Use of γ-hexachlorocyclohexane as a terminal electron acceptor by an anaerobic enrichment culture

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.

Comparative insight of pesticide transformations between river and wetland systems

The widespread use of pesticides threatens the environment and ecosystems. Despite the positive effects of plant protection products, pesticides also have unexpected negative effects on nontarget organisms. The microbial biodegradation of pesticides is one of the major pathways for reducing their risks at aquatic ecosystems. The objective of this study was to compare the biodegradability of pesticides in simulated wetland and river systems. Parallel experiments were conducted with 17 pesticides based on the OECD 309 guidelines. A comprehensive analytical method, such as target screening combined with suspect and non-target screening, was performed to evaluate the biodegradation via identification of transformation products (TPs) using LC-HRMS. As evidence of biodegradation, we identified 97 TPs for 15 pesticides. Metolachlor and dimethenamid had 23 and 16 TPs, respectively, including Phase II glutathione conjugates. The analysis of 16S rRNA sequences for microbials characterized operational taxonomic units. Rheinheimera and Flavobacterium, which have the potential for glutathione S-transferase, were dominant in wetland systems. Estimation of toxicity, biodegradability, and hydrophobicity using QSAR prediction indicated lower environmental risks of detected TPs. We conclude that the wetland system is more favorable for pesticide degradation and risk mitigation mainly attributed to the abundance and variety of the microbial communities.

Anaerobic biodegradation of chloroform and dichloromethane with a Dehalobacter enrichment culture

Chloroform (CF) and dichloromethane (DCM) are among the more commonly identified chlorinated aliphatic compounds found in contaminated soil and groundwater. Complete dechlorination of CF has been reported under anaerobic conditions by microbes that respire CF to DCM and others that biodegrade DCM. The objectives of this study were to ascertain if a commercially available bioaugmentation enrichment culture (KB-1® Plus CF) uses an oxidative or fermentative pathway for biodegradation of DCM; and to determine if the products from DCM biodegradation can support organohalide respiration of CF to DCM in the absence of an exogenous electron donor. In various treatments with the KB-1^® Plus CF culture to which ¹⁴C-CF was added, the predominant product was ¹⁴CO₂, indicating that oxidation is the predominant pathway for DCM. Recovery of ¹⁴C-DCM when biodegradation was still in progress confirmed that CF first undergoes reductive dechlorination to DCM. ¹⁴C-labeled organic acids, including acetate and propionate, were also recovered, suggesting that synthesis of organic acids provides a sink for the electron equivalents from oxidation of DCM. When the biomass was washed to remove organic acids from prior additions of exogenous electron donor and only CF and DCM were added, the culture completely dechlorinated CF. The total amount of DCM added was not sufficient to provide the electron equivalents needed to reduce CF to DCM. Thus, the additional reducing power came via the DCM generated from CF reduction. Nevertheless, the rate of CF consumption was considerably slower in comparison to treatments that received an exogenous electron donor. IMPORTANCE Chloroform (CF) and dichloromethane (DCM) are among the more commonly identified chlorinated aliphatic compounds found in contaminated soil and groundwater. One way to address this problem is to add microbes to the subsurface that can biodegrade these compounds. While microbes are known that can accomplish this task, less is known about the pathways used under anaerobic conditions. Some use an oxidative pathway, resulting mainly in carbon dioxide. Others use a fermentative pathway, resulting in formation of organic acids. In this study, a commercially available bioaugmentation enrichment culture (KB-1^® Plus CF) was evaluated using carbon-14 labelled chloroform. The main product formed was carbon dioxide, indicating the use of an oxidative pathway. The reducing power gained from oxidation was shown to support reductive dechlorination of CF to DCM. The results demonstrate the potential to achieve full dechlorination of CF and DCM to nonhazardous products that are difficult to identify in the field.

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.

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.

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.

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

The biotransformation of hexachlorocyclohexane isomers (HCH) by two Dehalococcoides mccartyi strains (195 and BTF08) and an enrichment culture was investigated and compared to conversion by the obligate anaerobic strain Clostridium pasteurianum strain DSMZ 525. The D. mccartyi strains preferentially transformed γ-HCH over α-HCH and δ-HCH isomers while β-HCH biotransformation was not significant. In case of the enrichment culture, γ-HCH was preferentially transformed over the δ-HCH, β-HCH and α-HCH isomers. Major observed metabolites in both cases were tetrachlorocyclohexene and as end products monochlorobenzene (MCB) and benzene. Dechlorination of the γ-HCH isomer was linked to an increase in cell numbers for strain 195. γ-HCH transformation was linked to considerable carbon stable isotope fractionation with the enrichment factor ε_c = - 5.5 ± 0.8‰ for D. mccartyi strain 195, ε_c = - 3.1 ± 0.4‰ for the enrichment culture and ε_c = - 4.1 ± 0.6‰ for co-metabolic transformation by C. pasteurianum.