Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

Mechanistic insights into chloroethene dechlorination by Dehalococcoides mccartyi strain CWV2: A multi-omics perspective

This study provides an in-depth investigation of the novel Dehalococcoides mccartyi (Dhc) strain CWV2, isolated from Taiwan, for its effectiveness in dechlorinating various chloroethenes, including PCE, TCE, DCEs, and VC, to ethene. Through multi-omics analyses, including genomic, transcriptomic, translatomic and proteomic profiling, we uncovered the mechanisms behind TCE dechlorination by strain CWV2. The genomic analysis identified key reductive dehalogenase (RDase) genes, pceA and vcrA, which enhance our understanding of the versatile dechlorination pathways in Dhc. Ribosome profiling provided detailed insights into the translational regulation of vcrA, revealing sophisticated genetic control over protein synthesis. Complementary BN-PAGE and proteomic analyses identified key RDase VcrA, offering further insights into the activity of the organohalide respiration (OHR) complex within CWV2 and its metabolic pathways. Multi-omics analyses provide a comprehensive understanding of the mechanisms behind TCE dechlorination and organohalide respiration, offering valuable insights to advance bioremediation strategies for chloroethene-contaminated environments.

Predicting trichloroethene attenuation in aquifers with reduced iron minerals under oxygen perturbation: From kinetic model to reactive transport model

Pollutant attenuation in aquifers due to the reactivity of in situ reduced iron minerals (RIM) under dynamic redox conditions has gained popularity because of its value in designing green and sustainable strategies for soil and groundwater remediation. In this study, a novel approach that integrates RIM-based kinetic modeling in reactive transport modeling was initiated to predict trichloroethene (TCE) attenuation in RIM-containing aquifers. The kinetic model was optimized and simplified based on previous efforts and verified with data from batch experiments (R2 > 0.85). Using the refined kinetic model as a reaction module, a multi-scale and multi-process reactive transport model (RTM) was developed, which comprehensively linked geochemical reactions, dispersion, and advection of TCE in the aquifers with RIM and accurately simulated the variations of TCE concentrations in experiments (R2 >0.95). Notably, the importance of RIM oxygenation in TCE attenuation during O2 perturbation was demonstrated by the model; the ratio of contributions from RIM-based anaerobic dichlorination, hydrodynamics, and RIM-based aerobic degradation to TCE removal in low-permeability zones after 10 d was approximately 1:7:21. Besides, the RTM helped to elucidate the effects of variations in key elements (i.e., Fe(II) species, O2) on the TCE attenuation system. This study can facilitate the evaluation and prediction of the TCE fate in aquifers with RIM under O2 perturbation, so that remediation strategies can be formulated. Meanwhile, the limitations of the study motivate future work to determine the effects of more elements (e.g., organic matters, microbes), environmental changes, and scale differences to develop more sophisticated models.

Biogenic amorphous FeOOH activated additional intracellular electron flow pathways for accelerating reductive dechlorination of tetrachloroethylene

Dissimilatory iron-reducing bacteria (DIRB) with extracellular electron transfer (EET) capabilities have shown significant potential for bioremediating halogenated hydrocarbon contaminated sites rich in iron and humic substances. However, the role and microbial molecular mechanisms of iron-humic acid (Fe-HA) complexes in the reductive dehalogenation process of DIRB remains inadequately elucidated. In this study, we developed a sustainable carbon cycling approach using Fe-HA complexes to modulate the electron flux from sawdust (SD), enabling almost complete reductive dechlorination by most DIRB (e.g., Shewanella oneidensis MR-1) that lack complex iron-sulfur molybdo enzymes. The SD-Fe-HA/MR-1 system achieved a 96.52% removal efficiency of tetrachloroethylene (PCE) at concentrations up to 250 μmol/L within 60 days. Material characterization revealed that DIRB facilitated the hydrolysis of macromolecular carbon sources by inducing the formation of amorphous ferrihydrite (FeOOH) in Fe-HA complexes. More importantly, the bioavailable FeOOH activated additional intracellular electron flow pathways, increasing the activity of potential dehalogenases. Transcriptome further highlight the innovative role of biogenic amorphous FeOOH in integrating intracellular redox metabolism with extracellular charge exchange to facilitate reductive dechlorination in DIRB. These findings provide novel insights into accelerating reductive dechlorination in-situ contaminated sites lacking obligate dehalogenating bacteria.

