Biodegradation of trichloroethylene (TCE) by methanotrophic community

Quantitative analysis of TCE biodegradation pathway in landfill cover utilizing continuous monitoring, droplet digital PCR and multi-omics sequencing technology

The remediation of volatile chlorinated hydrocarbons in the quasi-vadose zone has become a significant challenge. We applied an integrated approach to assess the biodegradability of trichloroethylene to identify the biotransformation mechanism. The formation of the functional zone biochemical layer was assessed by analyzing the distribution of landfill gas, physical and chemical properties of cover soil, spatial-temporal variations of micro-ecology, biodegradability of landfill cover soil and distributional difference metabolic pathway. Real-time online monitoring showed that trichloroethylene continuously undergoes anaerobic dichlorination and simultaneous aerobic/anaerobic conversion-aerobic co-metabolic degradation on the vertical gradient of the landfill cover system and reduction in trans-1,2-dichloroethylene in the anoxic zone but not 1,1-dichloroethylene. PCR and diversity sequencing revealed the abundance and spatial distribution of known dichlorination-related genes within the landfill cover, with 6.61 ± 0.25 × 104-6.78 ± 0.09 × 106 and 1.17 ± 0.78 × 103-7.82 ± 0.07 × 105 copies per g/soil of pmoA and tceA, respectively. In addition, dominant bacteria and diversity were significantly linked with physicochemical factors, and Mesorhizobium, Pseudoxanthomonas and Gemmatimonas were responsible for biodegradation in the aerobic, anoxic and anaerobic zones. Metagenome sequencing identified 6 degradation pathways of trichloroethylene that may occur in the landfill cover; the main pathway was incomplete dechlorination accompanied by cometabolic degradation. These results indicate that the anoxic zone is important for trichloroethylene degradation.

Acidophilic methanotrophs: Occurrence, diversity, and possible bioremediation applications

Methanotrophs have been identified and isolated from acidic environments such as wetlands, acidic soils, peat bogs, and groundwater aquifers. Due to their methane (CH₄ ) utilization as a carbon and energy source, acidophilic methanotrophs are important in controlling the release of atmospheric CH₄ , an important greenhouse gas, from acidic wetlands and other environments. Methanotrophs have also played an important role in the biodegradation and bioremediation of a variety of pollutants including chlorinated volatile organic compounds (CVOCs) using CH₄ monooxygenases via a process known as cometabolism. Under neutral pH conditions, anaerobic bioremediation via carbon source addition is a commonly used and highly effective approach to treat CVOCs in groundwater. However, complete dechlorination of CVOCs is typically inhibited at low pH. Acidophilic methanotrophs have recently been observed to degrade a range of CVOCs at pH < 5.5, suggesting that cometabolic treatment may be an option for CVOCs and other contaminants in acidic aquifers. This paper provides an overview of the occurrence, diversity, and physiological activities of methanotrophs in acidic environments and highlights the potential application of these organisms for enhancing contaminant biodegradation and bioremediation.

Recent advances and trends of trichloroethylene biodegradation: A critical review

Trichloroethylene (TCE) is a ubiquitous chlorinated aliphatic hydrocarbon (CAH) in the environment, which is a Group 1 carcinogen with negative impacts on human health and ecosystems. Based on a series of recent advances, the environmental behavior and biodegradation process on TCE biodegradation need to be reviewed systematically. Four main biodegradation processes leading to TCE biodegradation by isolated bacteria and mixed cultures are anaerobic reductive dechlorination, anaerobic cometabolic reductive dichlorination, aerobic co-metabolism, and aerobic direct oxidation. More attention has been paid to the aerobic co-metabolism of TCE. Laboratory and field studies have demonstrated that bacterial isolates or mixed cultures containing Dehalococcoides or Dehalogenimonas can catalyze reductive dechlorination of TCE to ethene. The mechanisms, pathways, and enzymes of TCE biodegradation were reviewed, and the factors affecting the biodegradation process were discussed. Besides, the research progress on material-mediated enhanced biodegradation technologies of TCE through the combination of zero-valent iron (ZVI) or biochar with microorganisms was introduced. Furthermore, we reviewed the current research on TCE biodegradation in field applications, and finally provided the development prospects of TCE biodegradation based on the existing challenges. We hope that this review will provide guidance and specific recommendations for future studies on CAHs biodegradation in laboratory and field applications.

