Anaerobic biotransformation of hexachlorocyclohexane isomers by Dehalococcoides species and an enrichment culture

Distribution of microbial taxa and genes degrading halogenated organic pollutants in the mangroves

Anthropogenic activities have led to serious contamination of halogenated organic pollutants (HOPs), such as PCBs, PBDEs, and HBCDs, in the mangrove wetland. Biodegradation of HOPs is generally driven by environmental microorganisms harboring dehalogenase genes. However, little is known if HOPs can affect the distributions of HOPs-degrading bacteria and dehalogenase genes in the mangrove wetlands. Historical data suggested that HOPs contamination has been persistent and even deteriorated in the mangrove wetlands in China. We found that the organohalides-respiring bacteria Dehalococcoidia and reductive dehalogenase genes were more prevalent in the subsurface layer sediments (20-30 cm depth; 1.935-9.876 % relative abundance; 71-286 contigs) than the surface layer (0-5 cm depth; 0.174-2.020 % relative abundance; 7-130 contigs). While the genes of haloacid and haloalkane dehalogenases were more abundant in the surface layer (30-100 and 18-138 contigs) than the subsurface layer (22-56 and 50-101 contigs). The abundance of HOPs-degrading genes of reductive dehalogenase, haloacid dehalogenases, AtzA, AtzB, TrzA, TrzN, PcpB, were determined by GeoChip 5.0. Their total abundance ranged from 444.760 to 880.909. Their distributions were mainly associated with the contamination levels of HOPs and strength of anthropogenic activities around the mangrove wetlands. Therefore, the distribution of bacterial taxa and genes involved in HOPs degradation was related to the depth of sediments and affected by the selective stress from HOPs.

Enhanced aerobic bioremediation of an aquifer heavily contaminated with a mixture of chlorobenzenes and hexachlorocyclohexanes at the Sardas landfill (Spain)

The groundwater at the Sardas landfill in Huesca, Spain, is contaminated with benzene, chlorobenzenes, and hexachlorocyclohexane (HCH) isomers due to illegal waste dumping from a former lindane factory. In this study, microcosms using field-derived groundwater to evaluate in situ bioremediation were constructed. Anaerobic biostimulation with lactate successfully transformed α-, β-, δ-, and γ-HCH within two weeks, but failed to degrade benzene and less chlorinated benzenes, even with nutrient addition. In contrast, aerobic biostimulation led to rapid degradation of benzene, chlorobenzenes, and α-, δ-, and γ-HCH. Notably, adding a phosphorus source significantly increased the degradation rates. Following these laboratory results, an in situ pilot test using the oxygen-releasing compound CaO2 was conducted at two site injection wells. The field results mirrored those from the microcosms, showing a marked reduction in contaminants at both the injection wells and surrounding wells. Bacterial community analysis based on the 16S rRNA genes in samples derived from aerobic microcosms and groundwater before and after the biostimulation test revealed a marked increase in the genus Pseudomonas, suggesting its potential role as biodegrading agent. This study illustrates the effectiveness of biostimulation as a viable strategy for treating groundwater contaminated with HCH isomers, benzene, and chlorobenzenes.

Degradation study of δ-hexachlorocyclohexane by iron sulfide nanoparticles: Elucidation of reaction pathway using compound specific isotope analysis and pH variation

pH influences the reactivity of iron (II) minerals towards halogenated pollutants like hexachlorocyclohexanes (HCHs). To explore these incompletely understood interactions, we investigated the carbon isotope fractionation of the δ-HCH isomer during dehalogenation by iron sulfide at pHs spanning a pH range across slightly acidic to alkaline domains (5.8-9.6). The δ-1,3,4,5,6-pentachlorocyclohex-1-ene (δ-PCCH) was the intermediate degradation product, while benzene, monochlorobenzene (MCB), but especially 1,2-dichlorobenzene (1,2-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), were the main degradation products of δ-HCH. These degradation products suggested dehydrochlorination as the main degradation pathway of δ-HCH by iron sulfide. Different kinetic experiments indicate that the rate constants (ka) during dechlorination of δ-HCH by iron sulfide rose with pH: 0.003 d-1 (pH 5.8) < 0.034 d-1 (pH 8) < 0.085 (pH 9.3) < 0.286 d-1 (pH 9.6). Upon Rayleigh model calculations, an enrichment factor (εC) of -7.8 ± 1.0 ‰ was calculated for δ-HCH dehalogenation by FeS at pH 8.0. This suggests an apparent kinetic isotope effect (AKIEC) value of 1.049 ± 0.006 for dehydrohalogenation. The magnitude of the isotope effect from this paper furthermore supports dehydrohalogenation and opens the possibility to study the degradation of HCHs by iron (II) minerals containing FeS as mackinawite in oxygen-deprived environments.

