Grape pomace compost harbors organohalide-respiring Dehalogenimonas species with novel reductive dehalogenase genes

Pilot tests for the optimization of the bioremediation strategy of a multi-layered aquifer at a multi-focus site impacted with chlorinated ethenes

A multi-layered aquifer in an industrial area in the north of the Iberian Peninsula is severely contaminated with the chlorinated ethenes (CEs) tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, and vinyl chloride. Both shallow and deep aquifers are polluted, with two differentiated north and south CEs plumes. Hydrogeochemical and isotopic data (δ13C of CEs) evidenced natural attenuation of CEs. To select the optimal remediation strategy to clean-up the contamination plumes, laboratory treatability studies were performed, which confirmed the intrinsic biodegradation potential of the north and south shallow aquifers to fully dechlorinate CEs to ethene after injection of lactate, but also the combination of lactate and sulfidized mZVI as an alternative treatment for the north deep aquifer. In the lactate-amended microcosms, full dechlorination of CEs was accompanied by an increase in 16S rRNA gene copies of Dehalococcoides and Dehalogenimonas, and the tceA, vcrA and bvcA reductive dehalogenases. Three in situ pilot tests were implemented, which consisted in injections of lactate in the north and south shallow aquifers, and injections of lactate and sulfidized mZVI in the north deep aquifer. The hydrogeochemical, isotopic and molecular analyses used to monitor the pilot tests evidenced that results obtained mimicked the laboratory observations, albeit at different dechlorination rates. It is likely that the efficiency of the injections was affected by the amendment distribution. In addition, monitoring of the pilot tests in the shallow aquifers showed the release of CEs due to back diffusion from secondary sources, which limited the use of isotopic data for assessing treatment efficiency. In the pilot test that combined the injection of lactate and sulfidized mZVI, both biotic and abiotic pathways contributed to the production of ethene. This study demonstrates the usefulness of integrating different chemical, isotopic and biomolecular approaches for a more robust selection and implementation of optimal remediation strategies in CEs polluted sites.

Anaerobic biodegradation of perfluorooctane sulfonate (PFOS) and microbial community composition in soil amended with a dechlorinating culture and chlorinated solvents

Perfluorooctane sulfonate (PFOS), one of the most frequently detected per- and polyfluoroalkyl substances (PFAS) occurring in soil, surface water, and groundwater near sites contaminated with aqueous film-forming foam (AFFF), has proven to be recalcitrant to many destructive remedies, including chemical oxidation. We investigated the potential to utilize microbially mediated reduction (bioreduction) to degrade PFOS and other PFAS through addition of a known dehalogenating culture, WBC-2, to soil obtained from an AFFF-contaminated site. A substantial decrease in total mass of PFOS (soil and water) was observed in microcosms amended with WBC-2 and chlorinated volatile organic compound (cVOC) co-contaminants - 46.4 ± 11.0 % removal of PFOS over the 45-day experiment. In contrast, perfluorooctanoate (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS) concentrations did not decrease in the same microcosms. The low or non-detectable concentrations of potential metabolites in full PFAS analyses, including after application of the total oxidizable precursor assay, indicated that defluorination occurred to non-fluorinated compounds or ultrashort-chain PFAS. Nevertheless, additional research on the metabolites and degradation pathways is needed. Population abundances of known dehalorespirers did not change with PFOS removal during the experiment, making their association with PFOS removal unclear. An increased abundance of sulfate reducers in the genus Desulfosporosinus (Firmicutes) and Sulfurospirillum (Campilobacterota) was observed with PFOS removal, most likely linked to initiation of biodegradation by desulfonation. These results have important implications for development of in situ bioremediation methods for PFAS and advancing knowledge of natural attenuation processes.

Dual C-Cl isotope fractionation offers potential to assess biodegradation of 1,2-dichloropropane and 1,2,3-trichloropropane by Dehalogenimonas cultures

1,2-dichloropropane (1,2-DCP) and 1,2,3-trichloropropane (1,2,3-TCP) are hazardous chemicals frequently detected in groundwater near agricultural zones due to their historical use in chlorinated fumigant formulations. In this study, we show that the organohalide-respiring bacterium Dehalogenimonas alkenigignens strain BRE15 M can grow during the dihaloelimination of 1,2-DCP and 1,2,3-TCP to propene and allyl chloride, respectively. Our work also provides the first application of dual isotope approach to investigate the anaerobic reductive dechlorination of 1,2-DCP and 1,2,3-TCP. Stable carbon and chlorine isotope fractionation values for 1,2-DCP (ƐC = -13.6 ± 1.4 ‰ and ƐCl = -27.4 ± 5.2 ‰) and 1,2,3-TCP (ƐC = -3.8 ± 0.6 ‰ and ƐCl = -0.8 ± 0.5 ‰) were obtained resulting in distinct dual isotope slopes (Λ12DCP = 0.5 ± 0.1, Λ123TCP = 4 ± 2). However direct comparison of ΛC-Cl among different substrates is not possible and investigation of the C and Cl apparent kinetic isotope effects lead to the hypothesis that concerted dichloroelimination mechanism is more likely for both compounds. In fact, whole cell activity assays using cells suspensions of the Dehalogenimonas-containing culture grown with 1,2-DCP and methyl viologen as electron donor suggest that the same set of reductive dehalogenases was involved in the transformation of 1,2-DCP and 1,2,3-TCP. This study opens the door to the application of isotope techniques for evaluating biodegradation of 1,2-DCP and 1,2,3-TCP, which often co-occur in groundwaters near agricultural fields.

Metagenomic 16S rRNA analysis and predictive functional profiling revealed intrinsic organohalides respiration and bioremediation potential in mangrove sediment

BACKGROUND

Mangrove sediment microbes are increasingly attracting scientific attention due to their demonstrated capacity for diverse bioremediation activities, encompassing a wide range of environmental contaminants.

MATERIALS AND METHODS

The microbial communities of five Avicennia marina mangrove sediment samples collected from Al Rayyis White Head, Red Sea (KSA), were characterized using Illumina amplicon sequencing of the 16S rRNA genes.

RESULTS

Our study investigated the microbial composition and potential for organohalide bioremediation in five mangrove sediments from the Red Sea. While Proteobacteria dominated four microbiomes, Bacteroidetes dominated the fifth. Given the environmental concerns surrounding organohalides, their bioremediation is crucial. Encouragingly, we identified phylogenetically diverse organohalide-respiring bacteria (OHRB) across all samples, including Dehalogenimonas, Dehalococcoides, Anaeromyxobacter, Desulfuromonas, Geobacter, Desulfomonile, Desulfovibrio, Shewanella and Desulfitobacterium. These bacteria are known for their ability to dechlorinate organohalides through reductive dehalogenation. PICRUSt analysis further supported this potential, predicting the presence of functional biomarkers for organohalide respiration (OHR), including reductive dehalogenases targeting tetrachloroethene (PCE) and 3-chloro-4-hydroxyphenylacetate in most sediments. Enrichment cultures studies confirmed this prediction, demonstrating PCE dechlorination by the resident microbial community. PICRUSt also revealed a dominance of anaerobic metabolic processes, suggesting the microbiome's adaptation to the oxygen-limited environment of the sediments.

CONCLUSION

This study provided insights into the bacterial community composition of five mangrove sediments from the Red Sea. Notably, diverse OHRB were detected across all samples, which possess the metabolic potential for organohalide bioremediation through reductive dehalogenation pathways. Furthermore, PICRUSt analysis predicted the presence of functional biomarkers for OHR in most sediments, suggesting potential intrinsic OHR activity by the enclosed microbial community.

Unveiling complete natural reductive dechlorination mechanisms of chlorinated ethenes in groundwater: Insights from functional gene analysis

Monitored natural attenuation (MNA) of chlorinated ethenes (CEs) has proven to be a cost-effective and environment-friendly approach for groundwater remediation. In this study, the complete dechlorination of CEs with formation of ethene under natural conditions, were observed at two CE-contaminated sites, including a pesticide manufacturing facility (PMF) and a fluorochemical plant (FCP), particularly in the deeply weathered bedrock aquifer at the FCP site. Additionally, a higher abundance of CE-degrading bacteria was identified with heightened dechlorination activities at the PMF site, compared to the FCP site. The reductive dehalogenase genes and Dhc 16 S rRNA gene were prevalent at both sites, even in groundwater where no CE dechlorination was observed. vcrA and bvcA was responsible for the complete dechlorination at the PMF and FCP site, respectively, indicating the distinct contributions of functional microbial species at each site. The correlation analyses suggested that Sediminibacterium has the potential to achieve the complete dechlorination at the FCP site. Moreover, the profiles of CE-degrading bacteria suggested that dechlorination occurred under Fe3+/sulfate-reducing and nitrate-reducing conditions at the PMF and FCP site, respectively. Overall these findings provided multi-lines of evidence on the diverse mechanisms of CE-dechlorination under natural conditions, which can provide valuable guidance for MNA strategies implementation.

