Metabolic Synergy of Dehalococcoides Populations Leading to Greater Reductive Dechlorination of Polychlorinated Biphenyls

Insights into anaerobic biotransformation of polychlorinated biphenyls in Dehalococcoides mccartyi CG1 through kinetic and stable isotopic analysis

Microbial degradation processes largely govern the fate of organic contaminants in the environment. Therefore, reliable evaluation of in situ biodegradation is essential for effective on-site contaminant management. Although compound-specific isotope analysis (CSIA) shows significant potential for assessing in situ attenuation and evaluating chemical and biodegradation mechanisms, empirical evidence supporting its application in the microbial degradation of polychlorinated biphenyls (PCBs) is still lacking. Microbial degradation of trace persistent organic pollutants is a multifaceted process influenced by various factors, with substrate concentration being a key factor affecting isotopic fractionation. Herein, to the best of our knowledge, for the first time, batch biodegradation experiments were conducted for analyzing the kinetics and carbon/chlorine isotope fractionation of chiral substrates (-)/(+)-PCB132 by Dehalococcoides mccartyi CG1 at varying substrate concentrations (0.3, 1.7, 2.4, 3.5, and 4.7 μM). The dechlorination of (-)/(+)-PCB132 was predominantly consistent with pseudo-first-order kinetics (kobs) in most cases. However, when the ratio of substrate concentration to the density of functional microorganisms falls below a specific threshold (<5.3 × 10-3 μmol/( × 1010 CG1 cells)), a decline in observed kobs is noted as degradation time increases, ultimately approaching the lower limit of bioavailability (kobs = 0). Notably, substantial normal isotope fractionation was observed for the first time during the anaerobic degradation of (-)/(+)-PCB132, with the isotopic enrichment factor (ƐC) varying from -1.27 ± 0.18‰ to -2.22 ± 0.01 for (-)/(+)-PCB132. Our findings indicate that, in addition to the effect of substrate concentration, the observed isotope fractionation of (-)/(+)-PCB132 was considerably affected by putative biodegradation activity. Enhanced activity within the anaerobic degradation system resulted in pronounced isotope masking. This study aims to contribute to a foundational understanding of bacterial reductive dehalogenation of PCBs at differing substrate concentrations while considering bioavailability.

Microbial and chemically induced reductive dechlorination of polychlorinated biphenyls in the environment

Polychlorinated biphenyls (PCBs) are persistent organic pollutants and are emitted during e-waste activities. Once they enter into the environment, PCBs could pose toxic effects to environmental compartments and public health. Reductive dechlorination offers a sustainable solution to manage the PCBs-contaminated environment. Under anaerobic conditions, reductive dechlorination of PCBs occurs, and PCBs congeners serve as potential electron acceptors which stimulate the growth of PCBs-dechlorinating microorganisms. In this review, microbial and chemically induced reductive dechlorination was summarized. During anaerobic conditions, highly chlorinated PCBs undergo reductive dechlorination and are converted into less chlorinated PCBs. The mechanisms involved in reductive dechlorination are mainly attacks on meta and/or para chlorines of PCBs mixtures in a contaminated environment and ortho dechlorination of PCBs. Based on methods, PCBs removal efficiency was as chemical > biological. Activated carbon (90%) showed more treatment efficiency than bacterial (84%). The review suggested that microbial remediation is a slow process; however, efficiency could be enhanced after amendments. Different microorganisms appear to be responsible for different dechlorination activities and the occurrence of various dehalogenation routes. However, PCBs dechlorination rate, extent, and route are influenced by pH, temperature, availability of carbon sources, and the presence or absence of H2 or competing electron acceptors and other electron donors.

Dual C and Cl compound-specific isotope analysis and metagenomic insights into the degradation of the pesticide methoxychlor

This study investigates the use of multi-element compound-specific isotope analysis (ME-CSIA) to monitor degradation processes of methoxychlor, a persistent organochlorine insecticide. Laboratory experiments examined the kinetics, release of transformation products, and carbon and chlorine isotope effects during methoxychlor degradation through alkaline hydrolysis, oxidation with alkaline-activated persulfate, and biotic reductive dechlorination. Results showed that hydrolysis and oxidation did not cause significant carbon and chlorine isotope fractionation, indicating that C-H rather than C-Cl bond cleavage was the rate-determining step. Conversely, biotic reductive dechlorination by a field-derived microcosm under strictly anoxic conditions displayed significant carbon (εC = -0.9 ± 0.3 ‰) and chlorine (εCl = -1.9 ± 1.0 ‰) isotope fractionation. Its corresponding calculated dual isotope slope (ΛC/Cl = 0.4 ± 0.1) and apparent kinetic isotope effects (AKIEC = 1.014 ± 0.005 and AKIECl = 1.006 ± 0.003) indicate a C-Cl bond cleavage as the rate-determining step, highlighting the difference with respect to the other studied degradation mechanisms. Changes in the microbial community diversity revealed that families such as Dojkabacteria, Anaerolineaceae, Dysgonomonadaceae, Bacteroidetes vadinHA17, Pseudomonadaceae, and Spirochaetaceae, may be potential agents of methoxychlor reductive dechlorination under anoxic conditions. This study advances the understanding of degradation mechanisms of methoxychlor and improves the ability to track its transformation in contaminated environments, including for the first time an isotopic perspective.