Multi-element (2H, 13C, 37Cl) isotope analysis to characterize reductive transformation of α-, β-, γ-, and δ-HCH isomers by cobalamin and Fe0 nanoparticles

Hexachlorocyclohexane (HCH), a typical persistent organic pollutant, poses a serious threat to both human health and the environment. The degradability of HCH isomers (α, β, γ, and δ) varies significantly under anoxic aqueous conditions and the corresponding reductive transformation mechanisms remain elusive. This work employed multi-element (2H, 13C, 37Cl) stable isotope analysis to characterize the reductive dehalogenation mechanisms of HCH isomers using cobalamin (vitamin B₁₂) reduced with Ti3+ and Fe0 nanoparticles. The isotopic fractionation of HCH isomers varied from -2.8 ± 0.5 to -7.0 ± 0.7 ‰ for carbon (εC), from not significant to -62.4 ± 5.2 ‰ for hydrogen (εH), and from -1.4 ± 0.2 to -4.7 ± 0.5 ‰ for chlorine (εCl), respectively. Dual C-Cl isotopic fractionation values (ΛC/Cl) for α-, β-, γ- and δ-HCH during the transformation by B12 were determined to be 2.0 ± 0.2, 1.5 ± 0.2, 1.1 ± 0.1, and 1.9 ± 0.3, respectively. The ΛC/Cl values of β- and δ-HCH in the reaction with Fe0 nanoparticles were found to be similar (1.9 ± 0.3 and 1.9 ± 0.2). However, the apparent kinetic isotope effect AKIEC/AKIECl values suggested that the bond cleavage mechanism of δ-HCH may differ from that of other isomers. The comparison of the angles θ by multi-element isotope plot showed a distinct differentiation between the pathways of anaerobic transformation of HCH isomers and aerobic pathways reported in the literature. Therefore, multi-element isotope analysis could offer a new perspective for characterizing the fate of HCH isomers.

Effectiveness of Biological Approaches for Removing Persistent Organic Pollutants from Wastewater: A Mini-Review

Persistent organic pollutants (POPs), including organochlorine pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzo-p-furans, pose significant hazards to the environment and living organisms. This concise review aims to consolidate knowledge on the biological processes involved in removing POPs from wastewater, an area less explored compared to conventional physico-chemical methods. The focus is on the potential of various aerobic and anaerobic microorganisms, fungi, and bacteria for efficient bioremediation, mitigating or eradicating the deleterious effects of these chemicals. The review scrutinizes individual bacterial strains and mixed cultures engaged in breaking down persistent organic pollutants in water, highlighting promising results from laboratory investigations that could be scaled for practical applications. The review concludes by underscoring the opportunities for exploring and advancing more sophisticated bioremediation techniques and optimized bioreactors. The ultimate goal is to enhance the efficiency of microbial-based strategies, implicitly reducing the environmental impact of persistent chemicals.

Characterization of anaerobic biotransformation of hexachlorocyclohexanes by novel microbial consortia enriched from channel and river sediments

The microbial biotransformation of hexachlorocyclohexane (HCH) by novel anaerobic microbial consortia enriched from sediments of an industrial effluent channel and the river Ravi in Pakistan was examined. The anaerobic consortia were capable of biotransforming α-, β-, γ-, and δ-HCH through reductive dichloroelimination, resulting in the formation of benzene and monochlorobenzene. Concerning γ-HCH biotransformation by the channel and river cultures, isotopic fractionations for carbon (εC) were - 5.3 ± 0.4 (‰) and - 10.6 ± 1.2 (‰), while isotopic fractionations for chlorine (εCl) were - 4.4 ± 0.4 (‰) and - 7.8 ± 0.9 (‰), respectively. Furthermore, lambda values (Λ), representing the correlation of δ13C and δ37Cl fractionation, were determined to be 1.1 ± 0.1 and 1.3 ± 0.1 for γ-HCH biotransformation, suggesting a reductive dichloroelimination as the initial step of HCH biotransformation in both cultures. Amplicon sequencing targeting the 16S rRNA genes revealed that Desulfomicrobium populations were considerably increased in both cultures, indicating their possible involvement in the degradation process. These findings suggest that Desulfomicrobium-like populations may have an important role in biotransformation of HCH and novel anaerobic HCH-degrading microbial consortia could be useful bioaugmentation agents for the bioremediation of HCH-contaminated sites in Pakistan.