Current advances and research prospects for agricultural and industrial uses of microbial strains available in world collections

Microorganisms are an important component of the ecosystem and have an enormous impact on human lives. Moreover, microorganisms are considered to have desirable effects on other co-existing species in a variety of habitats, such as agriculture and industries. In this way, they also have enormous environmental applications. Hence, collections of microorganisms with specific traits are a crucial step in developing new technologies to harness the microbial potential. Microbial culture collections (MCCs) are a repository for the preservation of a large variety of microbial species distributed throughout the world. In this context, culture collections (CCs) and microbial biological resource centres (mBRCs) are vital for the safeguarding and circulation of biological resources, as well as for the progress of the life sciences. Ex situ conservation of microorganisms tagged with specific traits in the collections is the crucial step in developing new technologies to harness their potential. Type strains are mainly used in taxonomic study, whereas reference strains are used for agricultural, biotechnological, pharmaceutical research and commercial work. Despite the tremendous potential in microbiological research, little effort has been made in the true sense to harness the potential of conserved microorganisms. This review highlights (1) the importance of available global microbial collections for man and (2) the use of these resources in different research and applications in agriculture, biotechnology, and industry. In addition, an extensive literature survey was carried out on preserved microorganisms from different collection centres using the Web of Science (WoS) and SCOPUS. This review also emphasizes knowledge gaps and future perspectives. Finally, this study provides a critical analysis of the current and future roles of microorganisms available in culture collections for different sustainable agricultural and industrial applications. This work highlights target-specific potential microbial strains that have multiple important metabolic and genetic traits for future research and use.

Cometabolic degradation mechanism and microbial network response of methanotrophic consortia to chlorinated hydrocarbon solvents

The cometabolism mechanism of chlorinated hydrocarbon solvents (CHSs) in mixed consortia remains largely unknown. CHS biodegradation characteristics and microbial networks in methanotrophic consortia were studied for the first time. The results showed that all CHSs can efficiently be degraded via cometabolism with a maximum degradation rate of 4.8 mg/(h·gcell). Chloroalkane and chloroethylene were more easily degraded than chlorobenzenes by methanotrophic consortia, especially nonfully chlorinated aliphatic hydrocarbons, which were converted to Cl- with a production rate of 0.29-0.36 mg/(h·gcell). In addition, the microecological response results indicated that Methylocystaceae (49.0%), Methylomonas (65.3%) and Methylosarcina (41.9%) may be the major functional degraders in methanotrophic consortia. Furthermore, the results of the microbial correlation network suggested that interactive relationships constructed by type I methanotrophs and heterotrophs determined biodegradability. Additionally, PICRUSt analysis showed that CHSs could increase the relative abundance of CHS degradation genes and reduce the relative abundance of methane oxidation genes, which was in good agreement with the experimental results.

Liquid-phase hydrodechlorination of trichloroethylene driven by nascent H2 under an open system: Hydrogenation activity, solvent effect and sulfur poisoning

Hydrodechlorination is a promising technology for the remediation of water body contaminated with trichloroethylene (TCE). In this work, the liquid-phase hydrogenation of TCE by Raney Ni (R-Ni) and Pd/C under an open system have been studied, in which nascent H₂ (Nas-H₂) generated in situ from the cathode acted as a hydrogen source. Experimental results showed that TCE was completely eliminate from the solution through the synergistic effects of hydrodechlorination and air flotation due to the formation of continuous micro/nano-sized Nas-H₂ bubbles from the cathode. Furthermore, the effects of inorganic anions and organic solvents on R-Ni and Pd/C hydrogenation activity were investigated, respectively. The results showed that NO₃^- and acetonitrile can form a competitive reaction with TCE; Sulfur with lone-pair electrons will cause irreversible poisoning to these two catalysts, and have a stronger inhibitory effect on Pd/C. This work helps to realize the separation of volatile halogenated compounds from water environment and provides certain data support for the choice of catalyst in the actual liquid-phase hydrogenation system.