Response of TCE biodegradation to elevated H2 and O2: Implication for electrokinetic-enhanced bioremediation

The study investigated the influences of pure H2 and O2 introduction, simulating gases produced from the electrokinetic-enhanced bioremediation (EK-Bio), on TCE degradation, and the dynamic changes of the indigenous microbial communities. The dissolved hydrogen (DH) and oxygen (DO) concentrations ranged from 0.2 to 0.7 mg/L and 2.6 to 6.6 mg/L, respectively. The biological analysis was conducted by 16S rRNA sequencing and functional gene analyses. The results showed that the H2 introduction enhanced TCE degradation, causing a 90.4% TCE removal in the first 4 weeks, and 131.1 μM was reduced eventually. Accordingly, cis-dichloroethylene (cis-DCE) was produced as the only product. The following three ways should be responsible for this promoted TCE degradation. Firstly, the high DH rapidly reduced the oxidation-reduction potential (ORP) value to around -500 mV, beneficial to TCE microbial dechlorination. Secondly, the high DH significantly changed the community and promoted the enrichment of TCE anaerobic dechlorinators, such as Sulfuricurvum, Sulfurospirillum, Shewanella, Geobacter, and Desulfitobacterium, and increased the abundance of dechlorination gene pceA. Thirdly, the high DH promoted preferential TCE dechlorination and subsequent sulfate reduction. However, TCE bio-remediation did not occur in a high DO environment due to the reduced aerobic function or lack of functional bacteria or co-metabolic substrate. The competitive dissolved organic carbon (DOC) consumption and unfriendly microbe-microbe interactions also interpreted the non-degradation of TCE in the high DO environment. These results provided evidence for the mechanism of EK-Bio. Providing anaerobic obligate dechlorinators, and aerobic metabolic bacteria around the electrochemical cathodes and anodes, respectively, or co-metabolic substrates to the anode can be feasible methods to promote remediation of TCE-contaminated shallow aquifer under EK-Bio technology.

Global prevalence of organohalide-respiring bacteria dechlorinating polychlorinated biphenyls in sewage sludge

BACKGROUND

Massive amounts of sewage sludge are generated during biological sewage treatment and are commonly subjected to anaerobic digestion, land application, and landfill disposal. Concurrently, persistent organic pollutants (POPs) are frequently found in sludge treatment and disposal systems, posing significant risks to both human health and wildlife. Metabolically versatile microorganisms originating from sewage sludge are inevitably introduced to sludge treatment and disposal systems, potentially affecting the fate of POPs. However, there is currently a dearth of comprehensive assessments regarding the capability of sewage sludge microbiota from geographically disparate regions to attenuate POPs and the underpinning microbiomes.

RESULTS

Here we report the global prevalence of organohalide-respiring bacteria (OHRB) known for their capacity to attenuate POPs in sewage sludge, with an occurrence frequency of ~50% in the investigated samples (605 of 1186). Subsequent laboratory tests revealed microbial reductive dechlorination of polychlorinated biphenyls (PCBs), one of the most notorious categories of POPs, in 80 out of 84 sludge microcosms via various pathways. Most chlorines were removed from the para- and meta-positions of PCBs; nevertheless, ortho-dechlorination of PCBs also occurred widely, although to lower extents. Abundances of several well-characterized OHRB genera (Dehalococcoides, Dehalogenimonas, and Dehalobacter) and uncultivated Dehalococcoidia lineages increased during incubation and were positively correlated with PCB dechlorination, suggesting their involvement in dechlorinating PCBs. The previously identified PCB reductive dehalogenase (RDase) genes pcbA4 and pcbA5 tended to coexist in most sludge microcosms, but the low ratios of these RDase genes to OHRB abundance also indicated the existence of currently undescribed RDases in sewage sludge. Microbial community analyses revealed a positive correlation between biodiversity and PCB dechlorination activity although there was an apparent threshold of community co-occurrence network complexity beyond which dechlorination activity decreased.

CONCLUSIONS

Our findings that sludge microbiota exhibited nearly ubiquitous dechlorination of PCBs indicate widespread and nonnegligible impacts of sludge microbiota on the fate of POPs in sludge treatment and disposal systems. The existence of diverse OHRB also suggests sewage sludge as an alternative source to obtain POP-attenuating consortia and calls for further exploration of OHRB populations in sewage sludge. Video Abstract.

Application of dissimilatory iron-reducing bacteria for the remediation of soil and water polluted with chlorinated organic compounds: Progress, mechanisms, and directions

Chlorinated organic compounds are widely used as solvents, but they are pollutants that can have adverse effects on the environment and human health. Dissimilatory iron-reducing bacteria (DIRB) such as Shewanella and Geobacter have been applied to treat a wide range of halogenated organic compounds due to their specific biological properties. Until now, there has been no systematic review on the mechanisms of direct or indirect degradation of halogenated organic compounds by DIRB. This work summarizes the discussion of DIRB's ability to enhance the dechlorination of reaction systems through different pathways, both biological and biochemical. For biological dechlorination, some DIRB have self-dechlorination capabilities that directly dechlorinate by hydrolysis. Adjustment of dechlorination genes through genetic engineering can improve the dechlorination capabilities of DIRB. DIRB can also adjust the capacity for the microbial community to dechlorinate and provide nutrients to enhance the expression of dechlorination genes in other bacteria. In biochemical dechlorination, DIRB bioconverts Fe(III) to Fe(II), which is capable of dichlorination. On this basis, the DIRB-driven Fenton reaction can efficiently degrade chlorinated organics by continuously maintaining anoxic conditions to generate Fe(II) and oxic conditions to generate H2O2. DIRB can drive microbial fuel cells due to their electroactivity and have a good dechlorination capacity at low levels of energy consumption. The contribution of DIRB to the removal of pesticides, antibiotics and POPs is summarized. Then the DIRB electron transfer mechanism is discussed, which is core to their ability to dechlorinate. Finally, the prospect of future work on the removal of chlorine-containing organic pollutants by DIRB is presented, and the main challenges and further research directions are suggested.

Genome-Centric Metatranscriptomic Characterization of a Humin-Facilitated Anaerobic Tetrabromobisphenol A-Dehalogenating Consortium

Tetrabromobisphenol A (TBBPA), a widely used brominated flame retardant in electronics manufacturing, has caused global contamination due to improper e-waste disposal. Its persistence, bioaccumulation, and potential carcinogenicity drive studies of its transformation and underlying (a)biotic interactions. This study achieved an anaerobic enrichment culture capable of reductively dehalogenating TBBPA to the more bioavailable bisphenol A. 16S rRNA gene amplicon sequencing and quantitative PCR confirmed that successive dehalogenation of four bromide ions from TBBPA was coupled with the growth of both Dehalobacter sp. and Dehalococcoides sp. with growth yields of 5.0 ± 0.4 × 108 and 8.6 ± 4.6 × 108 cells per μmol Br- released (N = 3), respectively. TBBPA dehalogenation was facilitated by solid humin and reduced humin, which possessed the highest organic radical signal intensity and reducing groups -NH2, and maintained the highest dehalogenation rate and dehalogenator copies. Genome-centric metatranscriptomic analyses revealed upregulated putative TBBPA-dehalogenating rdhA (reductive dehalogenase) genes with humin amendment, cprA-like Dhb_rdhA1 gene in Dehalobacter species, and Dhc_rdhA1/Dhc_rdhA2 genes in Dehalococcoides species. The upregulated genes of lactate fermentation, de novo corrinoid biosynthesis, and extracellular electron transport in the humin amended treatment also stimulated TBBPA dehalogenation. This study provided a comprehensive understanding of humin-facilitated organohalide respiration.

Integrated Advanced Molecular Tools Predict In Situ cVOC Degradation Rates: Field Demonstration

Chlorinated volatile organic compound (cVOC) degradation rate constants are crucial information for site management. Conventional approaches generate rate estimates from the monitoring and modeling of cVOC concentrations. This requires time series data collected along the flow path of the plume. The estimates of rate constants are often plagued by confounding issues, making predictions cumbersome and unreliable. Laboratory data suggest that targeted quantitative analysis of Dehalococcoides mccartyi (Dhc) biomarker genes (qPCR) and proteins (qProt) can be directly correlated with reductive dechlorination activity. To assess the potential of qPCR and qProt measurements to predict rates, we collected data from cVOC-contaminated aquifers. At the benchmark study site, the rate constant for degradation of cis-dichloroethene (cDCE) extracted from monitoring data was 11.0 ± 3.4 yr-1, and the rate constant predicted from the abundance of TceA peptides was 6.9 yr-1. The rate constant for degradation of vinyl chloride (VC) from monitoring data was 8.4 ± 5.7 yr-1, and the rate constant predicted from the abundance of TceA peptides was 5.2 yr-1. At the other study sites, the rate constants for cDCE degradation predicted from qPCR and qProt measurements agreed within a factor of 4. Under the right circumstances, qPCR and qProt measurements can be useful to rapidly predict rates of cDCE and VC biodegradation, providing a major advance in effective site management.

Machine Learning Modeling Based on Microbial Community for Prediction of Natural Attenuation in Groundwater

Natural attenuation is widely adopted as a remediation strategy, and the attenuation potential is crucial to evaluate whether remediation goals can be achieved within the specified time. In this work, long-term monitoring of indigenous microbial communities as well as benzene, toluene, ethylbenzene, and xylene (BTEX) and chlorinated aliphatic hydrocarbons (CAHs) in groundwater was conducted at a historic pesticide manufacturing site. A machine learning approach for natural attenuation prediction was developed with random forest classification (RFC) followed by either random forest regression (RFR) or artificial neural networks (ANNs), utilizing microbiological information and contaminant attenuation rates for model training and cross-validation. Results showed that the RFC could accurately predict the feasibility of natural attenuation for both BTEX and CAHs, and it could successfully identify the key genera. The RFR model was sufficient for the BTEX natural attenuation rate prediction but unreliable for CAHs. The ANN model showed better performance in the prediction of the attenuation rates for both BTEX and CAHs. Based on the assessments, a composite modeling method of RFC and ANN was proposed, which could reduce the mean absolute percentage errors. This study reveals that the combined machine learning approach under the synergistic use of field microbial data has promising potential for predicting natural attenuation.