Tracking the transformation of persistent organic pollutants in food webs using multi element isotope and enantiomer fractionation

In order to track the transformation of persistent organic pollutants (POPs) in food webs, field experiments were conducted at two sites using stable isotope and enantiomer fractionation concepts. The enantiomers of α-hexachlorocyclohexane (α-HCH) were selected as representative compounds for POPs. Isotope and enantiomer fractionation allowed the characterization of α-HCH enantiomer biotransformation processes along trophic levels of the food web - from soil and plants to animal livers, fat tissues and milk. The enrichment of heavy isotopes in soils, plants and sediments as well as the changes of enantiomer fractionation indicate that the biotransformation of α-HCH occurred in these compartments. Moreover, the increase of carbon and chlorine isotopic compositions as well as the changes of enantiomer fractionation of liver, fat tissues and milk demonstrated that the overall HCH exposure was much higher than estimates based on concentration levels, while the isotope and enantiomer fractionation revealed the enantiomer specific enantiomer uptake across the blood-brain barriers. Dual element isotope analysis suggested that complex transformation processes have occurred along the potential food web from the HCH sources over different environmental compartments to animal livers, fat tissues and milk. The results imply that the analyses of stable isotope compositions and concentrations has potential to reconstruct the exposure of higher organisms to POPs.

Anaerobic biotransformation of hexachlorocyclohexane isomers in aqueous condition: Dual CCl isotope fractionation and impact on microbial community compositions

Hexachlorocyclohexane (HCH) isomers are persistent organic pollutants (POPs) with high toxicity, lipid solubility, chemical stability. Despite the current ban on usage of Lindane, residual contamination cannot be ignored, and HCH are frequently detected in groundwater and threaten human health. Cultures capable of degrading α-HCH, β-HCH, γ-HCH, and δ-HCH individually have been enriched in anoxic aqueous conditions. Compound-Specific Isotope Analysis (CSIA) was applied to examine the transformation mechanisms of different HCH isomers by the four enrichment cultures. 16S rRNA sequencing techniques were employed to examine the community composition of the enrichment cultures and detect changes in these communities resulting from adding individual HCH isomers. The results indicated that the ability of the enrichment cultures for dichloroelimination of HCH isomers was inconsistent. During dichloroelimination, different bond cleavage mode of β- and δ-HCH led to distinct isotopic effects. HCH isomers had significant impact on the microbial community, while different microbial communities showed comparable isotopic effects during the transformation of a specific HCH isomer. In addition, bacteria in the phyla Proteobacteria and Firmicutes were proposed as the dominant dechlorinators. This study provides a novel perspective on the mode of bond cleavage during HCH dichloroelimination and the effect of HCH on microbial communities, which could potentially support the evaluation of HCH transformation by CSIA and their effects on the microecosystems of groundwater.

Biodiversity in wetland+ system: a passive solution for HCH dump effluents

The hexachlorocyclohexane isomers (HCH) are long-banned pesticides. Even though their use has been prohibited for decades, their presence in the environment is still reported worldwide. Wetland + is a registered trademark of the remedial treatment technology consisting of an aerobic sedimentary tank, a permeable reactive barrier, a biosorption system, and an aerobic wetland. This proven method combines a reductive treatment known from PRBs with the natural wetland self-cleaning processes. The average efficiency of the system is 96.8% for chlorobenzenes (ClB) and 81.7% for HCH, during the first 12 months of the system operation. The presence of the genes encoding enzymes involved in the degradation of the HCH compounds indicates that the removal of HCH and ClB occurs not only by chemical removal but also through aerobic and anaerobic combining biodegradation. Changes in abundance and the composition of the diatom community were found to be suitable indicators of the water quality and of the impact of the Wetland + operation on the water ecosystem. The system's annual operation exhibited a markedly higher number of diatom species in the closing profiles of the Ostrovský Creek, the Wetland + effluent recipient.

Compound-specific stable isotope analysis for characterization of the transformation of γ-HCH induced by biochar

The role of biochar as the redox catalyst in the removal of reductive pollutants from soil and water system has been extensively studied recently, but there is still a lack of qualitative description of its specific mechanisms in redox processes. In this study, the mechanism of biochar in the transformation process of γ-HCH under anoxic condition was revealed by the compound-specific isotope analysis. The concentration and carbon isotopic composition (δ¹³C) of γ-HCH were detected in the treatments with different initial concentrations of γ-HCH and biochar materials with different redox properties and varied doses. The surface functional groups and electrochemical properties of biochar before and after the reaction were also characterized. The addition amount of biochar could affect the reduction of γ-HCH concentration, which were 59.1%, 34.6% and 22.4% in treatments with the addition of 5%, 1% and 0.2% biochar, respectively. Meanwhile, the δ¹³C value of γ-HCH also increased from -26.6 ± 0.2‰ to -23.8 ± 0.2‰ with the addition amount of biochar, especially in the treatment with 5% biochar. As evidenced by X-ray diffraction analysis and electrochemical analysis, biochar promoted the adsorption and transformation of γ-HCH simultaneously, and the oxygen-containing functional groups on the surface of biochar played an important role in the redox process. The isotopic fractionation value (ε_C) of γ-HCH transformation by biochar was first reported as -3.4 ± 0.4‰. The results will enable the quantitative description of the transformation degree of organic pollutants induced by biochar, and provide a new approach for evaluating the in-situ remediation effects of biochar in a complex environment.