Biotransformation of the insensitive munition constituents 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN) by aerobic methane-oxidizing consortia and pure cultures

We present the first report of biotransformation of 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN), replacements for the explosives 1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT), respectively, by methane-oxidizing cultures under aerobic conditions. Two consortia, dominated by Methylosinus spp., degraded both compounds with transient production of reduced NTO products, and non-stoichiometric production of reduced DNAN products. No release of inorganic nitrogen was observed with either compound, indicating that NTO and DNAN may be utilized as nitrogen sources by these consortia. The pure culture Methylosinus trichosporium OB3b also degraded both compounds. Degradation was observed in the presence of acetylene (a known inhibitor of methane monooxygenase; MMO) when methanol was supplied, indicating that MMO was not involved. Furthermore, studies with purified soluble MMO (sMMO) from OB3b indicated that neither compound was a substrate for sMMO. Degradation was inhibited by 2-iodosobenzoic acid, but not by dicoumarol, suggesting involvement of an oxygen- and dicoumarol-insensitive (nitro)reductase. These results indicate methanotrophs can aerobically degrade NTO and DNAN via one or more (nitro)reductases, with sMMO serving a supporting role deriving reducing equivalents from methane. This finding is important because methanotrophic bacteria are widely dispersed, and may represent a previously unrecognized route of NTO and DNAN biotransformation in aerobic environments.

A novel process of the isolation of nitrifying bacteria and their development in two different natural lab-scale packed-bed bioreactors for trichloroethylene bioremediation

Trichloroethylene (TCE) is a carcinogenic compound that is commonly present in groundwater and has been detected in drinking water sources for Mexican towns in the Mexico-US border area. Nitrifying bacteria, such as Nitrosomonas europaea, have been shown to be capable of degrading halogenated compounds, including TCE, but it is difficult to obtain high cell concentrations of these bacteria. The aim of the present study was to generate biomass of a nitrifying bacterial consortium from the sludge of an urban wastewater treatment plant (WWTP) and evaluate its capacity to biodegrade TCE in two different natural lab-scaled packed bed bioreactors. The consortium was isolated by a novel method using a continuous stirred-tank bioreactor inoculated with activated sludge from the Domos WWTP located in Cd. Obregón, Sonora, Mexico. The bioreactor was fed with specific media to cultivate ammonia-oxidizing bacteria at a dilution rate near the maximum specific growth rate reported for Nitrosomonas europaea. Optical density and suspended solids measurements were performed to determine the culture biomass production, and the presence of inorganic nitrogen species was determined by spectrophotometry. The presence of nitrifying ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) was confirmed by PCR amplification, and biofilm formation was observed by scanning electron microscopy. Batch-scale experiments confirmed the biodegradative activity of the isolated consortium, which was subsequently fixed in an inorganic carrier as zeolite and a synthetic carrier such as polyurethane to both be used as lab-scale packed-bed bioreactors, with up to 58.63% and 62.7% of TCE biodegradation achieved, respectively, demonstrating a possible alternative for TCE bioremediation in environmental and engineering systems.

A novel methanotrophic co-metabolic system with high soluble methane monooxygenase activity to biodegrade refractory organics in pulping wastewater

Pulping wastewater still contains massive refractory organics after biotreatment, with high colority, low biodegradability, and lasting biotoxicity. To eliminate refractory organics in pulping wastewater, a methanotrophic co-metabolic system in a gas cycle Sequencing Batch Biofilm Reactor (gcSBBR) seeded by soil at a ventilation opening of coal mine was quickly built on the 92nd day. The removal rate of COD, colority and TOC was 53.28%, 50.59% and 51.60%, respectively. Analysis of 3D-EEM indicated that glycolated protein-like, melanoidin-like or lignocellulose-like, and humic acid-like decreased by 7.85%, 5.02% and 1.74%, respectively. Moreover, this system exhibited high activity of soluble methane monooxygenase (sMMO) and mmoX encoding sMMO reached up to 7.89 × 10⁵ copies/μL. Methanotrophs, namely, Methylocaldum (8.28%), Methylococcus (6.06%) and Methylomonas (0.07%), were detected by 16S rRNA sequencing. And other bacteria were dominated by Denitratisoma, Anaerolineaceae_uncultured and Methylophilaceae_uncultured. Refractory organics was biodegraded through the synergy among microorganisms, and a postulated synergy pathway was put forward.

