conditions is feasible, and the isolated CT-degrader Pseudomonas sp. strain Stari2 has a great potential for in-situ bioremediation of CT-contaminated environments.

Efficient Reductive Defluorination of Branched PFOS by Metal-Porphyrin Complexes

Vitamin B₁₂ (VB₁₂) has been reported to degrade PFOS in the presence of Ti^III citrate at 70 °C. Porphyrin-based catalysts have emerged as VB₁₂ analogues and have been successfully used in various fields of research due to their interesting structural and electronic properties. However, there is inadequate information on the use of these porphyrin-based metal complexes in the defluorination of PFOS. We have therefore explored a series of porphyrin-based metal complexes for the degradation of PFOS. Co^II-5,10,15,20-tetraphenyl-21H,23H-porphyrin (Co^II-TPP), Co^II-5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (Co^II-M-TPP), and Co^III-M-TPP exhibited efficient reductive defluorination of the branched PFOS. Within 5-8 h, these compounds achieved the same level of PFOS defluorination as VB₁₂ achieved in 7-10 days. For branched isomers, the specific removal rate of the Co^II-TPP-Ti^III citrate system is 64-105 times higher than that for VB₁₂-Ti^III citrate. Moreover, the Co^II-TPP-Ti^III citrate system displayed efficient (51%) defluorination for the branched PFOS (br-PFOS) in 1 day even at room temperature (25 °C). The effects of the iron and cobalt metal centers, reaction pH, and several reductants (NaBH₄, nanosized zerovalent zinc (nZn⁰), and Ti^III citrate) were systematically investigated. Based on the analysis of the products and previously published reports, a new possible defluorination pathway of branched PFOS is also proposed.

2,4-Dichlorophenoxyacetic acid degradation in methanogenic mixed cultures obtained from Brazilian Amazonian soil samples

2,4-Dichlorophenoxyacetic acid (2,4-D) is the third most applied pesticide in Brazil to control broadleaf weeds in crop cultivation and pastures. Due to 2,4-D's high mobility and long half-life under anoxic conditions, this herbicide has high probability for groundwater contamination. Bioremediation is an attractive solution for 2,4-D contaminated anoxic environments, but there is limited understanding of anaerobic 2,4-D biodegradation. In this study, methanogenic enrichment cultures were obtained from Amazonian top soil (0-40 cm) and deep soil (50 -80 cm below ground) that biotransform 2,4-D (5 µM) to 4-chlorophenol and phenol. When these cultures were transferred (10% v/v) to fresh medium containing 40 µM or 160 µM 2,4-D, the rate of 2,4-D degradation decreased, and biotransformation did not proceed beyond 4-chlorophenol and 2,4-dichlorophenol in the top and deep soil cultures, respectively. 16S rRNA gene sequencing and qPCR of a selection of microbes revealed no significant enrichment of known organohalide-respiring bacteria. Furthermore, a member of the genus Cryptanaerobacter was identified as possibly responsible for phenol conversion to benzoate in the top soil inoculated culture. Overall, these results demonstrate the effect of 2,4-D concentration on biodegradation and microbial community composition, which are both important factors when developing pesticide bioremediation technologies.

Metagenomic- and Cultivation-Based Exploration of Anaerobic Chloroform Biotransformation in Hypersaline Sediments as Natural Source of Chloromethanes

Chloroform (CF) is an environmental contaminant that can be naturally formed in various environments ranging from forest soils to salt lakes. Here we investigated CF removal potential in sediments obtained from hypersaline lakes in Western Australia. Reductive dechlorination of CF to dichloromethane (DCM) was observed in enrichment cultures derived from sediments of Lake Strawbridge, which has been reported as a natural source of CF. No CF removal was observed in abiotic control cultures without artificial electron donors, indicating biotic CF dechlorination in the enrichment cultures. Increasing vitamin B₁₂ concentration from 0.04 to 4 µM in enrichment cultures enhanced CF removal and reduced DCM formation. In cultures amended with 4 µM vitamin B₁₂ and ¹³C labelled CF, formation of ¹³CO₂ was detected. Known organohalide-respiring bacteria and reductive dehalogenase genes were neither detected using quantitative PCR nor metagenomic analysis of the enrichment cultures. Rather, members of the order Clostridiales, known to co-metabolically transform CF to DCM and CO₂, were detected. Accordingly, metagenome-assembled genomes of Clostridiales encoded enzymatic repertoires for the Wood-Ljungdahl pathway and cobalamin biosynthesis, which are known to be involved in fortuitous and nonspecific CF transformation. This study indicates that hypersaline lake microbiomes may act as a filter to reduce CF emission to the atmosphere.