Characterization of Hexachlorocyclohexane Isomer Dehydrochlorination by LinA1 and LinA2 Using Multi-element Compound-Specific Stable Isotope Analysis

Dehydrochlorination is one of the main (thus far discovered) processes for aerobic microbial transformation of hexachlorocyclohexane (HCH) which is mainly catalyzed by LinA enzymes. In order to gain a better understanding of the reaction mechanisms, multi-element compound-specific stable isotope analysis was applied for evaluating α- and γ-HCH transformations catalyzed by LinA1 and LinA2 enzymes. The isotopic fractionation (ε_E) values for particular elements of (+)α-HCH (ε_C = -10.8 ± 1.0‰, ε_Cl = -4.2 ± 0.5‰, ε_H = -154 ± 16‰) were distinct from the values for (-)α-HCH (ε_C = -4.1 ± 0.7‰, ε_Cl = -1.6 ± 0.2‰, ε_H = -68 ± 10‰), whereas the dual-isotope fractionation patterns were almost identical for both enantiomers (Λ_C-Cl = 2.4 ± 0.4 and 2.5 ± 0.2, Λ_H-C = 12.9 ± 2.4 and 14.9 ± 1.1). The ε_E of γ-HCH transformation by LinA1 and LinA2 were -7.8 ± 1.0‰ and -7.5 ± 0.8‰ (ε_C), -2.7 ± 0.3‰ and -2.5 ± 0.4‰ (ε_Cl), -170 ± 25‰ and -150 ± 13‰ (ε_H), respectively. Similar Λ_C-Cl values (2.7 ± 0.2 and 2.9 ± 0.2) were observed as well as similar Λ_H-C values (20.1 ± 2.0 and 18.4 ± 1.9), indicating a similar reaction mechanism by both enzymes during γ-HCH transformation. This is the first data set on 3D isotope fractionation of α- and γ-HCH enzymatic dehydrochlorination, which gave a more precise characterization of the bond cleavages, highlighting the potential of multi-element compound-specific stable isotope analysis to characterize different transformation processes (e.g., dehydrochlorination and reductive dehalogenation).

Uptake and Metabolization of HCH Isomers in Trees Examined over an Annual Growth Period by Compound-Specific Isotope Analysis and Enantiomer Fractionation

To understand the role of plants for natural attenuation, a field study was conducted to characterize the fate of HCH in trees over an annual growth period using compound-specific isotope analysis and enantiomer fractionation. Stable and slightly higher δ¹³C and δ³⁷Cl values of HCH of host soil samples compared to the muck (consisting nearly exclusively of HCH) revealed that masking isotope effects caused by the limited bioavailability may underestimate the real extent of HCH transformation in soil. In contrast, an increase of δ¹³C and δ³⁷Cl values in trees indicated the transformation of HCH. A large variability of δ¹³C and δ³⁷Cl values in trees over the growth period was observed, representing different transformation extents among different growth times, which is further supported by the shift of the enantiomer fraction (EF), indicating the preferential transformation of enantiomers also varied over the different growth periods. Based on dual-element isotope analysis, different predominant transformation mechanisms were observed during the growing seasons. Our observation implies that plants are acting as biological pumps driving a cycle of uptake and metabolization of HCH and refeed during littering to soil catalyzing their transformation. The changes of the transformation mechanism in different seasons have implications for phytoscreening and shed new light on phytoremediation of HCH at field sites.

Mechanistic insight into co-metabolic dechlorination of hexachloro-1,3-butadiene in Dehalococcoides

