A Dehalogenimonas Population Respires 1,2,4-Trichlorobenzene and Dichlorobenzenes

A global systematic review and meta-analysis of innovative technologies for 1,2,4-trichlorobenzene remediation in soil and water

The 1,2,4-trichlorobenzene (1,2,4-TCB) is a persistent organic pollutant, which poses a serious concern due to its long-lasting and detrimental impact on soil and water quality. This study uses meta-analysis to investigate the effectiveness of various remediation methods for 1,2,4-TCB in water and soil. In water, the intimate coupling of photocatalysis and biodegradation (ICPB) demonstrated the highest removal rate (80%), followed by photocatalysis (PC, 69%), bioremediation (B, 53%), and photolysis (P, 42%). Optimal conditions for 1,2,4-TCB removal in water included short remediation times (< 5 days), higher temperatures (≥ 25 °C), neutral pH, and specific free radicals (H+ > •OH > •O2-). In soil, short-term remediation methods and suspended cultures showed higher removal rates. Topsoil depth layers (≤ 10 cm) exhibited better removal rates than subsoil (> 10 cm). Key factors influencing remediation effectiveness in water were hydraulic retention time (HRT), salinity, and water table depth, while in soil, remediation time and soil depth layer were the most significant. This research highlights the importance of optimizing remediation methods and environmental conditions to remove 1,2,4-TCB from contaminated sites effectively. Further investigation is needed to understand the underlying mechanisms and optimal conditions for these remediation methods, particularly in soil. Effective remediation of 1,2,4-TCB requires a tailored approach considering specific environmental conditions and challenges in water and soil. The ICPB shows promise, especially in aquatic environments. However, further research is essential to optimize these methods, particularly for soil remediation.

Effects of in vivo chlorobenzene exposure on bone tissue in a rat model

Calcipaenic bone disorders (e.g., osteoporosis) are becoming an epidemic as a significant public health concern. The underlying genetic, epigenetic, and homeostatic factors and the determinants of bone tissue expression are triggered by environmental exposures. Endocrine disruptor compounds are important in the development of pathological bone alterations. The aim of this study is to design an in vivo subtoxic chlorobenzene exposure model that can be used to explore certain bone changes and their consequences. Male Wistar rats were treated via gastric tube with a 1:1 mixture of hexachlorobenzene + 1,2,4-trichlorobenzene at a dose of 1.0 μg/kg bw; in a final volume of 1 mL, for 30, 60 and 90 days. Blood serum and bone samples were obtained from the femur diaphysis. The results of the treatments (n = 10/group) were interpreted as related to the controls. Serum levels of γGT, SGOT, SGPT were determined, along with bone tissue morphology, as well as the total mineral content of the bone and the mobilizable anorganic content. ANOVA was used to analyze the measurement data. As a result of the treatment protocol, histological examinations of bone morphology showed osteoid degeneration, as well as an altered state of the bone matrix. These findings are supported by the DEXA images, which showed a time-dependent decrease in surface mineral content, in parallel, an increase in the mobilizable anorganic content of the bone was detected. These results suggest that chlorobenzene administered may be a causal factor and changes in bone tissue structure can be traced.

How Pseudomonas conducts reductive dechlorination of 2,4,6-trichlorophenol: Insights into metabolic performance and organohalide respiration process

Organohalide-respiring bacteria (OHRB) play a key role in facilitating the detoxification of halogenated organics, but their slow growth and harsh growth conditions often limit their application in field remediation. In this study, we investigated the metabolic performance and organohalide respiration process of a non-obligate OHRB, Pseudomonas sp. CP-1, demonstrating favorable anaerobic reductive dechlorination ability of 2,4,6-trichlorophenol to 4-chlorophenol with a removal rate constant (k) of 0.46 d-1. Due to its facultative anaerobic nature, strain CP-1 exhibited unique metabolic properties. In aerobic conditions, strain CP-1 preferentially utilized oxygen for rapid proliferation, and anaerobic reductive dechlorination was initiated once the oxygen was depleted. The aerobic proliferation facilitated the subsequent reductive dechlorination process. Through multi-tool analysis, a modified tricarboxylic acid cycle was proposed to be linked to organohalide respiration when acetate served as the sole carbon source. A predictive model for the electron transport chain (ETC) for reductive dechlorination was constructed, with complex Ⅰ, complex Ⅱ, ubiquinone, complex Fix (flavoprotein), and reductive dehalogenase (RDase) as the major components. A specific RDase facilitating reductive dechlorination was identified. It shared a 64.35 % amino acid similarity with biochemically characterized RDases and was designated CprA-2. Its ortho-dechlorination catalytic process was proposed through molecular docking. The discovery of highly adaptable Pseudomonas with favorable dechlorination activity and the elucidation of its metabolic properties provide valuable insights into the understanding of non-obligate OHRBs and their application regulation.