Theoretical investigation of the mechanism for the reductive dehalogenation of methyl halides mediated by the CoI-based compounds cobalamin and cobaloxime

Theoretical calculations focusing on the cleavage of the C-X bond in methyl halides (CH₃X; X = Cl, Br, I) as mediated by Co^I-based systems have been carried out using the hybrid functional ωB97-XD together with the basis set 6-311++G(2d,2p). A total of seven Co^I-based compounds were evaluated: cob[I]alamin (Co^ICbl) in its base-on form and cobaloxime (Co^ICbx) with either no ligand or different ligands (either pyridine (PYR), tributylphosphine (TBP), dimethyl sulfide (DMS), cyclohexylisocyanide (CI), or 5,6-dimethylbenzimidazole (DMB)) at the lower axial position. For the large Co^ICbl system, an ONIOM scheme was employed, where the high layer was described at the DFT level and the low layer was computed using the semi-empirical method PM6. A full DFT model was employed for the Co^ICbx cases. An S_N2-like mechanism was evaluated in all cases. The intrinsic reaction coordinate profiles suggested early transition states with activation energies of ≈ 12 kcal/mol, ≈ 10 kcal/mol, and ≈ 5 kcal/mol for C-Cl, C-Br, and C-I cleavage, respectively, which is consistent with the leaving group abilities of these halides. The evolutions of the atomic charges in and the bond orders of Co-C and C-X were computed, and the results confirmed the existence of early transition states (δB_av≈ 40%), where the polarization C^δ+-X^δ- (%E_v ≈ 43%) is the determining factor in the reaction process. Finally, a comparison of all the determined parameters showed that the reaction in the DMB-Co^ICbx system resembles the process that occurs in the larger Co^ICbl, suggesting that the former system could be a reliable model for the study of reductive dehalogenation mediated by vitamin B₁₂, which is key to the anaerobic microbiological treatment of halocarbon contaminants.

Vitamin B12 effects on chlorinated methanes-degrading microcosms: Dual isotope and metabolically active microbial populations assessment

Field-derived anoxic microcosms were used to characterize chloroform (CF) and carbon tetrachloride (CT) natural attenuation to compare it with biostimulation scenarios in which vitamin B₁₂ was added (B₁₂/pollutant ratio of 0.01 and 0.1) by means of by-products, carbon and chlorine compound-specific stable-isotope analysis, and the active microbial community through 16S rRNA MiSeq high-throughput sequencing. Autoclaved slurry controls discarded abiotic degradation processes. B₁₂ catalyzed CF and CT biodegradation without the accumulation of dichloromethane, carbon disulphide, or CF. The carbon isotopic fractionation value of CF (ƐC_CF) with B₁₂ was -14±4‰, and the value for chlorine (ƐCl_CF) was -2.4±0.4‰. The carbon isotopic fractionation values of CT (ƐC_CT) were -16±6 with B₁₂, and -13±2‰ without B₁₂; and the chlorine isotopic fractionation values of CT (ƐCl_CT) were -6±3 and -4±2‰, respectively. Acidovorax, Ancylobacter, and Pseudomonas were the most metabolically active genera, whereas Dehalobacter and Desulfitobacterium were below 0.1% of relative abundance. The dual C-Cl element isotope slope (Λ=Δδ¹³C/Δδ³⁷Cl) for CF biodegradation (only detected with B₁₂, 7±1) was similar to that reported for CF reduction by Fe(0) (8±2). Several reductive pathways might be competing in the tested CT scenarios, as evidenced by the lack of CF accumulation when B₁₂ was added, which might be linked to a major activity of Pseudomonas stutzeri; by different chlorine apparent kinetic isotope effect values and Λ which was statistically different with and without B₁₂ (5±1 vs 6.1±0.5), respectively. Thus, positive B₁₂ effects such as CT and CF degradation catalyst were quantified for the first time in isotopic terms, and confirmed with the major activity of species potentially capable of their degradation. Moreover, the indirect benefits of B₁₂ on the degradation of chlorinated ethenes were proved, creating a basis for remediation strategies in multi-contaminant polluted sites.