Hexachloro-1,3-butadiene (HCBD) as one of emerging persistent organic pollutants (POPs) poses potential risk to human health and ecosystems. Organohalide-respiring bacteria (OHRB)-mediated reductive dehalogenation represents a promising strategy to remediate HCBD-contaminated sites. Nonetheless, information on the HCBD-dechlorinating OHRB and their dechlorination pathways remain unknown. In this study, both in vivo and in vitro experiments, as well as quantum chemical calculation, were employed to successfully identify and characterize the reductive dechlorination of HCBD by Dehalococcoides. Results showed that some Dehalococcoides extensively dechlorinated HCBD to (E)-1,2,3-tri-CBD via (E)-1,1,2,3,4-penta-CBD and (Z,E)-1,2,3,4-tetra-CBD in a co-metabolic way. Both qPCR and 16S rRNA gene amplicon sequencing analyses suggested that the HCBD-dechlorinating Dehalococcoides coupled their cell growth with dechlorination of perchloroethene (PCE), rather than HCBD. The in vivo and in vitro ATPase assays indicated ≥78.89% decrease in ATPase activity upon HCBD addition, which suggested HCBD inhibition on ATPase-mediated energy harvest and provided rationality on the Dehalococcoides-mediated co-metabolic dechlorination of HCBD. Interestingly, dehalogenation screening of organohalides with the HCBD-dechlorinating enrichment cultures showed that debromination of bromodichloromethane (BDCM) was active in the in vitro RDase assays but non-active in the in vivo experiments. Further in vitro assays of hydrogenase activity suggested that significant inhibition of BDCM on the hydrogenase activity could block electron derivation from H₂ for consequent reduction of organohalides in the in vivo experiments. Therefore, our results provided unprecedented insight into metabolic, co-metabolic and RDase-active-only dehalogenation of varied organohalides by specific OHRB, which could guide future screening of OHRB for remediation of sites contaminated by HCBD and other POPs.

Soil from a Hexachlorocyclohexane Contaminated Field Site Inoculates Wheat in a Pot Experiment to Facilitate the Microbial Transformation of β-Hexachlorocyclohexane Examined by Compound-Specific Isotope Analysis

β-Hexachlorocyclohexane (β-HCH) is a remnant from former HCH pesticide production. Its removal from the environment gained attention in the last few years since it is the most stable HCH isomer. However, knowledge about the transformation of β-HCH in soil-plant systems is still limited. Therefore, experiments with a contaminated field soil were conducted to investigate the transformation of β-HCH in soil-plant systems by compound specific isotope analysis (CSIA). The results showed that the δ¹³C and δ³⁷Cl values of β-HCH in the soil of the planted control remained stable, revealing no transformation due to a low bioavailability. Remarkably, an increase of the δ¹³C and δ³⁷Cl values in soil and plant tissues of the spiked treatments were observed, indicating the transformation of β-HCH in both the soil and the plant. This was surprising as previously it was shown that wheat is unable to transform β-HCH when growing in hydroponic culture or garden soil. Thus, results of this work indicate for the first time that a microbial community of the soil inoculated the wheat and then facilitated the transformation of β-HCH in the wheat, which may have implications for the development of phytoremediation concepts. A high abundance of HCH degraders belonging to Sphingomonas sp., Mycobacterium sp., and others was detected in the β-HCH-treated bulk and rhizosphere soil, potentially supporting the biotransformation.

Biodegradation of Lindane (γ-Hexachlorocyclohexane) To Nontoxic End Products by Sequential Treatment with Three Mixed Anaerobic Microbial Cultures

The γ isomer of hexachlorocyclohexane (HCH), also known as lindane, is a carcinogenic persistent organic pollutant. Lindane was used worldwide as an agricultural insecticide. Legacy soil and groundwater contamination with lindane and other HCH isomers is still a big concern. The biotic reductive dechlorination of HCH to nondesirable and toxic lower chlorinated compounds such as monochlorobenzene (MCB) and benzene, among others, has been broadly documented. Here, we demonstrate that complete biodegradation of lindane to nontoxic end products is attainable using a sequential treatment approach with three mixed anaerobic microbial cultures referred to as culture I, II, and III. Biaugmentation with culture I achieved dechlorination of lindane to MCB and benzene. Culture II was able to dechlorinate MCB to benzene, and finally, culture III carried out methanogenic benzene degradation. Distinct Dehalobacter populations, corresponding to different 16S rRNA amplicon sequence variants in culture I and culture II, were responsible for lindane and MCB dechlorination, respectively. This study continues to highlight key roles of Dehalobacter as chlorobenzene- and HCH -respiring bacteria and demonstrates that sequential treatment with specialized anaerobic cultures may be explored at field sites in order to address legacy soil and groundwater contamination with HCH.

Recent Advanced Technologies for the Characterization of Xenobiotic-Degrading Microorganisms and Microbial Communities