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.

Dehalogenimonas Strain W from Estuarine Sediments Dechlorinates 1,2-Dichloroethane under Elevated Salinity

Organohalide-respiring bacteria (OHRB) have been found in various environments and play an indispensable role in the biogeochemical cycling and detoxification of halogenated organic compounds (HOCs). Currently, few ORHB have been reported to perform reductive dechlorination under high salinity conditions, indicating a knowledge gap on the diversity of OHRB and the survival strategy of OHRB in saline environments (e.g., estuarine, marine). This study reports the characterization of an enrichment culture dominated by a new Dehalogenimonas population strain W derived from estuarine sediments, which demonstrates the capability to dechlorinate 1,2-dichloroethane (1,2-DCA) to ethene under elevated salinity (≥5.1% NaCl, w/v). Metagenomic and proteomic analyses revealed that the distinctive high-salinity dechlorination of strain W is primarily attributed to a putative reductive dehalogenase (RDase) DdeA, which shares >91.4% amino acid identity with the dihaloeliminating RDase DcpA from other Dehalogenimonas strains. Additionally, ectoine biosynthesis enzymes (EctABC) contribute to the strain's salt tolerance. These findings underscore the potential of OHRB, particularly Dehalogenimonas, to detoxify HOCs in high-salinity environments, such as estuarine and marine ecosystems, by employing compatible solutes as an adaptive mechanism.

Environmental fate and transformation mechanisms of chlorinated organic pollutants from the petrochemical industry: Insights for pollution control and remediation

Chlorinated organic pollutants (Cl-OPs), highly toxic and environmentally persistent, have become the spotlight, particularly from petrochemical industry. This study focuses on environmental fate of Cl-OPs from petrochemical industry, and transformation mechanisms in multi-media, aiming to enhance pollution control and remediation strategies. Emitted from leakage and waste discharge, Cl-OPs, encompassing chlorinated volatile organic compounds (Cl-VOCs), traditional and emerging persistent organic pollutants (POPs), were prevalent with average concentrations of 10-6-103 μg/m3 in the atmosphere, 10-2-105 μg/kg in soil and 100-105 μg/L in groundwater. Significantly, emerging POPs, particularly hexachlorobutadiene (HCBD) and short-chain chlorinated paraffins (SCCPs), with concentrations comparable to Cl-VOCs, urgently need attention. Once into the environment, Cl-OPs are naturally transformed primarily through atmospheric oxidation and water photolysis induced by hydroxyl radical (‧OH), and microbial degradation. Despite challenges in atmospheric complete degradation, ‧OH in water effectively photolytically degrade chlorinated benzenes and paraffins facilitated by dissolved oxygen and organic matter. Microbial degradation, influenced by oxygen, temperature, and pH, is essential for Cl-OPs removal from water and soil, where oxidation make complete mineralization possible whereas dechlorination may generate higher toxic intermediates. Hence, Cl-OPs control necessitates an attentive to leakage and waste management. Furthermore, advanced ‧OH oxidation and microbial treatment are of effective remediation prospect.