Improve Niche Colonization and Microbial Interactions for Organohalide-Respiring-Bacteria-Mediated Remediation of Chloroethene-Contaminated Sites

Organohalide-respiring bacteria (OHRB)-mediated reductive dehalogenation is promising in in situ bioremediation of chloroethene-contaminated sites. The bioremediation efficiency of this approach is largely determined by the successful colonization of fastidious OHRB, which is highly dependent on the presence of proper growth niches and microbial interactions. In this study, based on two ecological principles (i.e., Priority Effects and Coexistence Theory), three strategies were developed to enhance niche colonization of OHRB, which were tested both in laboratory experiments and field applications: (i) preinoculation of a niche-preparing culture (NPC, being mainly constituted of fermenting bacteria and methanogens); (ii) staggered fermentation; and (iii) increased inoculation of CE40 (a Dehalococcoides-containing tetrachloroethene-to-ethene dechlorinating enrichment culture). Batch experimental results show significantly higher dechlorination efficiencies, as well as lower concentrations of volatile fatty acids (VFAs) and methane, in experimental sets with staggered fermentation and niche-preconditioning with NPC for 4 days (CE40_NPC-4) relative to control sets. Accordingly, a comparatively higher abundance of Dehalococcoides as major OHRB, together with a lower abundance of fermenting bacteria and methanogens, was observed in CE40_NPC-4 with staggered fermentation, which indicated the balanced syntrophic and competitive interactions between OHRB and other populations for the efficient dechlorination. Further experiments with microbial source tracking analyses suggested enhanced colonization of OHRB by increasing the inoculation ratio of CE40. The optimized conditions for enhanced colonization of OHRB were successfully employed for field bioremediation of trichloroethene (TCE, 0.3-1.4 mM)- and vinyl chloride (VC, ∼0.04 mM)-contaminated sites, resulting in 96.6% TCE and 99.7% VC dechlorination to ethene within 5 and 3 months, respectively. This study provides ecological principles-guided strategies for efficient bioremediation of chloroethene-contaminated sites, which may be also employed for removal of other emerging organohalide pollutants.

Short-chain fatty acids facilitated long-term dechlorination of PCBs in Taihu Lake sediment microcosms: Evidence from PCB congener and microbial community analyses

Microbial reductive dechlorination hosts great promise as an in situ bioremediation strategy for polychlorinated biphenyls (PCBs) contamination. However, the slow dechlorination in sediments limits natural attenuation. Short-chain fatty acids, as preferred carbon sources and electron donors for dechlorinating microorganisms, might stimulate PCB dechlorination. Herein, two sets of short-chain fatty acids, sole acetate and a fatty acid mixture (acetate, propionate, and butyrate), were amended periodically into Taihu Lake (China) sediment microcosms containing nine PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170) after 24 weeks of incubation. Short-chain fatty acids facilitated the long-term PCB dechlorination and the promoting effect of the fatty acid mixture compared favorably with that of sole acetate. By the end of 108 weeks, the total PCB mass concentrations in acetate amended and fatty acid mixture amended microcosms significantly declined by 7.6% and 10.3% compared with non-amended microcosms (P < 0.05), respectively. Short-chain fatty acids selectively favored the removal of flanked meta and single-flanked para chlorines. Notably, a rare ortho dechlorination pathway, PCB25 (24-3-CB) to PCB13 (3-4-CB), was enhanced. Supplementary fatty acids significantly increased reductive dehalogenases (RDase) gene pcbA5 instead of improving the growth of Dehalococcoides. These findings highlight the merits of low cost short-chain fatty acids on in situ biostimulation in treating PCBs contamination.

Alleviating Chlorinated Alkane Inhibition on Dehalococcoides to Achieve Detoxification of Chlorinated Aliphatic Cocontaminants

Cocontamination by multiple chlorinated solvents is a prevalent issue in groundwater, presenting a formidable challenge for effective remediation. Despite the recognition of this issue, a comprehensive assessment of microbial detoxification processes involving chloroethenes and associated cocontaminants, along with the underpinning microbiome, remains absent. Moreover, strategies to mitigate the inhibitory effects of cocontaminants have not been reported. Here, we revealed that chloroform exhibited the most potent inhibitory effects, followed by 1,1,1-trichloroethane and 1,1,2-trichloroethane, on dechlorination of dichloroethenes (DCEs) in Dehalococcoides-containing consortia. The observed inhibition could be attributed to suppression of biosynthesis and enzymatic activity of reductive dehalogenases and growth of Dehalococcoides. Notably, cocontaminants more profoundly inhibited Dehalococcoides populations harboring the vcrA gene than those possessing the tceA gene, thereby explaining the accumulation of vinyl chloride under cocontaminant stress. Nonetheless, we successfully ameliorated cocontaminant inhibition by augmentation with Desulfitobacterium sp. strain PR owing to its ability to attenuate cocontaminants, resulting in concurrent detoxification of DCEs, trichloroethanes, and chloroform. Microbial community analyses demonstrated obvious alterations in taxonomic composition, structure, and assembly of the dechlorinating microbiome in the presence of cocontaminants, and introduction of strain PR reshaped the dechlorinating microbiome to be similar to its original state in the absence of cocontaminants. Altogether, these findings contribute to developing bioremediation technologies to clean up challenging sites polluted with multiple chlorinated solvents.

Mimicking reductive dehalogenases for efficient electrocatalytic water dechlorination

Electrochemical technology is a robust approach to removing toxic and persistent chlorinated organic pollutants from water; however, it remains a challenge to design electrocatalysts with high activity and selectivity as elaborately as natural reductive dehalogenases. Here we report the design of high-performance electrocatalysts toward water dechlorination by mimicking the binding pocket configuration and catalytic center of reductive dehalogenases. Specifically, our designed electrocatalyst is an assembled heterostructure by sandwiching a molecular catalyst into the interlayers of two-dimensional graphene oxide. The electrocatalyst exhibits excellent dechlorination performance, which enhances reduction of intermediate dichloroacetic acid by 7.8 folds against that without sandwich configuration and can selectively generate monochloro-groups from trichloro-groups. Molecular simulations suggest that the sandwiched inner space plays an essential role in tuning solvation shell, altering protonation state and facilitating carbon-chlorine bond cleavage. This work demonstrates the concept of mimicking natural reductive dehalogenases toward the sustainable treatment of organohalogen-contaminated water and wastewater.

Intermittent Oxygen Supply Facilitates Codegradation of Trichloroethene and Toluene by Anaerobic Consortia

Biodegradation is commonly employed for remediating trichloroethene- or toluene-contaminated sites. However, remediation methods using either anaerobic or aerobic degradation are inefficient for dual pollutants. We developed an anaerobic sequencing batch reactor system with intermittent oxygen supply for the codegradation of trichloroethylene and toluene. Our results showed that oxygen inhibited anaerobic dechlorination of trichloroethene, but dechlorination rates remained comparable to that at dissolved oxygen levels of 0.2 mg/L. Intermittent oxygenation engendered reactor redox fluctuations (-146 to -475 mV) and facilitated rapid codegradation of targeting dual pollutants, with trichloroethene degradation constituting only 27.5% of the noninhibited dechlorination. Amplicon sequencing analysis revealed the predominance of Dehalogenimonas (16.0% ± 3.5%) over Dehalococcoides (0.3% ± 0.2%), with ten times higher transcriptomic activity in Dehalogenimonas. Shotgun metagenomics revealed numerous genes related to reductive dehalogenases and oxidative stress resistance in Dehalogenimonas and Dehalococcoides, as well as the enrichment of diversified facultative populations with functional genes related to trichloroethylene cometabolism and aerobic and anaerobic toluene degradation. These findings suggested that the codegradation of trichloroethylene and toluene may involve multiple biodegradation mechanisms. Overall results of this study demonstrate the effectiveness of intermittent micro-oxygenation in aiding trichloroethene-toluene degradation, suggesting the potential for the bioremediation of sites with similar organic pollutants.

Sustained detoxification of 1,2-dichloroethane to ethylene by a symbiotic consortium containing Dehalococcoides species

1,2-Dichloroethane (1,2-DCA) is a ubiquitous volatile halogenated organic pollutant in groundwater and soil, which poses a serious threat to the ecosystem and human health. Microbial reductive dechlorination has been recognized as an environmentally-friendly strategy for the remediation of sites contaminated with 1,2-DCA. In this study, we obtained an anaerobic microbiota derived from 1,2-DCA contaminated groundwater, which was able to sustainably convert 1,2-DCA into non-toxic ethylene with an average dechlorination rate of 30.70 ± 11.06 μM d^-1 (N = 6). The microbial community profile demonstrated that the relative abundance of Dehalococcoides species increased from 0.53 ± 0.08% to 44.68 ± 3.61% in parallel with the dechlorination of 1,2-DCA. Quantitative PCR results showed that the Dehalococcoides species 16S rRNA gene increased from 2.40 ± 1.71 × 10⁸ copies∙mL^-1 culture to 4.07 ± 2.45 × 10⁸ copies∙mL^-1 culture after dechlorinating 110.69 ± 30.61 μmol of 1,2-DCA with a growth yield of 1.55 ± 0.93 × 10⁸ cells per μmol Cl^- released (N = 6), suggesting that Dehalococcoides species used 1,2-DCA for organohalide respiration to maintain cell growth. Notably, the relative abundances of Methanobacterium sp. (p = 0.0618) and Desulfovibrio sp. (p = 0.0001995) also increased significantly during the dechlorination of 1,2-DCA and were clustered in the same module with Dehalococcoides species in the co-occurrence network. These results hinted that Dehalococcoides species, the obligate organohalide-respiring bacterium, exhibited potential symbiotic relationships with Methanobacterium and Desulfovibrio species. This study illustrates the importance of microbial interactions within functional microbiota and provides a promising microbial resource for in situ bioremediation in sites contaminated with 1,2-DCA.