Global environmental contamination with a complex mixture of xenobiotics has become a major environmental issue worldwide. Many xenobiotic compounds severely impact the environment due to their high toxicity, prolonged persistence, and limited biodegradability. Microbial-assisted degradation of xenobiotic compounds is considered to be the most effective and beneficial approach. Microorganisms have remarkable catabolic potential, with genes, enzymes, and degradation pathways implicated in the process of biodegradation. A number of microbes, including Alcaligenes, Cellulosimicrobium, Microbacterium, Micrococcus, Methanospirillum, Aeromonas, Sphingobium, Flavobacterium, Rhodococcus, Aspergillus, Penecillium, Trichoderma, Streptomyces, Rhodotorula, Candida, and Aureobasidium, have been isolated and characterized, and have shown exceptional biodegradation potential for a variety of xenobiotic contaminants from soil/water environments. Microorganisms potentially utilize xenobiotic contaminants as carbon or nitrogen sources to sustain their growth and metabolic activities. Diverse microbial populations survive in harsh contaminated environments, exhibiting a significant biodegradation potential to degrade and transform pollutants. However, the study of such microbial populations requires a more advanced and multifaceted approach. Currently, multiple advanced approaches, including metagenomics, proteomics, transcriptomics, and metabolomics, are successfully employed for the characterization of pollutant-degrading microorganisms, their metabolic machinery, novel proteins, and catabolic genes involved in the degradation process. These technologies are highly sophisticated, and efficient for obtaining information about the genetic diversity and community structures of microorganisms. Advanced molecular technologies used for the characterization of complex microbial communities give an in-depth understanding of their structural and functional aspects, and help to resolve issues related to the biodegradation potential of microorganisms. This review article discusses the biodegradation potential of microorganisms and provides insights into recent advances and omics approaches employed for the specific characterization of xenobiotic-degrading microorganisms from contaminated environments.

Genetic Analysis of Citrobacter sp.86 Reveals Involvement of Corrinoids in Chlordecone and Lindane Biotransformations

Chlordecone (Kepone®) and γ-hexachlorocyclohexane (γ-HCH or lindane) have been used for decades in the French West Indies (FWI) resulting in long-term soil and water pollution. In a previous work, we have identified a new Citrobacter species (sp.86) that is able to transform chlordecone into numerous products under anaerobic conditions. No homologs to known reductive dehalogenases or other candidate genes were found in the genome sequence of Citrobacter sp.86. However, a complete anaerobic pathway for cobalamin biosynthesis was identified. In this study, we investigated whether cobalamin or intermediates of cobalamin biosynthesis was required for chlordecone microbiological transformation. For this purpose, we constructed a set of four Citrobacter sp.86 mutant strains defective in several genes belonging to the anaerobic cobalamin biosynthesis pathway. We monitored chlordecone and its transformation products (TPs) during long-term incubation in liquid cultures under anaerobic conditions. Chlordecone TPs were detected in the case of cobalamin-producing Citrobacter sp.86 wild-type strain but also in the case of mutants able to produce corrinoids devoid of lower ligand. In contrast, mutants unable to insert the cobalt atom in precorrin-2 did not induce any transformation of chlordecone. In addition, it was found that lindane, previously shown to be anaerobically transformed by Citrobacter freundii without evidence of a mechanism, was also degraded in the presence of the wild-type strain of Citrobacter sp.86. The lindane degradation abilities of the various Citrobacter sp.86 mutant strains paralleled chlordecone transformation. The present study shows the involvement of cobalt-containing corrinoids in the microbial degradation of chlorinated compounds with different chemical structures. Their increased production in contaminated environments could accelerate the decontamination processes.

Dual C-Cl isotope analysis for characterizing the anaerobic transformation of α, β, γ, and δ-hexachlorocyclohexane in contaminated aquifers

Hexachlorocyclohexanes (HCHs) are widespread and persistent environmental pollutants, which cause heavy contamination in soil, sediment and groundwater. An anaerobic consortium, which was enriched on β-HCH using a soil sample from a contaminated area of a former pesticide factory, was capable to transform α, β, γ, and δ-HCH via tetrachlorocyclohexene isomers stoichiometrically to benzene and chlorobenzene. The carbon and chlorine isotope enrichment factors (ε_C and ε_Cl) of the dehalogenation of the four isomers ranged from -1.9 ± 0.3 to -6.4 ± 0.7‰ and from -1.6 ± 0.2 to -3.2 ± 0.6‰, respectively, and the correlation of δ³⁷Cl and δ¹³C (Λ values) of the four isomers ranged from 1.1 ± 0.1 to 2.4 ± 0.2. The evaluation of Λ and the apparent kinetic isotope effects (AKIE) for carbon and chlorine may lead to the hypothesis that the two eliminated chlorine atoms of α- and γ-HCH were in axial positions, the same as for the β-HCH conformer which has six chlorine atoms in axial positions after ring flip. The dichloroelimination of δ-HCH resulted in distinct AKIE and Λ values as one chlorine atom is in axial whereas the other chlorine atoms are in the equatorial positions. Significant chlorine and carbon isotope fractionations of HCH isomers were observed in the samples from a contaminated aquifer (Bitterfeld, Germany). The ³⁷Cl/³⁵Cl and ¹³C/¹²C isotope fractionation patterns of HCH isomers from laboratory experiments were used diagnostically in a model to characterize microbial dichloroelimination in the field study. The comparison of isotope fractionation patterns indicates that the transformation of HCH isomers at the field was mainly governed by microbial dichloroelimination transformation.