Natural attenuation of BTEX and chlorobenzenes in a formerly contaminated pesticide site in China: Examining kinetics, mechanisms, and isotopes analysis

Groundwater contamination from abandoned pesticide sites is a prevalent issue in China. To address this problem, natural attenuation (NA) of pollutants has been increasingly employed as a management strategy for abandoned pesticide sites. However, limited studies have focused on the long-term NA process of co-existing organic pollutants in abandoned pesticide sites by an integrated approach. In this study, the NA of benzene, toluene, ethylbenzene, and xylene (BTEX), and chlorobenzenes (CBs) in groundwater of a retired industry in China was systematically investigated during the monitoring period from June 2016 to December 2021. The findings revealed that concentrations of BTEX and CBs were effectively reduced, and their NA followed first-order kinetics with different rate constants. The sulfate-reducing bacteria, nitrate-reducing bacteria, fermenting bacteria, aromatic hydrocarbon metabolizing bacteria, and reductive dechlorinating bacteria were detected in groundwater. It was observed that distinct environmental parameters played a role in shaping both overall and key bacterial communities. ORP (14.72%) and BTEX (12.89%) were the main drivers for variations of the whole and key functional microbial community, respectively. Moreover, BTEX accelerated reductive dechlorination. Furthermore, BTEX and CBs exhibited significant enrichment of 13C, ranging from +2.9 to +27.3‰, demonstrating their significance in situ biodegradation. This study provides a scientific basis for site management.

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.

Responses of Anabaena sp. PCC7120 to lindane: Physiological effects and differential expression of potential lin genes

Lindane (γ-HCH) is an organochlorine pesticide that causes huge environmental concerns worldwide due to its recalcitrance and toxicity. The use of the cyanobacterium Anabaena sp. PCC 7120 in aquatic lindane bioremediation has been suggested but information relative to this process is scarce. In the present work, data relative to the growth, pigment composition, photosynthetic/respiration rate, and oxidative stress response of Anabaena sp. PCC 7120 in the presence of lindane at its solubility limit in water are shown. In addition, lindane degradation experiments revealed almost a total disappearance of lindane in the supernatants of Anabaena sp. PCC 7120 culture after 6 days of incubation. The diminishing in lindane concentration was in concordance with an increase in the levels of trichlorobenzene inside the cells. Furthermore, to identify potential orthologs of the linA, linB, linC, linD, linE, and linR genes from Sphingomonas paucimobilis B90A in Anabaena sp. PCC 7120, a whole genome screening was performed allowing the identification of five putative lin orthologs (all1353 and all0193 putative orthologs of linB, all3836 putative orthologs of linC, and all0352 and alr0353 putative orthologs of linE and linR, respectively) which could be involved in the lindane degradation pathway. Differential expression analysis of these genes in the presence of lindane revealed strong upregulation of one of the potential lin genes of Anabaena sp. PCC 7120.

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.

Degradation of chlorinated solvents with reactive iron minerals in subsurface sediments from redox transition zones

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10^-4 to 1.73 × 10^-3 Lg^-1h^-1 with pyrite (FeS₂), 4.97 × 10^-5 to 1.24 × 10^-3 Lg^-1h^-1with mackinawite (FeS), 1.25 × 10^-4 to 1.89 × 10^-4 Lg^-1h^-1 with siderite (FeCO₃), and 1.79 × 10^-4 to 1.10 × 10^-3 Lg^-1h^-1 with magnetite (Fe₃O₄). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.

Compound-specific isotope analysis (CSIA) evaluation of degradation of chlorinated benzenes (CBs) and benzene in a contaminated aquifer

Compound-specific isotope analysis (CSIA) has become a valuable tool in understanding the fate of organic contaminants at field sites. However, its application to chlorinated benzenes (CBs), a group of toxic and persistent groundwater contaminants, has received less attention. This study employed CSIA to investigate the occurrence of natural degradation of various CBs and benzene in a contaminated aquifer. Despite the complexity of the study area (e.g., installation of a sheet pile barrier and the presence of a complex set of contaminants), the substantial enrichments in δ¹³C values (i.e., >2‰) for all CBs and benzene across the sampling wells indicate in situ degradation of these compounds. In particular, the ¹³C enrichments for 1,2,4-trichlorobenzene (1,2,4-TCB) and 1,2-dichlorobenzene (1,2-DCB) display good correlations with decreasing groundwater concentrations, consistent with the effects of in situ biodegradation. Using the Rayleigh model, the extent of degradation (EoD) is estimated to be 47-99% for 1,2-DCB, and 21-73% for 1,2,4-TCB. The enrichments observed for the other CBs (1,4-DCB and chlorobenzene (MCB)) and benzene at the site are also suggestive of in situ biodegradation. Due to simultaneous degradation and production of 1,4-DCB (a major 1,2,4-TCB degradation product), MCB (from DCB degradation), and benzene (from MCB degradation), the estimation of EoD for these intermediate compounds is more complex but a modelling simulation supports in situ biodegradation of these daughter products. In particular, the fact that the δ¹³C values of MCB and benzene (i.e., daughter products of 1,2,4-TCB) are more enriched than the original δ¹³C value of their parent 1,2,4-TCB provides definitive evidence for the occurrence of in situ biodegradation of the MCB and benzene.