Dehalogenimonas etheniformans sp. nov., a formate-oxidizing, organohalide-respiring bacterium isolated from grape pomace

A strictly anaerobic, organohalide-respiring bacterium, designated strain GP^T, was characterized using a polyphasic approach. GP^T is Gram-stain-negative, non-spore-forming and non-motile. Cells are irregular cocci ranging between 0.6 and 0.9 µm in diameter. GP^T couples growth with the reductive dechlorination of 1,2-dichloroethane, vinyl chloride and all polychlorinated ethenes, except tetrachloroethene, yielding ethene and inorganic chloride as dechlorination end products. H₂ and formate serve as electron donors for organohalide respiration in the presence of acetate as carbon source. Major cellular fatty acids include C_16 : 0, C_18 : 1ω9c, C_16 : 1, C_14 : 0 and C_18 : 0. On the basis of 16S rRNA gene phylogeny, GP^T is most closely related to Dehalogenimonas formicexedens NSZ-14^T and Dehalogenimonas alkenigignens IP3-3^T with 99.8 and 97.4 % sequence identities, respectively. Genome-wide pairwise comparisons based on average nucleotide identity, average amino acid identity and digital DNA-DNA hybridization do not support the inclusion of GP^T in previously described species of the genus Dehalogenimonas with validly published names. On the basis of phylogenetic, physiological and phenotypic traits, GP^T represents a novel species within the genus Dehalogenimonas, for which the name Dehalogenimonas etheniformans sp. nov. is proposed. The type strain is GP^T (= JCM 39172^T = CGMCC 1.17861^T).

Disturbance and restoration of soil microbial communities after in-situ thermal desorption in a chlorinated hydrocarbon contaminated site

Thermal desorption technology has been widely used for the remediation of organic contaminated soil, but the heating process may alter the soil properties and its safety reutilization. After thermal remediation, the target pollutants including chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane, 1,2,3-trichloropropane and vinyl chloride in the chlorinated hydrocarbon contaminated site were reduced significantly. The soil microbial α-diversity was also reduced by more than half. Notably, the relative abundance of Chloroflexi decreased by 9.0%, while Firmicutes had a 9.0% increase after thermal remediation. By water regulation and exogenous microorganism addition, the soil microbial community could not be restored to its initial state before thermal remediation in a relatively short time (30 days). The relative abundance of Proteobacteria increased from 25.4% to 41.7% and 51.0% by water regulation and exogenous microorganism addition, respectively. The modularity of the microbial co-occurrence network was strengthened after microbial restoration, but the interaction among microorganisms was weakened. Thermal remediation might be conducive to the C- and N-cycle related processes, but severely weakened the sulfide oxidation processes. Notably, microbial restoration would benefit the recovery of the S-cycle functional groups. These results provided a new perspective for the safety reutilization of soil after thermal remediation.

Effect of wine pomace extract on dechlorination of chloroethenes in soil suspension

Chloroethenes are widely used as solvent in the metal industry and the dry cleaning industry, but their spillage into soil and groundwater due to improper handling has negatively impacted human health. Bioremediation using microorganisms is one of the technologies to clean up soil and groundwater contaminated with chloroethenes. In this study, we examined the bioremediation of chloroethene-contaminated soil using wine pomace extract (WPE). WPE is a liquid containing seven major carboxylic acids and other substances extracted from grape pomace produced in winemaking. WPE clearly promoted the anaerobic bioremediation of chloroethenes. In the tetrachloroethene (PCE) degradation test that used fractions derived from WPE, the water-eluted fraction containing L-lactic acid, L-tartaric acid, and others promoted the dechlorination of PCE, whereas the methanol-eluted fraction containing mainly syringic acid did not. In another PCE degradation test that used L-lactic acid, L-tartaric acid, and syringic acid test solutions, L-lactic acid and L-tartaric acid enhanced the dechlorination of PCE, but syringic acid did not. The results suggest that L-lactic acid and L-tartaric acid in WPE function as hydrogen donors in the anaerobic microbial degradation of chloroethene. This technology realizes environmental remediation through the effective use of food by-products.

Biohydrogenation of 1,3-Butadiene to 1-Butene under Acetogenic Conditions by Acetobacterium wieringae

The environmental fate and transformation mechanism(s) of 1,3-butadiene (BD) under anoxic conditions remain largely unexplored. Anaerobic consortia that can biohydrogenate BD to stoichiometric amounts of 1-butene at a maximum rate of 205.7 ± 38.6 μM day^-1 were derived from freshwater river sediment. The formation of 1-butene occurred only in the presence of both H₂ and CO₂ with concomitant acetate production, suggesting the dependence of BD biohydrogenation on acetogenesis. The 16S rRNA gene-targeted amplicon sequencing revealed the enrichment and dominance of a novel Acetobacterium wieringae population, designated as strain N, in the BD-biohydrogenating community. Multiple genes encoding putative ene-reductases, candidate catalysts for the hydrogenation of the C═C bond in diene compounds, were annotated on the metagenome-assembled genome of strain N, and thus attributed the BD biohydrogenation activity to strain N. Our findings emphasize an essential but overlooked role of certain Acetobacterium members (e.g., strain N) contributing to the natural attenuation of BD in contaminated subsurface environments (e.g., sediment and groundwater). Future efforts to identify and characterize the ene-reductase(s) responsible for BD biohydrogenation in strain N hold promise for the development of industrial biocatalysts capable of stereoselective conversion of BD to 1-butene.

Metabolome patterns identify active dechlorination in bioaugmentation consortium SDC-9™

Ultra-high performance liquid chromatography-high-resolution mass spectrometry (UPHLC-HRMS) is used to discover and monitor single or sets of biomarkers informing about metabolic processes of interest. The technique can detect 1000's of molecules (i.e., metabolites) in a single instrument run and provide a measurement of the global metabolome, which could be a fingerprint of activity. Despite the power of this approach, technical challenges have hindered the effective use of metabolomics to interrogate microbial communities implicated in the removal of priority contaminants. Herein, our efforts to circumvent these challenges and apply this emerging systems biology technique to microbiomes relevant for contaminant biodegradation will be discussed. Chlorinated ethenes impact many contaminated sites, and detoxification can be achieved by organohalide-respiring bacteria, a process currently assessed by quantitative gene-centric tools (e.g., quantitative PCR). This laboratory study monitored the metabolome of the SDC-9™ bioaugmentation consortium during cis-1,2-dichloroethene (cDCE) conversion to vinyl chloride (VC) and nontoxic ethene. Untargeted metabolomics using an UHPLC-Orbitrap mass spectrometer and performed on SDC-9™ cultures at different stages of the reductive dechlorination process detected ~10,000 spectral features per sample arising from water-soluble molecules with both known and unknown structures. Multivariate statistical techniques including partial least squares-discriminate analysis (PLSDA) identified patterns of measurable spectral features (peak patterns) that correlated with dechlorination (in)activity, and ANOVA analyses identified 18 potential biomarkers for this process. Statistical clustering of samples with these 18 features identified dechlorination activity more reliably than clustering of samples based only on chlorinated ethene concentration and Dhc 16S rRNA gene abundance data, highlighting the potential value of metabolomic workflows as an innovative site assessment and bioremediation monitoring tool.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

The importance of proper pH adjustment and control to achieve complete in situ enhanced reductive dechlorination

In situ bioremediation of chlorinated compounds such as perchloroethylene (PCE) and trichloroethylene (TCE) through enhanced reductive dechlorination (ERD) requires appropriate growth conditions for organohalide-respiring bacteria (OHRB). One of the most important factors controlling OHRB metabolism is groundwater pH. Dehalococcoides spp. (DHC) growth may be inhibited when pH is lower than 6.0, which can lead to the accumulation of toxic daughter compounds including cis-dichloroethylene (cDCE) and vinyl chloride (VC). Aquifer pH may decline as HCl is released during reductive dechlorination and from substrate fermentation to fatty acids and carbonic acid. In this article, we demonstrate that using proper pH adjustment and control in situ is an appropriate strategy to achieve complete ERD (i.e., complete conversion of PCE and TCE to nontoxic ethylene) in remediation sites with inherently low pH values and/or low buffering capacity. To analyze the effectiveness of this approach, field monitoring results are presented for a challenging site containing high concentrations of PCE and TCE (>10 000 µg/L and >1000 µg/L, respectively) and low aquifer pH (~4.9). Addition of a bioaugmentation culture, emulsified vegetable oil (EVO), and a colloidal buffer (CoBupH^TM ) to increase pH, stimulated rapid conversion of PCE and TCE to cDCE and VC. However, further conversion of cDCE and VC was very limited. To stimulate complete conversion to ethylene, additional CoBupH^TM and nutrients were injected, resulting in a rapid increase in metabolic rates, and maintained the aquifer pH at ~6.5 for more than five years, thus demonstrating that complete ERD can be achieved in sites with similar characteristics. Proper pH adjustment and control is needed to limit the accumulation of toxic intermediates, maintaining in situ bioremediation as an efficient, affordable, and environmentally friendly option to treat chlorinated compounds. Integr Environ Assess Manag 2022;00:1-6. © 2022 SETAC.

Resilience of organohalide-detoxifying microbial community to oxygen stress in sewage sludge

Organohalide pollutants are prevalent in the environment, causing harms to wildlife and human. Organohalide-respiring bacteria (OHRB) could detoxify these pollutants in anaerobic environments, but the most competent OHRB (i.e., Dehalococcoides) is susceptible to oxygen. This study reports exceptional resistance and resilience of sewage sludge microbial communities to oxygen stress for attenuation of structurally distinct organohalide pollutants, including tetrachloroethene, tetrabromobisphenol A, and polybrominated diphenyl ethers. The dehalogenation rate constant of these organohalide pollutants in oxygen-exposed sludge microcosms was maintained as 74-120% as that in the control without oxygen exposure. Subsequent top-down experiments clarified that sludge flocs and non-OHRB contributed to alleviating oxygen stress on OHRB. In the dehalogenating microcosms, multiple OHRB (Dehahlococcoides, Dehalogenimonas, and Sulfurospirillum) harboring distinct reductive dehalogenase genes (pceA, pteA, tceA, vcrA, and bdeA) collaborated to detoxify organohalide pollutants but responded differentially to oxygen stress. Comprehensive microbial community analyses (taxonomy, diversity, and structure) demonstrated certain resilience of the sludge-derived dehalogenating microbial communities to oxygen stress. Additionally, microbial co-occurrence networks were intensified by oxygen stress in most microcosms, as a possible stress mitigation strategy. Altogether the mechanistic and ecological findings in this study contribute to remediation of organohalide-contaminated sites encountering oxygen disturbance.