Theoretical analysis of C-F bond cleavage mediated by cob[I]alamin-based structures

In the present work, C-F bond cleavage mediated by the super-reduced form of cobalamin (i.e., Co^ICbl) was theoretically studied at the ONIOM(BP86/6-311++G(d,p):PM6) + SMD level of theory. Dispersion effects were introduced by employing Grimme's empirical dispersion at the ONIOM(BP86-D/6-311++G(d,p):PM6) + SMD level. In the first stage of the study, cobalamin was characterized in terms of the coordination number of the central cobalt atom. The ONIOM(BP86/6-311++G(d,p):PM6) results showed that the base-off form of the system is slightly more stable than its base-on counterpart (ΔE = E _base-off - E _base-on ~ -2 kcal/mol). The inclusion of dispersive forces in the description of the system stabilizes the base-on form, which becomes as stable as its base-off counterpart. Moreover, in the latter case, the energy barrier separating both structures was found to be negligible, with a computed value of 1.02 kcal/mol. In the second stage of the work, the reaction Co^ICbl + CH₃F → MeCbl + F^- was studied considering the base-off and the base-on forms of Co^ICbl. The reaction that occurs in the presence of the base-on form of Co^ICbl was found to be kinetically more favorable (ΔE ^≠ = 13.7 kcal/mol) than that occurring in the presence of the base-off form (ΔE ^≠ = 41.2 kcal/mol). Further reaction-force analyses of the processes showed that the energy barrier to C-F bond cleavage arises largely due to structural rearrangements when the reaction occurs on the base-on form of the Co^ICbl complex, but is mainly due to electronic rearrangements when the reaction takes place on the base-off form of the complex. The latter behavior emerges from differences in the synchronicity of the bond strengthening/weakening processes along the reaction path; the base-on mode of Co^ICbl is able to decrease the synchronicity of the chemical events. This work gives new molecular-level insights into the role of Cbl-based systems in the cleavage of C-F bonds. These insights have potential implications for research into processes for degrading fluorine-containing pollutants.

Carbon and Chlorine Isotope Fractionation Patterns Associated with Different Engineered Chloroform Transformation Reactions

To use compound-specific isotope analysis for confidently assessing organic contaminant attenuation in the environment, isotope fractionation patterns associated with different transformation mechanisms must first be explored in laboratory experiments. To deliver this information for the common groundwater contaminant chloroform (CF), this study investigated for the first time both carbon and chlorine isotope fractionation for three different engineered reactions: oxidative C-H bond cleavage using heat-activated persulfate, transformation under alkaline conditions (pH ∼ 12) and reductive C-Cl bond cleavage by cast zerovalent iron, Fe(0). Carbon and chlorine isotope fractionation values were -8 ± 1‰ and -0.44 ± 0.06‰ for oxidation, -57 ± 5‰ and -4.4 ± 0.4‰ for alkaline hydrolysis (pH 11.84 ± 0.03), and -33 ± 11‰ and -3 ± 1‰ for dechlorination, respectively. Carbon and chlorine apparent kinetic isotope effects (AKIEs) were in general agreement with expected mechanisms (C-H bond cleavage in oxidation by persulfate, C-Cl bond cleavage in Fe(0)-mediated reductive dechlorination and E1_CB elimination mechanism during alkaline hydrolysis) where a secondary AKIE_Cl (1.00045 ± 0.00004) was observed for oxidation. The different dual carbon-chlorine (Δδ¹³C vs Δδ³⁷Cl) isotope patterns for oxidation by thermally activated persulfate and alkaline hydrolysis (17 ± 2 and 13.0 ± 0.8, respectively) vs reductive dechlorination by Fe(0) (8 ± 2) establish a base to identify and quantify these CF degradation mechanisms in the field.

Recovery of Elemental Tellurium Nanoparticles by the Reduction of Tellurium Oxyanions in a Methanogenic Microbial Consortium