Understanding the sources, function, and irreplaceable role of cobamides in organohalide-respiring bacteria

Halogenated organic compounds are persistent pollutants that pose a serious threat to human health and the safety of ecosystems. Cobamides are essential cofactors for reductive dehalogenases (RDase) in organohalide-respiring bacteria (OHRB), which catalyze the dehalogenation process. This review systematically summarizes the impact of cobamides on organohalide respiration. The catalytic processes of cobamide in dehalogenation processes are also discussed. Additionally, we examine OHRB, which cannot synthesize cobamide and must obtain it from the environment through a salvage pathway; the co-culture with cobamide producer is more beneficial and possible. This review aims to help readers better understand the importance and function of cobamides in reductive dehalogenation. The presented information can aid in the development of bioremediation strategies.

Underexplored Organohalide-Respiring Bacteria in Sewage Sludge Debrominating Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants prevalent in the environment. Organohalide-respiring bacteria (OHRB) can attenuate PBDEs via reductive debromination, but often producing toxic end-products. Debromination of PBDEs to diphenyl ether remains a rare phenomenon and is so far specifically associated with Dehalococcoides isolated from e-waste polluted sites. The occurrence of PBDE debromination in other ecosystems and underpinning OHRB are underexplored. Here we found that debromination of PBDEs is a common trait of sewage sludge microbiota, and diphenyl ether was produced as the end-product at varying quantities (0.6-52.9% mol of the parent PBDEs) in 76 of 84 cultures established with bioreactor sludge. Diverse debromination pathways converting PBDEs to diphenyl ether, including several new routes, were identified. Although Dehalococcoides contributed to PBDE debromination, Dehalogenimonas, Dehalobacter, and uncultivated Dehalococcoidia likely played more important roles than previously recognized. Multiple reductive dehalogenase genes (including bdeA, pcbA4, pteA, and tceA) were also prevalent and coexisted in bioreactor sludge. Collectively, these findings contribute to enhancing our comprehension of the environmental fate of PBDEs, expanding the diversity of microorganisms catalyzing PBDE debromination, and developing consortia for bioremediation application.

Combatting multiple aromatic organohalide pollutants in sediments by bioaugmentation with a single Dehalococcoides

Dehalococcoides are capable of dehalogenating various organohalide pollutants under anaerobic conditions, and they have been applied in bioremediation. However, the presence of multiple aromatic organohalides, including polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and tetrabromobisphenol A (TBBPA), at contaminated sites may pose challenges to Dehalococcoides-mediated bioremediation due to the lack of knowledge about the influence of co-contamination on bioremediation. In this study, we investigated the bioremediation of aromatic organohalides present as individual and co-contaminants in sediments by bioaugmentation with a single population of Dehalococcoides. Bioaugmentation with Dehalococcoides significantly increased the dehalogenation rate of PCBs, PBDEs, and TBBPA in sediments contaminated with individual pollutants, being up to 19.7, 27.4 and 2.1 times as that in the controls not receiving bioinoculants. For sediments containing all the three classes of pollutants, bioaugmentation with Dehalococcoides also effectively enhanced dehalogenation, and the extent of enhancement depended on the bioinoculants and types of pollutants. Interestingly, in many cases co-contaminated sediments bioaugmented with Dehalococcoides mccartyi strain CG1 displayed a greater enhancement in dehalogenation rates compared to the sediments polluted with individual pollutant. For instance, when augmented with a low quantity of strain CG1, the dehalogenation rates of Aroclor1260 and PBDEs in co-contaminated sediments were approximately two times as that in sediments containing individual pollutants (0.428 and 9.03 vs. 0.195 and 4.20 × 10-3d-1). Additionally, D. mccartyi CG1 grew to higher abundances in co-contaminated sediments. These findings demonstrate that a single Dehalococcoides population can sustain dehalogenation of multiple aromatic organohalides in contaminated sediments, suggesting that co-contamination does not necessarily impede the use of Dehalococcoides for bioremediation. The study also underscores the significance of anaerobic organohalide respiration for effective bioremediation.