Complete Reductive Dechlorination of 4-Hydroxy-chlorothalonil by Dehalogenimonas Populations

Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile, TePN) is one of the most widely used fungicides all over the world. Its major environmental transformation product 4-hydroxy-chlorothalonil (4-hydroxy-2,5,6-trichloroisophthalonitrile, 4-OH-TPN) is more persistent, mobile, and toxic and is frequently detected at a higher concentration in various habitats compared to its parent compound TePN. Further microbial transformation of 4-OH-TPN has never been reported. In this study, we demonstrated that 4-OH-TPN underwent complete microbial reductive dehalogenation to 4-hydroxy-isophthalonitrile via 4-hydroxy-dichloroisophthalonitrile and 4-hydroxy-monochloroisophthalonitrile. 16S rRNA gene amplicon sequencing demonstrated that Dehalogenimonas species was enriched from 6% to 17-22% after reductive dechlorination of 77.24 μmol of 4-OH-TPN. Meanwhile, Dehalogenimonas copies increased by one order of magnitude and obtained a yield of 1.78 ± 1.47 × 10⁸ cells per μmol Cl^- released (N = 6), indicating that 4-OH-TPN served as the terminal electron acceptor for organohalide respiration of Dehalogenimonas species. A draft genome of Dehalogenimonas species was assembled through metagenomic sequencing, which harbors 30 putative reductive dehalogenase genes. Syntrophobacter, Acetobacterium, and Methanosarcina spp. were found to be the major non-dechlorinating populations in the microbial community, who might play important roles in the reductive dechlorination of 4-OH-TPN by the Dehalogenimonas species. This study first reports that Dehalogenimonas sp. can also respire on the seemingly dead-end product of TePN, paving the way to complete biotransformation of the widely present TePN and broadening the substrate spectrum of Dehalogenimonas sp. to polychlorinated hydroxy-benzonitrile.

Diversity of organohalide respiring bacteria and reductive dehalogenases that detoxify polybrominated diphenyl ethers in E-waste recycling sites

Widespread polybrominated diphenyl ethers (PBDEs) contamination poses risks to human health and ecosystems. Bioremediation is widely considered to be a less ecologically disruptive strategy for remediation of organohalide contamination, but bioremediation of PBDE-contaminated sites is limited by a lack of knowledge about PBDE-dehalogenating microbial populations. Here we report anaerobic PBDE debromination in microcosms established from geographically distinct e-waste recycling sites. Complete debromination of a penta-BDE mixture to diphenyl ether was detected in 16 of 24 investigated microcosms; further enrichment of these 16 microcosms implicated microbial populations belonging to the bacterial genera Dehalococcoides, Dehalogenimonas, and Dehalobacter in PBDE debromination. Debrominating microcosms tended to contain either both Dehalogenimonas and Dehalobacter or Dehalococcoides alone. Separately, complete debromination of a penta-BDE mixture was also observed by axenic cultures of Dehalococcoides mccartyi strains CG1, CG4, and 11a5, suggesting that this phenotype may be fairly common amongst Dehalococcoides. PBDE debromination in these isolates was mediated by four reductive dehalogenases not previously known to debrominate PBDEs. Debromination of an octa-BDE mixture was less prevalent and less complete in microcosms. The PBDE reductive dehalogenase homologous genes in Dehalococcoides genomes represent plausible molecular markers to predict PBDE debromination in microbial communities via their prevalence and transcriptions analysis.

Organohalide Respiration with Diclofenac by Dehalogenimonas

Diclofenac (DCF) is a pharmaceutically active contaminant frequently found in aquatic ecosystems. The transformation pathways and microbiology involved in the biodegradation of DCF, particularly under anoxic conditions, remain poorly understood. Here, we demonstrated microbially mediated reductive dechlorination of DCF in anaerobic enrichment culture derived from contaminated river sediment. Over 90% of the initial 76.7 ± 3.6 μM DCF was dechlorinated at a maximum rate of 1.8 ± 0.3 μM day^-1 during a 160 days' incubation. Mass spectrometric analysis confirmed that 2-(2-((2-chlorophenyl)amino)phenyl)acetic acid (2-CPA) and 2-anilinophenylacetic acid (2-APA) were formed as the monochlorinated and nonchlorinated DCF transformation products, respectively. A survey of microbial composition and Sanger sequencing revealed the enrichment and dominance of a new Dehalogenimonas population, designated as Dehalogenimonas sp. strain DCF, in the DCF-dechlorinating community. Following the stoichiometric conversion of DCF to 2-CPA (76.0 ± 2.1 μM) and 2-APA (3.7 ± 0.8 μM), strain DCF cell densities increased by 24.4 ± 4.4-fold with a growth yield of 9.0 ± 0.1 × 10⁸ cells per μmol chloride released. Our findings expand the metabolic capability in the genus Dehalogenimonas and highlight the relevant roles of organohalide-respiring bacteria for the natural attenuation of halogenated contaminants of emerging concerns (e.g., DCF).

Mini-review: effect of temperature on microbial reductive dehalogenation of chlorinated ethenes: a review

Temperature is a key factor affecting microbial activity and ecology. An increase in temperature generally increases rates of microbial processes up to a certain threshold, above which rates decline rapidly. In the subsurface, temperature of groundwater is usually stable and related to the annual average temperature at the surface. However, anthropogenic activities related to the use of the subsurface, e.g. for thermal heat management, foremost heat storage, will affect the temperature of groundwater locally. This mini-review intends to summarize the current knowledge on reductive dehalogenation activities of the chlorinated ethenes, common urban groundwater contaminants, at different temperatures. This includes an overview of activity and dehalogenation extent at different temperatures in laboratory isolates and enrichment cultures, the effect of shifts in temperature in micro- and mesocosm studies as well as observed biotransformation at different natural and induced temperatures at contaminated field sites. Furthermore, we address indirect effects on biotransformation, e.g. changes in fermentation, methanogenesis and sulfate reduction as competing or synergetic microbial processes. Finally, we address the current gaps in knowledge regarding bioremediation of chlorinated ethenes, microbial community shifts and bottlenecks for active combination with thermal energy storage, and necessities for bioaugmentation and/or natural re-populations after exposure to high temperature.

Dehalogenation of Chlorinated Ethenes to Ethene by a Novel Isolate, "Candidatus Dehalogenimonas etheniformans"

Dehalococcoides mccartyi strains harboring vinyl chloride (VC) reductive dehalogenase (RDase) genes are keystone bacteria for VC detoxification in groundwater aquifers, and bioremediation monitoring regimens focus on D. mccartyi biomarkers. We isolated a novel anaerobic bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of respiratory dechlorination of VC to ethene. This bacterium couples formate and hydrogen (H₂) oxidation to the reduction of trichloro-ethene (TCE), all dichloroethene (DCE) isomers, and VC with acetate as the carbon source. Cultures that received formate and H₂ consumed the two electron donors concomitantly at similar rates. A 16S rRNA gene-targeted quantitative PCR (qPCR) assay measured growth yields of (1.2 ± 0.2) × 10⁸ and (1.9 ± 0.2) × 10⁸ cells per μmol of VC dechlorinated in cultures with H₂ or formate as electron donor, respectively. About 1.5-fold higher cell numbers were measured with qPCR targeting cerA, a single-copy gene encoding a putative VC RDase. A VC dechlorination rate of 215 ± 40 μmol L^-1 day^-1 was measured at 30°C, with about 25% of this activity occurring at 15°C. Increasing NaCl concentrations progressively impacted VC dechlorination rates, and dechlorination ceased at 15 g NaCl L^-1. During growth with TCE, all DCE isomers were intermediates. Tetrachloroethene was not dechlorinated and inhibited dechlorination of other chlorinated ethenes. Carbon monoxide formed and accumulated as a metabolic by-product in dechlorinating cultures and impacted reductive dechlorination activity. The isolation of a new Dehalogenimonas species able to effectively dechlorinate toxic chlorinated ethenes to benign ethene expands our understanding of the reductive dechlorination process, with implications for bioremediation and environmental monitoring. IMPORTANCE Chlorinated ethenes are risk drivers at many contaminated sites, and current bioremediation efforts focus on organohalide-respiring Dehalococcoides mccartyi strains to achieve detoxification. We isolated and characterized the first non-Dehalococcoides bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of metabolic reductive dechlorination of TCE, all DCE isomers, and VC to environmentally benign ethene. In addition to hydrogen, the new isolate utilizes formate as electron donor for reductive dechlorination, providing opportunities for more effective electron donor delivery to the contaminated subsurface. The discovery that a broader microbial diversity can achieve detoxification of toxic chlorinated ethenes in anoxic aquifers illustrates the potential of naturally occurring microbes for biotechnological applications.