This research focuses on the microbial recovery of elemental tellurium (Te(0)) from aqueous streams containing soluble tellurium oxyanions, tellurate (Te(VI)), and tellurite (Te(IV)). An anaerobic mixed microbial culture occurring in methanogenic granular sludge was able to biocatalyze the reduction of both Te oxyanions to produce Te(0) nanoparticles (NPs) in sulfur-free medium. Te(IV) reduction was seven times faster than that of Te(VI), such that Te(IV) did not accumulate to a great extent during Te(VI) reduction. Endogenous substrates in the granular sludge provided the electron equivalents required to reduce Te oxyanions; however, the reduction rates were modestly increased with an exogenous electron donor such as H2. The effect of four redox mediators (anthraquinone-2,6-disulfonate, hydroxocobalamin, riboflavin, and lawsone) was also tested. Riboflavin increased the rate of Te(IV) reduction eleven-fold and also enhanced the fraction Te recovered as extracellular Te(0) NPs from 21% to 64%. Lawsone increased the rate of Te(VI) reduction five-fold, and the fraction of Te recovered as extracellular material increased from 49% to 83%. The redox mediators and electron donors also impacted the morphologies and localization of Te(0) NPs, suggesting that NP production can be tailored for a particular application.

Enhanced U(VI) bioreduction by alginate-immobilized uranium-reducing bacteria in the presence of carbon nanotubes and anthraquinone-2,6-disulfonate

Uranium-reducing bacteria were immobilized with sodium alginate, anthraquinone-2,6-disulfonate (AQDS), and carbon nanotubes (CNTs). The effects of different AQDS-CNTs contents, U(IV) concentrations, and metal ions on U(IV) reduction by immobilized beads were examined. Over 97.5% U(VI) (20 mg/L) was removed in 8 hr when the beads were added to 0.7% AQDS-CNTs, which was higher than that without AQDS-CNTs. This result may be attributed to the enhanced electron transfer by AQDS and CNTs. The reduction of U(VI) occurred at initial U(VI) concentrations of 10 to 100 mg/L and increased with increasing AQDS-CNT content from 0.1% to 1%. The presence of Fe(III), Cu(II) and Mn(II) slightly increased U(VI) reduction, whereas Cr(VI), Ni(II), Pb(II), and Zn(II) significantly inhibited U(VI) reduction. After eight successive incubation-washing cycles or 8 hr of retention time (HRT) for 48 hr of continuous operation, the removal efficiency of uranium was above 90% and 92%, respectively. The results indicate that the AQDS-CNT/AL/cell beads are suitable for the treatment of uranium-containing wastewaters.

Bioaugmentation with distinct Dehalobacter strains achieves chloroform detoxification in microcosms

Chloroform (CF) is a widespread groundwater contaminant not susceptible to aerobic degradation. Under anoxic conditions, CF can undergo abiotic and cometabolic transformation but detoxification is generally not achieved. The recent discovery of distinct Dehalobacter strains that respire CF to dichloromethane (DCM) and ferment DCM to nonchlorinated products promises that bioremediation of CF plumes is feasible. To track both strains, 16S rRNA gene-based qPCR assays specific for either Dehalobacter strain were designed and validated. A laboratory treatability study explored the value of bioaugmentation and biostimulation to achieve CF detoxification using anoxic microcosms established with aquifer material from a CF-contaminated site. Microcosms that received 6% (v/v) of the CF-to-DCM-dechlorinating culture Dhb-CF to achieve an initial Dehalobacter cell titer of 1.6 ± 0.9 × 10(4) mL(-1) dechlorinated CF to stoichiometric amounts of DCM. Subsequent augmentation with 3% (v/v) of the DCM-degrading consortium RM to an initial Dehalobacter cell abundance of 1.2 ± 0.2 × 10(2) mL(-1) achieved complete DCM degradation in microcosms amended with 10 mM bicarbonate. Growth of the CF-respiring and the DCM-degrading Dehalobacter populations and detoxification were also observed in microcosms that received both inocula simultaneously. These findings suggest that anaerobic bioremediation (e.g., bioaugmentation) is a possible remedy at CF- and DCM-contaminated sites without CT, which strongly inhibited CF organohalide respiration and DCM organohalide fermentation.

Effective and characteristics of anthraquinone-2,6-disulfonate (AQDS) on denitrification by Paracoccus versutus sp.GW1

The effects of anthraquinone-2,6-disulfonate (AQDS) on microbial populations and the biocatalysing characteristics of AQDS on denitrifying strain GW1 are discussed. The results showed that microbial population abundances were 30, 6, 12 and 18% during the different periods (1st, 5th, 10th and 20th days). Strain GW1 utilized AQDS as an electron acceptor in the respiration chain and reduced AQDS to hydroquinone (QH2). The nitrate removal efficiency of strain GW1 was increased 1.14-1.63 fold with 0.04-0.32 mmol/L AQDS. A linear correlation was found between the nitrate removal reaction constant k[gNO3(-)- N/(gVSS x d)] (where VSS is volatile suspended solids) and AQDS concentration (CAQDS), which was k = 19.332 C(AQDS) + 11.115 (R2 = 0.9749). The stabilized oxidation-reduction potential (ORP) values with AQDS were lower 22-92 mv than the control during the denitrification process. The concentration of nitrite sequentially accumulated up to around 250 mg/L when nitrate have removed completely. The results suggested that AQDS as redox mediator was capable of biocatalysing the nitrate and nitrite removal rate.