Dual C-Cl Isotope Analysis for Characterizing the Reductive Dechlorination of α- and γ-Hexachlorocyclohexane by Two Dehalococcoides mccartyi Strains and an Enrichment Culture

Hexachlorocyclohexanes (HCHs) are persistent organic contaminants that threaten human health. Microbial reductive dehalogenation is one of the most important attenuation processes in contaminated environments. This study investigated carbon and chlorine isotope fractionation of α- and γ-HCH during the reductive dehalogenation by three anaerobic cultures. The presence of tetrachlorocyclohexene (TeCCH) indicated that reductive dichloroelimination was the first step of bond cleavage. Isotope enrichment factors (ε_C and ε_Cl) were derived from the transformation of γ-HCH (ε_C, from -4.0 ± 0.5 to -4.4 ± 0.6 ‰; ε_Cl, from -2.9 ± 0.4 to -3.3 ± 0.4 ‰) and α-HCH (ε_C, from -2.4 ± 0.2 to -3.0 ± 0.4 ‰; ε_Cl, from -1.4 ± 0.3 to -1.8 ± 0.2 ‰). During α-HCH transformation, no enantioselectivity was observed, and similar ε_c values were obtained for both enantiomers. The correlation of ¹³C and ³⁷Cl fractionation (Λ = Δδ¹³C/Δδ³⁷Cl ≈ ε_C/ε_Cl) of γ-HCH (from 1.1 ± 0.3 to 1.2 ± 0.1) indicates similar bond cleavage during the reductive dichloroelimination by the three cultures, similar to α-HCH (1.7 ± 0.2 to 2.0 ± 0.3). The different isotope fractionation patterns during reductive dichloroelimination and dehydrochlorination indicates that dual-element stable isotope analysis can potentially be used to evaluate HCH transformation pathways at contaminated field sites.

Insights Into the Biodegradation of Lindane (γ-Hexachlorocyclohexane) Using a Microbial System

Lindane (γ-hexachlorocyclohexane) is an organochlorine pesticide that has been widely used in agriculture over the last seven decades. The increasing residues of lindane in soil and water environments are toxic to humans and other organisms. Large-scale applications and residual toxicity in the environment require urgent lindane removal. Microbes, particularly Gram-negative bacteria, can transform lindane into non-toxic and environmentally safe metabolites. Aerobic and anaerobic microorganisms follow different metabolic pathways to degrade lindane. A variety of enzymes participate in lindane degradation pathways, including dehydrochlorinase (LinA), dehalogenase (LinB), dehydrogenase (LinC), and reductive dechlorinase (LinD). However, a limited number of reviews have been published regarding the biodegradation and bioremediation of lindane. This review summarizes the current knowledge regarding lindane-degrading microbes along with biodegradation mechanisms, metabolic pathways, and the microbial remediation of lindane-contaminated environments. The prospects of novel bioremediation technologies to provide insight between laboratory cultures and large-scale applications are also discussed. This review provides a theoretical foundation and practical basis to use lindane-degrading microorganisms for bioremediation.

Biotransformation of hexachlorocyclohexanes contaminated biomass for energetic utilization demonstrated in continuous anaerobic digestion system

Lindane, the γ-hexachlorocyclohexane (HCH) isomer, was among the most used pesticides worldwide. Although it was banned in 2009, residues of Lindane and other HCH-isomers are still found with high concentrations in contaminated fields. For clean-up, phytoremediation combined with anaerobic digestion (AD) of contaminated biomass to produce biogas and fertilizer could be a promising strategy and was tested in two 15 L laboratory-scale continuous stirred tank reactors. During operation over one year by adding HCH isomers (γ, α and β) consecutively, no negative influence on conventional reactor parameters was observed. The γ- and α-HCH isomers were transformed to chlorobenzene and benzene, and transformation became faster along with time, while β-HCH was not removed. Genus Methanosaeta and order Clostridiales, showing significant enhancement on abundance with HCH addition, may be used as bioindicators for HCH dehalogenation in AD process. The potential for HCH degradation in AD system was restricted to axial Cl atoms of HCH and it showed slight enantioselective preference towards transformation of (+) α-HCH. Moreover, metabolite benzene was mineralized to CO₂ and methane, deducing from tracer experiments with benzene-¹³C₆. Overall, AD appears to be a feasible option for treatment of γ and α-HCHs contaminated biomass.