Microbial Community Response to a Bioaugmentation Test to Degrade Trichloroethylene in a Fractured Rock Aquifer, Trenton, N.J

Bioaugmentation is a promising strategy for enhancing trichloroethylene (TCE) degradation in fractured rock. However, slow or incomplete biodegradation can lead to stalling at degradation byproducts such as 1,2-dichloroethene (cis-DCE) and vinyl chloride (VC). Over the course of 7 years, we examined the response of groundwater microbial populations in a bioaugmentation test where an emulsified vegetable oil solution (EOS®) and a dechlorinating consortium (KB-1®), containing the established dechlorinator Dehalococcoides, were injected into a TCE-contaminated fractured rock aquifer. Indigenous microbial communities responded within 2 days to added substrate and outcompeted KB-1®, and over the years of monitoring, several other notable turnover events were observed. Concentrations of ethene, the end product in reductive dechlorination, had the strongest correlations (p< 0.05) with members of Candidatus Colwellbacteria but their involvement in reductive dechlorination is unknown and warrants further investigation. Dehalococcoides never exceeded 0.6% relative abundance of groundwater microbial communities, despite its previously presumed importance at the site. Increased concentrations of carbon dioxide, acetic acid, and methane were positively correlated with increasing ethene concentrations; however, concentrations of cis-DCE and VC remained high by the end of the monitoring period suggesting preferential enrichment of indigenous partial dechlorinators over bioaugmented complete dechlorinators. This study highlights the importance of characterizing in situ microbial populations to understand how they can potentially enhance or inhibit augmented TCE degradation.

Dehalococcoides mccartyi NIT01, a novel isolate, dechlorinates high concentrations of chloroethenes by expressing at least six different reductive dehalogenases

This study presents the isolation of a novel strain of Dehalococcoides mccartyi, NIT01, which can completely dechlorinate up to 4.0 mM of trichloroethene to ethene via 1,2-cis-dichroroethene and vinyl chloride within 25 days. Strain NIT01 dechlorinated chloroethenes (CEs) at a temperature range of 25-32 °C and pH range of 6.5-7.8. The activity of the strain was inhibited by salt at more than 1.3% and inactivated by 1 h exposure to 2.0% air or 0.5 ppm hypochlorous acid. The genome of NIT01 was highly similar to that of the Dehalococcoides strains DCMB5, GT, 11a5, CBDB1, and CG5, and all included identical 16S rRNA genes. Moreover, NIT01 had 19 rdhA genes including NIT01-rdhA7 and rdhA13, which are almost identical to vcrA and pceA that encode known dehalogenases for tetrachloroethene and vinyl chloride, respectively. We also extracted RdhAs from the membrane fraction of NIT01 using 0.5% n-dodecyl-β-d-maltoside and separated them by anion exchange chromatography to identify those involved in CE dechlorination. LC/MS identification of the LDS-PAGE bands and RdhA activities in the fractions indicated cellular expression of six RdhAs. NIT01-RdhA7 (VcrA) and NIT01-RdhA15 were highly detected and NIT01-RdhA6 was the third-most detected. Among these three RdhAs, NIT01-RdhA15 and NIT01-RdhA6 had no biochemically identified relatives and were suggested to be novel functional dehalogenases for CEs. The expression of multiple dehalogenases may support bacterial tolerance to high concentrations of CEs.

Improved assessment and performance monitoring of a biowall at a chlorinated solvent site using high-resolution passive sampling

This study contrasts the use of high-resolution passive sampling and traditional groundwater monitoring wells (GWMW) to characterize a chlorinated solvent site and assess the effectiveness of a biowall (mulch, compost and sand) that was installed to remediate trichloroethene (TCE), the primary contaminant of concern. High-resolution passive profilers (HRPPs) were direct driven hydraulically upgradient, within, and hydraulically downgradient of the biowall and in close proximity to existing GWMWs. Compared with hydraulically upgradient locations, the biowall was highly reducing, there were higher densities of bacteria/genes capable of reductive dechlorination, and TCE was being reductively transformed, but not completely, as cis-1,2-dichloroethene (cis-DCE) was detected within and hydraulically downgradient of the biowall. However, based on the high-resolution data, there were a number of important findings which were not discoverable using data from GWMWs alone. Data from the HRPPs indicate that the biowall was completely transforming TCE to ethene (C₂H₄) except within a high velocity interval, where the concentrations were reduced, but breakthrough of cis-DCE was apparent. Hydraulically upgradient of the biowall, concentrations of TCE increased with depth where a very low permeability zone exists that will likely remain as a long-term source. In addition, although low concentrations of cis-DCE were present downgradient of the biowall, surfacing into a downgradient stream was not detected. This study demonstrates the advantages of high-resolution passive sampling of aquifers to assess the performance of remediation techniques compared to traditional methods such as GWMWs.

Geobacter sp. Strain IAE Dihaloeliminates 1,1,2-Trichloroethane and 1,2-Dichloroethane

Chlorinated ethanes, including 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA), are widespread groundwater contaminants. Enrichment cultures XR_DCA and XR_TCA derived from river sediment dihaloeliminated 1,2-DCA to ethene and 1,1,2-TCA to vinyl chloride (VC), respectively. The XR_TCA culture subsequently converted VC to ethene via hydrogenolysis. Microbial community profiling demonstrated the enrichment of Geobacter 16S rRNA gene sequences in both the XR_DCA and XR_TCA cultures, and Dehalococcoides mccartyi (Dhc) sequences were only detected in the ethene-producing XR_TCA culture. The presence of a novel Geobacter population, designated as Geobacter sp. strain IAE, was identified by the 16S rRNA gene-targeted polymerase chain reaction and Sanger sequencing. Time-resolved population dynamics attributed the dihaloelimination activity to strain IAE, which attained the growth yields of 0.93 ± 0.06 × 10⁷ and 1.18 ± 0.14 × 10⁷ cells per μmol Cl^- released with 1,2-DCA and 1,1,2-TCA as electron acceptors, respectively. In contrast, Dhc growth only occurred during VC-to-ethene hydrogenolysis. Our findings discover a Geobacter sp. strain capable of respiring multiple chlorinated ethanes and demonstrate the involvement of a broader diversity of organohalide-respiring bacteria in the detoxification of 1,2-DCA and 1,1,2-TCA.

Field study of microbial community structure and dechlorination activity in a multi-solvents co-contaminated site undergoing natural attenuation

BTEX and chlorinated aliphatic hydrocarbons (CAHs) are the common pollutants found at contaminated sites, and natural attenuation (NA) of CAHs was widely observed where they coexist. In this work, the groundwater in a site co-contaminated with BTEX and CAHs was monitored for 1 year. The compositions and activities of the microfloras, especially dechlorinators and their relationships with the contaminants, geochemical properties, seasons and depth were evaluated. The results are consistent with the well-known NA conceptual model where CAHs are not able to stimulate the enrichment of dechlorinators alone, but BTEX does promote dechlorination. The higher temperature, rather than ORP in the deeper groundwater of the wet season became a key factor to promote the abundance of dechlorinators, but only when BTEX was available, indicating that the substrates from the BTEX biodegradation played an important role in the dechlorinator enrichment. The elevated ORP in the shallower groundwater exceeded the optimum conditions for reductive dechlorination and no significant seasonal variation of dechlorinators was found. The co-occurrence network revealed the cooperative interactions among the functional microfloras in which dechlorinators, BTEX degraders, and fermentative bacteria jointly promoted the dechlorination. These findings provided us a further understanding of the NA processes in a commingled plume.

Microbiome composition resulting from different substrates influences trichloroethene dechlorination performance

Hydrogen-releasing substrates can stimulate the reductive dechlorination of trichloroethene (TCE) mediated by organohalide-respiring bacteria (OHRB) at contaminated sites. However, how the substrate affects microbiome assembly and the accompanying influences on the growth of OHRB and reductive TCE dechlorination remains unclear. We evaluated the effects of microbial community structures and potential functions on the reductive dechlorination of TCE in three anaerobic reactors with acetate, soybean oil, or molasses as the substrate and no cobalamin or amino acid supplementation. The molasses-fed reactor exhibited superior performance and dechlorination of TCE loadings to ethene, and the oil-fed reactor exhibited a high growth rate of the key OHRB, Dehalococcoides. This finding suggests an effect of the substrate on reductive dechlorination and the growth of Dehalococcoides. The three reactors developed distinct microbial community structures and the predicted metagenomes were distinguished on the basis of vitamin and amino acid metabolisms as well as fermentation pathways. In addition to the diversified hydrogen-producing pathways, the molasses-induced microbiome exhibited high potential to synthesize the cobalamin, which may account for its high Dehalococcoides activity and thus effective dechlorination performance. The substrate dependence of microbiomes may provide insight into strategies of exogenous amino acid supplementation to benefit Dehalococcoides growth. This study adds novel insight into the interplay of hydrogen-releasing substrates and OHRB. The results may contribute to the development of tailored and cost-effective management for the reductive dechlorination of chlorinated solvents in bioremediation.

Long-Term Dechlorination of Polychlorinated Biphenyls (PCBs) in Taihu Lake Sediment Microcosms: Identification of New Pathways, PCB-Driven Shifts of Microbial Communities, and Insights into Dechlorination Potential

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) is regarded as an alternative approach for in situ remediation and detoxification in the environment. To better understand the process of PCB dechlorination in freshwater lake sediment, a long-term (108 weeks) dechlorination study was performed in Taihu Lake sediment microcosms with nine parent PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170). Within 108 weeks, the total PCBs declined by 32.8%, while parent PCBs declined by 84.8%. PCB dechlorinators preferred to attack meta- and para-chlorines, principally para-flanked meta and single-flanked para chlorines. A total of 58 dechlorination pathways were observed, and 20 of them were not in 8 processes, suggesting the broad spectrum of PCB dechlorination in the environment. Rare ortho dechlorination was confirmed to target the unflanked ortho chlorine, indicating a potential for complete dechlorination. PCBs drove the shifts of the microbial community structures, and putative dechlorinating bacteria were growth-linked to PCB dechlorination. The distinct jump of RDase genes ardA, rdh12, pcbA4, and pcbA5 was found to be consistent with the commencement of dechlorination. The maintained high level of putative dechlorinating phylum Chloroflexi (including Dehalococcoides and o-17/DF-1), genus Dehalococcoides, and four RDase genes at the end of incubation revealed the long-term dechlorination potential. This work provided insights into dechlorination potential for long-term remediation strategies at PCB-contaminated sites.