Transcriptional analysis of Clostridium beijerinckii NCIMB 8052 to elucidate role of furfural stress during acetone butanol ethanol fermentation

BACKGROUND

Furfural is the prevalent microbial inhibitor generated during pretreatment and hydrolysis of lignocellulose biomass to monomeric sugars, but the response of acetone butanol ethanol (ABE) producing Clostridium beijerinckii NCIMB 8052 to this compound at the molecular level is unknown. To discern the effect of furfural on C. beijerinckii and to gain insight into molecular mechanisms of action and detoxification, physiological changes of furfural-stressed cultures during acetone butanol ethanol (ABE) fermentation were studied, and differentially expressed genes were profiled by genome-wide transcriptional analysis.

RESULTS

A total of 5,003 C. beijerinckii NCIMB 8052 genes capturing about 99.7% of the genome were examined. About 111 genes were differentially expressed (up- or down-regulated) by C. beijerinckii when it was challenged with furfural at acidogenic growth phase compared with 721 genes that were differentially expressed (up- or down-regulated) when C. beijerinckii was challenged with furfural at solventogenic growth phase. The differentially expressed genes include genes related to redox and cofactors, membrane transporters, carbohydrate, amino sugar and nucleotide sugar metabolisms, heat shock proteins, DNA repair, and two-component signal transduction system. While C. beijerinckii exposed to furfural stress during the acidogenic growth phase produced 13% more ABE than the unstressed control, ABE production by C. beijerinckii ceased following exposure to furfural stress during the solventogenic growth phase.

CONCLUSION

Genome-wide transcriptional response of C. beijerinckii to furfural stress was investigated for the first time using microarray analysis. Stresses emanating from ABE accumulation in the fermentation medium; redox balance perturbations; and repression of genes that code for the phosphotransferase system, cell motility and flagellar proteins (and combinations thereof) may have caused the premature termination of C. beijerinckii 8052 growth and ABE production following furfural challenge at the solventogenic phase.This study provides insights into basis for metabolic engineering of C. beijerinckii NCIMB 8052 for enhanced tolerance of lignocellulose-derived microbial inhibitory compounds, thereby improving bioconversion of lignocellulose biomass hydrolysates to biofuels and chemicals. Indeed, two enzymes encoded by Cbei_3974 and Cbei_3904 belonging to aldo/keto reductase (AKR) and short-chain dehydrogenase/reductase (SDR) families have been identified to be involved in furfural detoxification and tolerance.

Microbial degradation of chloroform

Chloroform (CF) is largely produced by both anthropogenic and natural sources. It is detected in ground and surface water sources and it represents the most abundant halocarbon in the atmosphere. Microbial CF degradation occurs under both aerobic and anaerobic conditions. Apart from a few reports describing the utilization of CF as a terminal electron acceptor during growth, CF degradation was mainly reported as a cometabolic process. CF aerobic cometabolism is supported by growth on short-chain alkanes (i.e., methane, propane, butane, and hexane), aromatic hydrocarbons (i.e., toluene and phenol), and ammonia via the activity of monooxygenases (MOs) operatively divided into different families. The main factors affecting CF cometabolism are (1) the inhibition of CF degradation exerted by the growth substrate, (2) the need for reductant supply to maintain MO activity, and (3) the toxicity of CF degradation products. Under anaerobic conditions, CF degradation was mainly associated to the activity of methanogens, although some examples of CF-degrading sulfate-reducing, fermenting, and acetogenic bacteria are reported in the literature. Higher CF toxicity levels and lower degradation rates were shown by anaerobic systems in comparison to the aerobic ones. Applied physiological and genetic aspects of microbial cometabolism of CF will be presented along with bioremediation perspectives.