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).

Presence of bacteria capable of PCB biotransformation in stormwater bioretention cells

Core samples from bioretention cell media as well as surface stormwater sediment samples from seven urban areas were collected to assess the potential for biotransformation activity of polychlorinated biphenyls (PCBs). The presence of putative organohalide-respiring bacteria in these samples was studied. Based on extracted DNA, Dehalobacter, Dehalogenimonas and Dehalococcoides were detected. Other organohalide-respiring bacteria like Desulfitobacterium and Sulfurospirillum were not studied. Bacteria containing the genes encoding for biphenyl 2,3-dioxygenase (bphA) or 2,3-dihydroxybiphenyl 1,2-dioxygenase (bphC) were detected in 29 of the 32 samples. These genes are key factors in PCB aerobic degradation. Transcribed bacterial genes from putative organohalide-respiring bacteria as well as genes encoding for bphA and bphC were obtained from the microbial community, thus showing the potential of organohalide respiration of PCBs and aerobic PCB degradation under both aerobic and anaerobic conditions in the surface samples collected at the bioretention site. Presence and concentrations of 209 PCB congeners in the bioretention media were also assessed. The total PCB concentration ranged from 38.4 ± 2.3 ng/g at the top layer of the inlet to 11.6 ± 1.2 ng/g at 20-30 cm at 3 m from the inlet. These results provide documentation that bacteria capable of PCB transformation, including both anaerobic dechlorination and aerobic degradation, were present and active in the bioretention.

Groundwater contaminated with short-chain chlorinated paraffins and microbial responses

The vertical migrations of toxic and persistent short-chain chlorinated paraffins (SCCPs) in soils as well as the microbial responses have been reported, however, there is a paucity of data on the resulting groundwater contamination. Here, we determined the concentration and congener profile of SCCPs in the groundwater beneath a production plant of chlorinated paraffins (CPs) and characterized the microbial community to explore their responses to SCCPs. Results showed that SCCPs ranged from not detected to 70.3 μg/L, with C₁₃-CPs (11.2-65.8%) and Cl₇-CPs (27.2-50.6%), in mass ratio, as the dominant groups. Similar to the distribution pattern in soils, SCCPs in groundwater were distributed in hotspot pattern. CP synthesis was the source of SCCPs in groundwater and the entire contamination plume significantly migrated downgradient, while there was an apparent hysteresis of C₁₃-CP migration. Groundwater microbial community was likely shaped by both hydrogeological condition (pH and depth) and SCCPs. Specifically, the microbial community responded to the contamination by forming a co-occurrence network with "small world" feature, where Desulfobacca, Desulfomonile, Ferritrophicum, Methylomonas, Syntrophobacter, Syntrophorhabdus, Syntrophus, and Thermoanaerobaculum were the keystone taxa. Furthermore, the interrelations between bacterial taxa and SCCPs indicated that the microbial community might cooperate to achieve the dechlorination and mineralization of SCCPs through either anaerobic organohalide respiration mainly functioned by the keystone taxa, or cometabolic degradation processes functioned by Aquabacterium and Hydrogenophaga. Results of this study would provide a better understanding of the environmental behavior and ecological effects of SCCPs in groundwater systems.