Sustainable Abiotic-Biotic Dechlorination of Perchloroethene with Sulfidated Nanoscale Zero-Valent Iron as Electron Donor Source

Combining organohalide-respiring bacteria with nanoscale zero-valent iron (nZVI) represents a promising approach for remediating chloroethene-contaminated aquifers. However, limited information is available regarding their synergistic dechlorinating ability for chloroethenes when nZVI is sulfidated (S-nZVI) under the organic electron donor-limited conditions typically found in deep aquifers. Herein, we developed a combined system utilizing a mixed culture containing Dehalococcoides (Dhc) and S-nZVI particles, which achieved sustainable dechlorination with repeated rounds of spiking with 110 μM perchloroethene (PCE). The relative abundance of Dhc considerably increased from 5.2 to 91.5% after five rounds of spiking with PCE, as evidenced by 16S rRNA gene amplicon sequencing. S-nZVI corrosion generated hydrogen as an electron donor for Dhc and other volatile fatty acid (VFA)-producing bacteria. Electron balance analysis indicated that 68.1% of electrons from Fe0 consumed in S-nZVI were involved in dechlorination, and 6.2, 1.1, and 3.2% were stored in formate, acetate, and other VFAs, respectively. The produced acetate possibly served as a carbon source for Dhc. Metagenomic analysis revealed that Desulfovibrio, Syntrophomonas, Clostridium, and Mesotoga were likely involved in VFA production. These findings provide valuable insights into the synergistic mechanisms of biotic and abiotic dechlorination, with important implications for sustainable remediation of electron donor-limited aquifers contaminated by chloroethenes.

Burning question: Rethinking organohalide degradation strategy for bioremediation applications

Organohalides are widespread pollutants that pose significant environmental hazards due to their high degree of halogenation and elevated redox potentials, making them resistant to natural attenuation. Traditional bioremediation approaches, primarily relying on bioaugmentation and biostimulation, often fall short of achieving complete detoxification. Furthermore, the emergence of complex halogenated pollutants, such as per- and polyfluoroalkyl substances (PFASs), further complicates remediation efforts. Therefore, there is a pressing need to reconsider novel approaches for more efficient remediation of these recalcitrant pollutants. This review proposes novel redox-potential-mediated hybrid bioprocesses, tailored to the physicochemical properties of pollutants and their environmental contexts, to achieve complete detoxification of organohalides. The possible scenarios for the proposed bioremediation approaches are further discussed. In anaerobic environments, such as sediment and groundwater, microbial reductive dehalogenation coupled with fermentation and methanogenesis can convert organohalides into carbon dioxide and methane. In environments with anaerobic-aerobic alternation, such as paddy soil and wetlands, a synergistic process involving reduction and oxidation can facilitate the complete mineralization of highly halogenated organic compounds. Future research should focus on in-depth exploration of microbial consortia, the application of ecological principles-guided strategies, and the development of bioinspired-designed techniques. This paper contributes to the academic discourse by proposing innovative remediation strategies tailored to the complexities of organohalide pollution.

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.

Coupling between 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) debromination and methanogenesis in anaerobic soil microcosms

Polybrominated diphenyl ethers (PBDEs) are persistent pollutants that may undergo microbial-mediated debromination in anoxic environments, where diverse anaerobic microbes such as methanogenic archaea co-exist. However, current understanding of the relations between PBDE pollution and methanogenic process is far from complete. To address this knowledge gap, a series of anaerobic soil microcosms were established. BDE-47 (2, 2', 4, 4'-tetrabromodiphenyl ether) was selected as a model pollutant, and electron donors were supplied to stimulate the activity of anaerobes. Debromination and methane production were monitored during the 12 weeks incubation, while obligate organohalide-respiring bacteria (OHRBs), methanogenic, and the total bacterial communities were examined at week 7 and 12. The results demonstrated slow debromination of BDE-47 in all microcosms, with considerable growth of Dehalococcoides and Dehalogenimonas over the incubation observed in most BDE-47 spiked treatments. In addition, the accumulation of intermediate metabolites positively correlated with the abundance of Dehalogenimonas at week 7, suggesting potential role of these OHRBs in debromination. Methanosarcinaceae were identified as the primary methanogenic archaea, and their abundance were correlated with the production of debrominated metabolites at week 7. Furthermore, it was observed for the first time that BDE-47 considerably enhanced methane production and increased the abundance of mcrA genes, highlighting the potential effects of PBDE pollution on climate change. This might be related to the inhibition of reductive N- and S-transforming microbes, as revealed by the quantitative microbial element cycling (QMEC) analysis. Overall, our findings shed light on the intricate interactions between PBDE and methanogenic processes, and contribute to a better understanding of the environmental fate and ecological implication of PBDE under anaerobic settings.