Successful microcosm demonstration of a strategy for biodegradation of a mixture of carbon tetrachloride and perchloroethene harnessing sulfate reducing and dehalorespiring bacteria

Carbon tetrachloride (CT) is known to inhibit the transformation of perchloroethene (PCE) to ethene by dehalorespiring bacteria, creating a challenge for the bioremediation of environments contaminated with both compounds. We report on the sequential use of sulfate reduction and dehalorespiration as a microbial strategy for the transformation of a mixture of CT (10 μM) and PCE (14 μM). Sulfide production in Desulfovibrio vulgaris cultures led to complete CT disappearance in as little as 12 days. The addition of amorphous ferric oxide decreased the proportion of chloroform (CF) produced from 65% to 30%. CT conversion rates were enhanced more than 13-fold where vitamin B(12) (5 μM) was added. In vitamin B(12)-containing D. vulgaris cultures, no chlorinated products were detected and carbon disulfide was the major product of CT transformation. PCE concentrations were not impacted upon by D. vulgaris activity. The subsequent inoculation of a PCE-respiring enrichment culture resulted in microbial PCE dechlorination to ethene.

Chloroform respiration to dichloromethane by a Dehalobacter population

Chloroform (CF), or trichloromethane, is an ubiquitous environmental pollutant because of its widespread industrial use, historically poor disposal and recalcitrance to biodegradation. Chloroform is a potent inhibitor of metabolism and no known organism uses it as a growth substrate. We discovered that CF was rapidly and sustainably dechlorinated in the course of investigating anaerobic reductive dechlorination of 1,1,1-trichloroethane in a Dehalobacter-containing culture. Like 1,1,1-trichloroethane dechlorination in this culture, CF dechlorination was a growth-linked respiratory process, requiring H(2) as an electron donor and CF as an electron acceptor. Moreover, the same specific reductive dehalogenase likely catalyzed both reactions. This Dehalobacter population appears specialized for substrates with three halogen substituents on the same carbon atom, with widespread implications for bioremediation.

Impact and application of electron shuttles on the redox (bio)transformation of contaminants: a review

During the last two decades, extensive research has explored the catalytic effects of different organic molecules with redox mediating properties on the anaerobic (bio)transformation of a wide variety of organic and inorganic compounds. The accumulated evidence points at a major role of electron shuttles in the redox conversion of several distinct contaminants, both by chemical and biological mechanisms. Many microorganisms are capable of reducing redox mediators linked to the anaerobic oxidation of organic and inorganic substrates. Electron shuttles can also be chemically reduced by electron donors commonly found in anaerobic environments (e.g. sulfide and ferrous iron). Reduced electron shuttles can transfer electrons to several distinct electron-withdrawing compounds, such as azo dyes, polyhalogenated compounds, nitroaromatics and oxidized metalloids, among others. Moreover, reduced molecules with redox properties can support the microbial reduction of electron acceptors, such as nitrate, arsenate and perchlorate. The aim of this review paper is to summarize the results of reductive (bio)transformation processes catalyzed by electron shuttles and to indicate which aspects should be further investigated to enhance the applicability of redox mediators on the (bio)transformation of contaminants.

Enhanced anaerobic biotransformation of carbon tetrachloride with precursors of vitamin B(12) biosynthesis

Relatively low concentrations of Vitamin B(12) are known to accelerate the anaerobic biotransformation of carbon tetrachloride (CT) and chloroform (CF). However, the addition of vitamin B(12) for field-scale bioremediation is expected to be costly. The present study considered a strategy to generate vitamin B(12) by addition of biosynthetic precursors. One of the precursors, porphobilinogen (PB) involved in the formation of the corrin ring, significantly increased the CT biotransformation rates by 2.7-, 8.8- and 10.9-fold when supplemented at 160, 500 and 900 microM, respectively. A positive control with 10 microM of vitamin B(12) resulted in a 5.9-fold increase in the CT-bioconversion rate. PB additions provided high molar yields of inorganic chloride (57% of CT organochlorine), comparable to that obtained with vitamin B(12) supplemented cultures. The primary substrate, methanol, known to induce vitamin B(12) production in methanogens and acetogens, was required for PB to have a significant impact on CT conversion. The observation suggests that PB's role was due to stimulating vitamin B(12) biosynthesis. The present study therefore provides insights on how to achieve vitamin B(12) enhanced rates of CT bioremediation through the use of less complex compounds that are precursors of vitamin B(12). Although PB is a costly chemical, its large impact points to corrin ring formation as the rate-limiting step.