Genome Sequence, Proteome Profile, and Identification of a Multiprotein Reductive Dehalogenase Complex in Dehalogenimonas alkenigignens Strain BRE15M

Bacteria of the genus Dehalogenimonas respire with vicinally halogenated alkanes via dihaloelimination. We aimed to describe involved proteins and their supermolecular organization. Metagenomic sequencing of a Dehalogenimonas-containing culture resulted in a 1.65 Mbp draft genome of Dehalogenimonas alkenigignens strain BRE15M. It contained 31 full-length reductive dehalogenase homologous genes (rdhA), but only eight had cognate rdhB gene coding for membrane-anchoring proteins. Shotgun proteomics of cells grown with 1,2-dichloropropane as an electron acceptor identified 1152 proteins representing more than 60% of the total proteome. Ten RdhA proteins were detected, including a DcpA ortholog, which was the strongest expressed RdhA. Blue native gel electrophoresis (BNE) demonstrating maximum activity was localized in a protein complex of 146–242 kDa. Protein mass spectrometry revealed the presence of DcpA, its membrane-anchoring protein DcpB, two hydrogen uptake hydrogenase subunits (HupL and HupS), an iron–sulfur protein (HupX), and subunits of a redox protein with a molybdopterin-binding motif (OmeA and OmeB) in the complex. BNE after protein solubilization with different detergent concentrations revealed no evidence for an interaction between the putative respiratory electron input module (HupLS) and the OmeA/OmeB/HupX module. All detected RdhAs comigrated with the organohalide respiration complex. Based on genomic and proteomic analysis, we propose quinone-independent respiration in Dehalogenimonas.

Recent advances in the biodegradation of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are typical lasting organic pollutants. Persistence and recalcitrance to biodegradation of PCBs have hampered the transformation of PCB congeners from the environment. Biological transformation of polychlorinated biphenyls could take place through anaerobic dechlorination, aerobic microbial degradation, and a combination of transformation of anaerobic dechlorination and aerobic degradation. Under anaerobic conditions, microbial dechlorination is an important degradation mode for PCBs, especially high-chlorinated congeners. The low-chlorinated compounds formed after reductive dechlorination could be further aerobically degraded and completely mineralized. This paper reviews the recent advances in biological degradation of PCBs, introduces the functional bacteria and enzymes involved in the anaerobic and aerobic degradation of PCBs, and discusses the synergistic action of anaerobic reduction and aerobic degradation. In addition, the different ways to the microbial remediation of PCBs-contaminated environments are discussed. This review provides a theoretical foundation and practical basis to use PCBs-degrading microorganisms for bioremediation.

Sequential biodegradation of 1,2,4-trichlorobenzene at oxic-anoxic groundwater interfaces in model laboratory columns

Halogenated organic solvents such as chlorobenzenes (CBs) are frequent groundwater contaminants due to legacy spills. When contaminated anaerobic groundwater discharges into surface water through wetlands and other transition zones, aeration can occur from various physical and biological processes at shallow depths, resulting in oxic-anoxic interfaces (OAIs). This study investigated the potential for 1,2,4-trichlorobenzene (1,2,4-TCB) biodegradation at OAIs. A novel upflow column system was developed to create stable anaerobic and aerobic zones, simulating a natural groundwater OAI. Two columns containing (1) sand and (2) a mixture of wetland sediment and sand were operated continuously for 295 days with varied doses of 0.14-1.4 mM sodium lactate (NaLac) as a model electron donor. Both column matrices supported anaerobic reductive dechlorination and aerobic degradation of 1,2,4-TCB spatially separated between anaerobic and aerobic zones. Reductive dechlorination produced a mixture of di- and monochlorobenzene daughter products, with estimated zero-order dechlorination rates up to 31.3 μM/h. Aerobic CB degradation, limited by available dissolved oxygen, occurred for 1,2,4-TCB and all dechlorinated daughter products. Initial reductive dechlorination did not enhance the overall observed extent or rate of subsequent aerobic CB degradation. Increasing NaLac dose increased the extent of reductive dechlorination, but suppressed aerobic CB degradation at 1.4 mM NaLac due to increased oxygen demand. 16S-rRNA sequencing of biofilm microbial communities revealed strong stratification of functional anaerobic and aerobic organisms between redox zones including the sole putative reductive dechlorinator detected in the columns, Dehalobacter. The sediment mixture column supported enhanced reductive dechlorination compared to the sand column at all tested NaLac doses and growth of Dehalobacter populations up to 4.1 × 10⁸ copies/g (51% relative abundance), highlighting the potential benefit of sediments in reductive dechlorination processes. Results from these model systems suggest both substantial anaerobic and aerobic CB degradation can co-occur along the OAI at contaminated sites where bioavailable electron donors and oxygen are both present.