Microbial Communities Associated with Sustained Anaerobic Reductive Dechlorination of α-, β-, γ-, and δ-Hexachlorocyclohexane Isomers to Monochlorobenzene and Benzene

Intensive historical and worldwide use of pesticide formulations containing hexachlorocyclohexane (HCH) has led to widespread contamination. We derived four anaerobic enrichment cultures from HCH-contaminated soil capable of sustainably dechlorinating each of α-, β-, γ-, and δ-HCH isomers stoichiometrically to benzene and monochlorobenzene (MCB). For each isomer, the dechlorination rates, inferred from production rates of the dechlorinated products, MCB and benzene, increased progressively from <3 to ∼12 μM/day over 2 years. The molar ratio of benzene to MCB produced was a function of the substrate isomer and ranged from β (0.77 ± 0.15), α (0.55 ± 0.09), γ (0.13 ± 0.02), to δ (0.06 ± 0.02) in accordance with pathway predictions based on prevalence of antiperiplanar geometry. Data from 16S rRNA gene amplicon sequencing and quantitative PCR revealed significant increases in the absolute abundances of Pelobacter and Dehalobacter, most notably in the α-HCH and δ-HCH cultures. Cultivation with a different HCH isomer resulted in distinct bacterial communities, but similar archaeal communities. This study provides the first direct comparison of shifts in anaerobic microbial communities induced by the dechlorination of distinct HCH isomers. It also uncovers candidate microorganisms responsible for the dechlorination of α-, β-, γ-, and δ-HCH, a key step toward better understanding and monitoring of natural attenuation processes and improving bioremediation technologies for HCH-contaminated sites.

Restoring HCHs polluted land as one of the priority activities during the UN-International Decade on Ecosystem Restoration (2021-2030): A call for global action

The United Nations General Assembly has recently declared 2021-2030 as the 'International Decade on Ecosystem Restoration' for facilitating the restoration of degraded and destroyed terrestrial and marine systems for regaining biodiversity and ecosystem services, creating job opportunities and also to fight against climate change. One of the prime focus is the restoration of ~350 mha of degraded land across the world for attaining the UN-Sustainable Development Goals. Pesticides are one of the major causes of land pollution and hexachlorocyclohexanes (HCHs, including technical-HCH and γ-HCH) is one of the widely used organochlorine pesticides during the past seven decades before α-, β-, and γ-HCH was listed in the Stockholm Convention in 2009. The widespread pollution of HCHs has been reported from every sphere of the environment and ~7 Mt of HCHs residues have been dumped worldwide near the production sites. HCHs isomers have higher volatility, water solubility and long-range atmospheric transport ability which further facilitates its entry into various environmental compartments. Therefore, the restoration and management of HCHs polluted land is urgently required. Despite various pilot-scale studies have been reported for the remediation of HCHs polluted land, they are not successfully established under the field conditions. This is mainly due to the high concentration of HCHs residues in the contaminated soil and also due to its toxicity and highly persistent nature, which increases the complexity of the onsite remediation. Here we provide a novel approach i.e. sequential and integrated remediation approach (SIRA) for the restoration of HCHs contaminated land by the integrated use of agroresidues along with the application of HCHs degrading microorganisms and chemical amendments followed by the plant-based clean-up techniques using grasses, herbs, shrubs and trees in a sequential manner. SIRA provides cost effective solution with enhanced ecological and socioeconomic benefits for the sustainable restoration of HCHs contaminated sites.

Isotope fractionation approach to characterize the reactive transport processes governing the fate of hexachlorocyclohexanes at a contaminated site in India

The transformation processes of hexachlorocyclohexane isomers (HCHs) from production sites of Lindane across the landscape and along the food web were studied as an example to understand the fate of POPs in the environment. Therefore, we studied the concentration and isotope composition of HCHs in different matrices in the vicinity of a dumpsite and a chemical plant producing HCHs in India. Carbon isotope compositions (δ¹³C) of HCHs and the enantiomer fraction (EF) of α-HCH were used as indicators to characterize in situ degradation in soil, groundwater, and sediment as well as along the food web. The HCHs were detected in plants growing on contaminated soil. Elevated concentrations of HCHs were found in a number of crops, which indicates an important transfer pathway of HCHs entering food webs. The EF value of α-HCH and the δ¹³C signature of HCHs indicated that degradation processes occurred in the rhizosphere or within the plants potentially attenuating the contamination of HCHs. The isotope enrichment of HCHs in dung and milk samples showed that degradation of HCHs may take place in the digestive track of cow and buffalo as well as during their metabolism. The δ¹³C of HCHs was used to analyze the potential dispersion routes on the landscape scale in order to understand the reactive transport pathways starting at the source of HCHs. In this study, the potential of carbon isotope fractionation and EF for characterizing uptake of HCHs into plants and accumulation in the food web were examined. To the best of our knowledge, this is the first study using the combination of stable isotope fractionation and EF to track the reactive transport processes in a complex environment including the food web.