Temperature dependence of sequential chlorinated ethenes dechlorination and the dynamics of dechlorinating microorganisms

Thermally enhanced bioremediation is a promising approach to shorten the bioremediation period of tetrachloroethene (PCE) and trichloroethene (TCE). To clarify the influence that temperature has on stepwise PCE dechlorination and associated microorganisms, this study conducted dechlorination experiments using contaminated soil and groundwater under five distinct temperature conditions (i.e., 15, 20, 25, 30, and 35 °C). PCE and TCE were dechlorinated most rapidly at 25-35 °C, whereas the preferable temperatures for the dechlorination of cis-1,2- dichloroethene (cis-1,2-DCE) and vinyl chloride (VC) were 25-30 °C and 25 °C, respectively. Microbial community analysis revealed that Sulfurospirillum and Geobacter may have a dominant contribution to the dechlorination of PCE to cis-1,2-DCE, whereas Dehalococcoides harboring VC reductase genes are likely major contributors to the dechlorination of cis-1,2-DCE and VC. These results suggest that temperature influences various microbial groups, including major dechlorinating microorganisms, resulting in the different extent of PCE dechlorination. In addition, the microbial community structure greatly changed after the onset of the experiment, whereas the temperature influence of 15-30 °C on the microbial community structure was minor; however, the microbial community was significantly impacted at 35 °C. Collectively, these results suggest that thermally enhanced anaerobic dechlorination at 25 °C is useful for successful dechlorination of chlorinated ethenes in a short period.

Ultrastructure of organohalide-respiring Dehalococcoidia revealed by cryo-electron tomography

Dehalococcoides mccartyi (Dhc) and Dehalogenimonas spp. (Dhgm) are members of the class Dehalococcoidia, phylum Chloroflexi, characterized by streamlined genomes and a strict requirement for organohalogens as electron acceptors. Here, we used cryo-electron tomography to reveal morphological and ultrastructural features of Dhc strain BAV1 and 'Candidatus Dehalogenimonas etheniformans' strain GP cells at unprecedented resolution. Dhc cells were irregularly shaped discs (890 ± 110 nm long, 630 ± 110 nm wide and 130 ± 15 nm thick) with curved and straight sides that intersected at acute angles, whereas Dhgm cells appeared as slightly flattened cocci (760 ± 85 nm). The cell envelopes were composed of a cytoplasmic membrane (CM), a paracrystalline surface layer (S-layer) with hexagonal symmetry and ∼22 nm spacing between repeating units, and a layer of unknown composition separating the CM and the S-layer. Cell surface appendages were only detected in Dhc cells, whereas both cell types had bundled cytoskeletal filaments. Repetitive globular structures, ∼5 nm in diameter and ∼9 nm apart, were observed associated with the outer leaflet of the CM. We hypothesized that those represent organohalide respiration (OHR) complexes and estimated ∼30,000 copies per cell. In Dhgm cultures, extracellular lipid vesicles (20 - 110 nm in diameter) decorated with putative OHR complexes but lacking an S-layer were observed. The new findings expand our understanding of the unique cellular ultrastructure and biology of organohalide-respiring Dehalococcoidia. Importance: Dehalococcoidia respire organohalogen compounds and play relevant roles in bioremediation of groundwater, sediments and soils impacted with toxic chlorinated pollutants. Using advanced imaging tools, we have obtained 3-dimensional images at macromolecular resolution of whole Dehalococcoidia cells revealing their unique structural components. Our data detail the overall cellular shape, cell envelope architecture, cytoskeletal filaments, the likely localization of enzymatic complexes involved in reductive dehalogenation, and the structure of extracellular vesicles. The new findings expand our understanding of the cell structure-function relationship in Dehalococcoidia with implications for Dehalococcoidia biology and bioremediation.

Metagenomic Analysis Reveals Microbial Interactions at the Biocathode of a Bioelectrochemical System Capable of Simultaneous Trichloroethylene and Cr(VI) Reduction

Bioelectrochemical systems (BES) are attractive and versatile options for the bioremediation of organic or inorganic pollutants, including trichloroethylene (TCE) and Cr(VI), often found as co-contaminants in the environment. The elucidation of the microbial players’ role in the bioelectroremediation processes for treating multicontaminated groundwater is still a research need that attracts scientific interest. In this study, 16S rRNA gene amplicon sequencing and whole shotgun metagenomics revealed the leading microbial players and the primary metabolic interactions occurring in the biofilm growing at the biocathode where TCE reductive dechlorination (RD), hydrogenotrophic methanogenesis, and Cr(VI) reduction occurred. The presence of Cr(VI) did not negatively affect the TCE degradation, as evidenced by the RD rates estimated during the reactor operation with TCE (111±2 μeq/Ld) and TCE/Cr(VI) (146±2 μeq/Ld). Accordingly, Dehalococcoides mccartyi, the primary biomarker of the RD process, was found on the biocathode treating both TCE (7.82E+04±2.9E+04 16S rRNA gene copies g⁻¹ graphite) and TCE/Cr(VI) (3.2E+07±2.37E+0716S rRNA gene copies g⁻¹ graphite) contamination. The metagenomic analysis revealed a selected microbial consortium on the TCE/Cr(VI) biocathode. D. mccartyi was the sole dechlorinating microbe with H₂ uptake as the only electron supply mechanism, suggesting that electroactivity is not a property of this microorganism. Methanobrevibacter arboriphilus and Methanobacterium formicicum also colonized the biocathode as H₂ consumers for the CH₄ production and cofactor suppliers for D. mccartyi cobalamin biosynthesis. Interestingly, M. formicicum also harbors gene complexes involved in the Cr(VI) reduction through extracellular and intracellular mechanisms.

Acceleration of polychlorinated biphenyls remediation in soil via sewage sludge amendment

Bioremediation of polychlorinated biphenyls (PCBs) is impeded by difficulties in massively cultivating bioinoculant. Meanwhile, sewage sludge is rich in pollutant-degrading microorganisms and nutrients, drawing our attention to investigate their potential to be used as a supplement for bioremediation of PCBs. Here we reported extensive microbial reductive dechlorination of PCBs by waste activated sludge (WAS) and digestion sludge (DS), which were identified to harbor multiple putative organohalide-respiring bacteria (i.e., Dehalococcoides, Dehalogenimonas, Dehalobacter, and uncultivated Dehalococcoidia) and PCB reductive dehalogenase genes (i.e., pcbA4 and pcbA5). Consequently, amendment of 1-20% (w/w) fresh WAS/DS enhanced the attenuation of PCBs by 126-544% in a soil microcosm compared with the control soil, with the fastest dechlorination of PCBs being achieved when spiked with 20% fresh WAS. Notably, dechlorination pathways of PCBs were also changed by sludge amendment. Microbial and physicochemical analyses revealed that the enhanced dechlorination of PCBs by sludge amendment was largely attributed to the synergistic effects of sludge-derived nutrients, PCB-dechlorinating bacteria, and stimulated growth of beneficial microorganisms (e.g., fermenters). Finally, risk assessment of heavy metals suggests low potential ecological risks of sludge amendment in soil. Collectively, our study demonstrates that sewage sludge amendment could be an efficient, cost-effective and environment-friendly approach for in situ bioremediation of PCBs.

Enhanced dechlorination of 1,2-dichloropropane to propene in a bioelectrochemical system mediated by Dehalogenimonas

Bioelectrochemical systems (BES) are promising technologies to enhance the growth of organohalide-respiring bacteria and to treat chlorinated aliphatic hydrocarbons. In this study, two carbon-based cathodic electrode materials, a graphite brush and a carbon cloth, were used as hydrogen suppliers to couple growth of Dehalogenimonas and dechlorination of 1,2-DCP to nontoxic propene in the cathode vessel. The BES with graphite brush electrode consumed ~4000 µM 1,2-DCP during 110 days and exhibited a degradation rate 5.6-fold higher than the maximum value obtained with the carbon cloth electrode, with a cathode potential set at -0.7 V. Quantitative PCR confirmed that Dehalogenimonas gene copies increased by two orders of magnitude in the graphite brush BES, with an average yield of 1.2·10⁸±5·10⁷ cells per µmol of 1,2-DCP degraded. The use of a pulsed voltage operation (cathode potential set at -0.6 V for 16 h and -1.1 V for 8 h) increased the coulombic efficiency and degradation of 1,2-DCP when compared with a continuous voltage operation of -1.1 V. Bacterial cell aggregates were observed in the surface of the graphite brush electrodes by electron scanning microscopy, suggesting biofilm formation. This study expands the range of chlorinated compounds degradable and organohalide-respiring bacteria capable of growing in BES.