Sequential biodegradation of 1,2,4-trichlorobenzene at oxic-anoxic groundwater interfaces in model laboratory columns

Halogenated organic solvents such as chlorobenzenes (CBs) are frequent groundwater contaminants due to legacy spills. When contaminated anaerobic groundwater discharges into surface water through wetlands and other transition zones, aeration can occur from various physical and biological processes at shallow depths, resulting in oxic-anoxic interfaces (OAIs). This study investigated the potential for 1,2,4-trichlorobenzene (1,2,4-TCB) biodegradation at OAIs. A novel upflow column system was developed to create stable anaerobic and aerobic zones, simulating a natural groundwater OAI. Two columns containing (1) sand and (2) a mixture of wetland sediment and sand were operated continuously for 295 days with varied doses of 0.14-1.4 mM sodium lactate (NaLac) as a model electron donor. Both column matrices supported anaerobic reductive dechlorination and aerobic degradation of 1,2,4-TCB spatially separated between anaerobic and aerobic zones. Reductive dechlorination produced a mixture of di- and monochlorobenzene daughter products, with estimated zero-order dechlorination rates up to 31.3 μM/h. Aerobic CB degradation, limited by available dissolved oxygen, occurred for 1,2,4-TCB and all dechlorinated daughter products. Initial reductive dechlorination did not enhance the overall observed extent or rate of subsequent aerobic CB degradation. Increasing NaLac dose increased the extent of reductive dechlorination, but suppressed aerobic CB degradation at 1.4 mM NaLac due to increased oxygen demand. 16S-rRNA sequencing of biofilm microbial communities revealed strong stratification of functional anaerobic and aerobic organisms between redox zones including the sole putative reductive dechlorinator detected in the columns, Dehalobacter. The sediment mixture column supported enhanced reductive dechlorination compared to the sand column at all tested NaLac doses and growth of Dehalobacter populations up to 4.1 × 108 copies/g (51% relative abundance), highlighting the potential benefit of sediments in reductive dechlorination processes. Results from these model systems suggest both substantial anaerobic and aerobic CB degradation can co-occur along the OAI at contaminated sites where bioavailable electron donors and oxygen are both present.

Insights into origins and function of the unexplored majority of the reductive dehalogenase gene family as a result of genome assembly and ortholog group classification

Organohalide respiring bacteria (OHRB) express reductive dehalogenases for energy conservation and growth. Some of these enzymes catalyze the reductive dehalogenation of chlorinated and brominated pollutants in anaerobic subsurface environments, providing a valuable ecosystem service. Dehalococcoides mccartyi strains have been most extensively studied owing to their ability to dechlorinate all chlorinated ethenes - most notably carcinogenic vinyl chloride - to ethene. The genomes of OHRB, particularly obligate OHRB, often harbour multiple putative reductive dehalogenase genes (rdhA), most of which have yet to be characterized. We recently sequenced and closed the genomes of eight new strains, increasing the number of available D. mccartyi genomes in NCBI from 16 to 24. From all available OHRB genomes, we classified predicted translations of reductive dehalogenase genes using a previously established 90% amino acid pairwise identity cut-off to identify Ortholog Groups (OGs). Interestingly, the majority of D. mccartyi dehalogenase gene sequences, once classified into OGs, exhibited a remarkable degree of synteny (gene order) in all genomes sequenced to date. This organization was not apparent without the classification. A high degree of synteny indicates that differences arose from rdhA gene loss rather than recombination. Phylogenetic analysis suggests that most rdhA genes have a long evolutionary history in the Dehalococcoidia with origin prior to speciation of Dehalococcoides and Dehalogenimonas. We also looked for evidence of synteny in the genomes of other species of OHRB. Unfortunately, there are too few closed Dehalogenimonas genomes to compare at this time. There is some partial evidence for synteny in the Dehalobacter restrictus genomes, but here too more closed genomes are needed for confirmation. Interestingly, we found that the rdhA genes that encode enzymes that catalyze dehalogenation of industrial pollutants are the only rdhA genes with strong evidence of recent lateral transfer - at least in the genomes examined herein. Given the utility of the RdhA sequence classification to comparative analyses, we are building a public web server () for the community to use, which allows users to add and classify new sequences, and download the entire curated database of reductive dehalogenases.

Highly Intensified Molecular Oxygen Activation on Bi@Bi2MoO6 via a Metallic Bi-Coordinated Facet-Dependent Effect

Construction of the semimetal/semiconductor composite interface is widely used to promote the O₂ molecule adsorption and charge transfer for boosting solar-driven molecular oxygen activation (MOA). Herein, a Bi@Bi₂MoO₆ heterostructure is fabricated via a two-step wet chemical method as a typical photocatalyst to investigate the underlying mechanism of Bi-coordinated facet-dependent MOA under visible-light illumination. Density functional theory and systematical characterization methods reveal the distinct charge transfer and O₂ activation processes on the surface of Bi nanoparticle-deposited Bi₂MoO₆ nanosheets with different facets exposed. By virtue of a particular and efficient [Bi₂O₂]²⁺ → Bi → MoO₄^2- interfacial charge-transfer channel, Bi deposited on the (001) facet of Bi₂MoO₆ can observably intensify MOA, thereby giving birth to more generation of reactive oxygen species and endowing the Bi@Bi₂MoO₆ with excellent photocatalytic performance in sodium pentachlorophenate (NaPCP) removal. The decomposition pathway of NaPCP is also proposed based on the intermediate determination and mineralization analysis. This work provides deep insights into the mechanism of facet-dependent MOA over a semimetal-semiconductor system and also sheds light on designing effective molecular oxygen-activated interface for environmental remediation.

Redox characteristics of humins and their coupling with potential PCB dechlorinators in southern Yellow Sea sediments

Natural attenuation of polychlorinated biphenyls (PCBs) by indigenous bacteria is an effective remediation strategy for polluted marine sediments. This study investigated the relationships between PCB concentrations in sediment pore water, humin electron transfer capacity, and potential PCB dechlorinators at eight sediment sampling sites in the southern Yellow Sea, China, with differential PCB contamination. Station A2 showed the highest PCB concentration (453.16 ng L^-1 for seven indicator PCBs), especially of less chlorinated PCB congeners (≤5 Cl atoms), humin redox activity, and Dehalococcoides abundance (p < 0.05). Statistical analyses revealed a highly positive correlation between Dehalococcoides abundance and PCB concentration (r = 0.836, p < 0.05) and the electron shuttling ability of humins (r = 0.952, p < 0.01), whereas this was not observed for total bacteria and other potential PCB dechlorinators, e.g., Dehalobacter and Dehalogenimonas. Based on these results, Dehalococcoides might play an important role in the in situ reductive dechlorination of PCBs involving humins in marine sediments, and the natural microbial PCB attenuation capacity at station A2 was high. Chemical characterizations, electrochemical properties, and Fourier transform infrared analysis suggested that humins at station A2 had the highest electron transfer capacity. Furthermore, quinones are likely to be the functional groups that shuttle electrons during PCB dechlorination. Overall, this study provides a useful foundation for evaluating the natural microbial attenuation potential and fates of PCBs in marine sediments and for determining the role of humins as redox mediators in in situ PCB dechlorination by putative indigenous dechlorinators.

Halogen bonding with the halogenabenzene bird structure, halobenzene, and halocyclopentadiene

The ability of the "bird-like" halogenabenzene molecule, referred to as X-bird (XCl to At), to form halogen-bonded complexes with the nucleophiles H₂ O and NH₃ was investigated using double-hybrid density functional theory and the aug-cc-pVTZ/aug-cc-pVTZ-PP basis set. The structures and interaction energies were compared with 5-halocyclopenta-1,3-diene (halocyclopentadiene; an isomer of halogenabenzene) and halobenzene, also complexed with H₂ O and NH₃ . The unusual structure of the X-bird, with the halogen bonded to two carbon atoms, results in two distinct σ-holes, roughly at the extension of the C-X bonds. Based on the behavior of the interaction energy (which increases for heavier halogens) and van der Waals (vdW) ratio (which decreases for heavier halogens), it is concluded that the X-bird forms proper halogen bonds with H₂ O and NH₃ . The interaction energies are larger than those of the halogen-bonded complexes involving halobenzene and halocyclopentadiene, presumably due to the presence of a secondary interaction. © 2019 Wiley Periodicals, Inc.