Microbial Chain Elongation and Subsequent Fermentation of Elongated Carboxylates as H2-Producing Processes for Sustained Reductive Dechlorination of Chlorinated Ethenes

In situ anaerobic groundwater bioremediation of trichloroethene (TCE) to nontoxic ethene is contingent on organohalide-respiring Dehalococcoidia, the most common strictly hydrogenotrophic Dehalococcoides mccartyi (D. mccartyi). The H₂ requirement for D. mccartyi is fulfilled by adding various organic substrates (e.g., lactate, emulsified vegetable oil, and glucose/molasses), which require fermenting microorganisms to convert them to H₂. The net flux of H₂ is a crucial controlling parameter in the efficacy of bioremediation. H₂ consumption by competing microorganisms (e.g., methanogens and homoacetogens) can diminish the rates of reductive dechlorination or stall the process altogether. Furthermore, some fermentation pathways do not produce H₂ or having H₂ as a product is not always thermodynamically favorable under environmental conditions. Here, we report on a novel application of microbial chain elongation as a H₂-producing process for reductive dechlorination. In soil microcosms bioaugmented with dechlorinating and chain-elongating enrichment cultures, near stoichiometric conversion of TCE (0.07 ± 0.01, 0.60 ± 0.03, and 1.50 ± 0.20 mmol L^-1 added sequentially) to ethene was achieved when initially stimulated by chain elongation of acetate and ethanol. Chain elongation initiated reductive dechlorination by liberating H₂ in the conversion of acetate and ethanol to butyrate and caproate. Syntrophic fermentation of butyrate, a chain-elongation product, to H₂ and acetate further sustained the reductive dechlorination activity. Methanogenesis was limited during TCE dechlorination in soil microcosms and absent in transfer cultures fed with chain-elongation substrates. This study provides critical fundamental knowledge toward the feasibility of chlorinated solvent bioremediation based on microbial chain elongation.

Increasing electron donor concentration does not accelerate complete microbial reductive dechlorination in contaminated sediment with native organic carbon

Experiments with Fe(III)-rich, chloroethene-contaminated sediment demonstrated that trichloroethylene (TCE) and vinyl chloride (VC) were completely reduced to ethene regardless of whether electron donor(s) were added at 1 × stoichiometry or 10 × stoichiometry relative to all-electron acceptors. Unamended controls uniformly reduced TCE to ethene with a mean time to complete dechlorination (operationally defined as the presence of stoichiometric ethene production) of 79 days. Adding 1 × and 10 × acetate hindered the rate and extent of TCE and VC reduction relative to unamended controls, with several only partially reduced when the experiments were terminated. Adding high molecular mass (soybean oil derivative) substrates did not increase microbial reductive dechlorination relative to unamended incubations, and in many cases, hindered microbial dechlorination in favor of methanogenesis. The mean time to complete dechlorination was comparable between low (× 1) and high (× 10) electron donor concentration for all lipid-based electron donors tested. Those tested included Newman Zone® Standard without sodium lactate (96 vs. 75 days, respectively), CAP 18 ME (85 vs. 94 days, respectively), EOS 598B42 (68 vs. 72 days, respectively), and acetate (134 vs. 125 days, respectively). These data suggest that the addition of an electron donor does not always increase the rate and extent of reductive dechlorination but will increase costs. In particular, increasing the concentration of electron donors higher than the stoichiometric demand only decreased complete microbial reductive dechlorination, which is the opposite of most standard "more time and more electrons" approaches. These data argue that site-specific electron donor demands must be evaluated, and in some cases, a monitored natural attenuation (MNA) approach is most favorable.

Assessment of chlorinated ethenes degradation after field scale injection of activated carbon and bioamendments: Application of isotopic and microbial analyses

Over the last decade, activated carbon amendments have successfully been applied to retain chlorinated ethene subsurface contamination. The concept of this remediation technology is that activated carbon and bioamendments are injected into aquifer systems to enhance biodegradation. While the scientific basis of the technology is established, there is a need for methods to characterise and quantify the biodegradation at field scale. In this study, an integrated approach was applied to assess in situ biodegradation after the establishment of a cross sectional treatment zone in a TCE plume. The amendments were liquid activated carbon, hydrogen release donors and a Dehalococcoides containing culture. The integrated approach included spatial and temporal evaluations on flow and transport, redox conditions, contaminant concentrations, biomarker abundance and compound-specific stable isotopes. This is the first study applying isotopic and microbial techniques to assess field scale biodegradation enhanced by liquid activated carbon and bioamendments. The injection enhanced biodegradation from TCE to primarily cis-DCE. The Dehalococcoides abundances facilitated characterisation of critical zones with insufficient degradation and possible explanations. A conceptual model of isotopic data together with distribution and transport information improved process understanding; the degradation of TCE was insufficient to counteract the contaminant input by inflow into the treatment zone and desorption from the sediment. The integrated approach could be used to document and characterise the in situ degradation, and the isotopic and microbial data provided process understanding that could not have been gathered from conventional monitoring tools. However, quantification of degradation through isotope data was restricted for TCE due to isotope masking effects. The combination of various monitoring tools, applied frequently at high-resolution, with system understanding, was essential for the assessment of biodegradation in the complex, non-stationary system. Furthermore, the investigations revealed prospects for future research, which should focus on monitoring contaminant fate and microbial distribution on the sediment and the activated carbon.

Effects of ferrous iron supplementation on reductive dechlorination of tetrachloroethene and on methanogenic microbial community

Chloroethenes are common soil and groundwater pollutants. Their dechlorination is impacted by environmental factors, such as the presence of metal ions. We here investigated the effect of ferrous iron on bacterial reductive dechlorination of chloroethenes and on methanogen community. Reductive dechlorination of tetrachloroethene was assayed with a groundwater sample originally containing 6.3 × 10³ copies mL⁻¹ of Dehalococcoides 16S rRNA gene and 2 mg L⁻¹ of iron. Supplementation with 28 mg L⁻¹ of ferrous iron enhanced the reductive dechlorination of cis-dichloroethene (cis-DCE) and vinyl chloride in the presence of methanogens. The supplementation shortened the time required for complete dechlorination of 1 mg L⁻¹ of tetrachloroethene to ethene and ethane from 84 to 49 d. Methanogens, such as Candidatus ‘Methanogranum’, Methanomethylovorans and Methanocorpusculum, were significantly more abundant in iron-supplemented cultures than in non-supplemented cultures (P < 0.01). Upon methanogen growth inhibition by 2-bromoethanesulfonate and in the absence of iron supplementation, cis-DCE was not dechlorinated. Further, iron supplementation induced 71.3% dechlorination of cis-DCE accompanied by an increase in Dehalococcoides 16S rRNA and dehalogenase vcrA gene copies but not dehalogenase tceA gene copies. These observations highlight the cooperative effect of iron and methanogens on the reductive dechlorination of chloroethenes by Dehalococcoides spp.

Cometabolic Vinyl Chloride Degradation at Acidic pH Catalyzed by Acidophilic Methanotrophs Isolated from Alpine Peat Bogs

Remediation of toxic chlorinated ethenes via microbial reductive dechlorination can lead to ethene formation; however, the process stalls in acidic groundwater, leading to the accumulation of carcinogenic vinyl chloride (VC). This study explored the feasibility of cometabolic VC degradation by moderately acidophilic methanotrophs. Two novel isolates, Methylomonas sp. strain JS1 and Methylocystis sp. strain MJC1, were obtained from distinct alpine peat bogs located in South Korea. Both isolates cometabolized VC with CH₄ as the primary substrate under oxic conditions at pH at or below 5.5. VC cometabolism in axenic cultures occurred in the presence (10 μM) or absence (<0.01 μM) of copper, suggesting that VC removal had little dependence on copper availability, which regulates expression and activity of soluble and particulate methane monooxygenases in methanotrophs. The model neutrophilic methanotroph Methylosinus trichosporium strain OB3b also grew and cometabolized VC at pH 5.0 regardless of copper availability. Bioaugmentation of acidic peat soil slurries with methanotroph isolates demonstrated enhanced VC degradation and VC consumption below the maximum concentration level of 2 μg L^-1. Community profiling of the microcosms suggested species-specific differences, indicating that robust bioaugmentation with methanotroph cultures requires further research.

Respiratory Vinyl Chloride Reductive Dechlorination to Ethene in TceA-Expressing Dehalococcoides mccartyi

Bioremediation of chlorinated ethenes in anoxic aquifers hinges on organohalide-respiring Dehalococcoidia expressing vinyl chloride (VC) reductive dehalogenase (RDase). The tceA gene encoding the trichloroethene-dechlorinating RDase TceA is frequently detected in contaminated groundwater but not recognized as a biomarker for VC detoxification. We demonstrate that tceA-carrying Dehalococcoides mccartyi (Dhc) strains FL2 and 195 grow with VC as an electron acceptor when sufficient vitamin B₁₂ (B₁₂) is provided. Strain FL2 cultures that received 50 μg L^-1 B₁₂ completely dechlorinated VC to ethene at rates of 14.80 ± 1.30 μM day^-1 and attained 1.64 ± 0.11 × 10⁸ cells per μmol of VC consumed. Strain 195 attained similar growth yields of 1.80 ± 1.00 × 10⁸ cells per μmol of VC consumed, and both strains could be consecutively transferred with VC as the electron acceptor. Proteomic analysis demonstrated TceA expression in VC-grown strain FL2 cultures. Resequencing of the strain FL2 and strain 195 tceA genes identified non-synonymous substitutions, although their consequences for TceA function are currently unknown. The finding that Dhc strains expressing TceA respire VC can explain ethene formation at chlorinated solvent sites, where quantitative polymerase chain reaction analysis indicates that tceA dominates the RDase gene pool.

Genome Sequence of "Candidatus Dehalogenimonas etheniformans" Strain GP, a Vinyl Chloride-Respiring Anaerobe

“Candidatus Dehalogenimonas etheniformans” strain GP couples growth with the reductive dechlorination of vinyl chloride and several polychlorinated ethenes. The genome sequence comprises a circular 2.07-Mb chromosome with a G+C content of 51.9% and harbors 50 putative reductive dehalogenase genes.

“Candidatus Dehalogenimonas etheniformans” strain GP couples growth with the reductive dechlorination of vinyl chloride and several polychlorinated ethenes. The genome sequence comprises a circular 2.07-Mb chromosome with a G+C content of 51.9% and harbors 50 putative reductive dehalogenase genes.