Methylotrophic bacteria in sustainable agriculture

Excessive use of chemical fertilizers to increase production from available land has resulted in deterioration of soil quality. To prevent further soil deterioration, the use of methylotrophic bacteria that have the ability to colonize different habitats, including soil, sediment, water, and both epiphytes and endophytes as host plants, has been suggested for sustainable agriculture. Methylotrophic bacteria are known to play a significant role in the biogeochemical cycle in soil ecosystems, ultimately fortifying plants and sustaining agriculture. Methylotrophs also improve air quality by using volatile organic compounds such as dichloromethane, formaldehyde, methanol, and formic acid. Additionally, methylotrophs are involved in phosphorous, nitrogen, and carbon cycling and can help reduce global warming. In this review, different aspects of the interaction between methylotrophs and host plants are discussed, including the role of methylotrophs in phosphorus acquisition, nitrogen fixation, phytohormone production, iron chelation, and plant growth promotion, and co-inoculation of these bacteria as biofertilizers for viable agriculture practices.

Analysis of trichloroethylene removal and bacterial community function based on pH-adjusted in an upflow anaerobic sludge blanket reactor

The study reported the upflow anaerobic sludge blanket (UASB) reactor performance in treating wastewater containing trichloroethylene (TCE) and characterized variations of bacteria composition and structure by changing the pH from 6.0 to 8.0. A slightly acidic environment (pH < 7.0) had a greater impact on the TCE removal. Illumina pyrosequencing was applied to investigate the bacterial community changes in response to pH shifts. The results demonstrated that pH greatly influenced the dominance and presence of specific populations. The potential TCE degradation pathway in the UASB reactor was proposed. Importantly, the genus Dehalobacter which was capable of reductively dechlorinating TCE was detected, and it was not found at pH of 6.0, which presumably is the reason why the removal efficiency of TCE was the lowest (80.73 %). Through Pearson correlation analyses, the relative abundance of Dehalobacter positively correlated with TCE removal efficiency (R = 0.912). However, the relative abundance of Lactococcus negatively correlated with TCE removal efficiency according to the results from Pearson correlation analyses and redundancy analysis (RDA).

Toxic and inhibitory effects of trichloroethylene aerobic co-metabolism on phenol-grown aerobic granules

Aerobic granule, a form of microbial aggregate, exhibits good potential in degrading toxic and recalcitrant substances. In this study, the inhibitory and toxic effects of trichloroethylene (TCE), a model compound for aerobic co-metabolism, on phenol-grown aerobic granules were systematically studied, using respiratory activities after exposure to TCE as indicators. High TCE concentration did not exert positive or negative effects on the subsequent endogenous respiration rate or phenol dependent specific oxygen utilization rate (SOUR), indicating the absence of solvent stress and induction effect on phenol-hydroxylase. Phenol-grown aerobic granules exhibited a unique response to TCE transformation product toxicity, that small amount of TCE transformation enhanced the subsequent phenol SOUR. Granules that had transformed between 1.3 and 3.7 mg TCE gSS(-1) showed at most 53% increase in the subsequent phenol SOUR, and only when the transformation exceeded 6.6 mg TCE gSS(-1) did the SOUR dropped below that of the control. This enhancing effect was found to sustain throughout several phenol dosages, and TCE transformation below the toxicity threshold also lessened the granules' sensitivity to higher phenol concentration. The unique toxic effect was possibly caused by the granule's compact structure as a protection barrier against the diffusive transformation product(s) of TCE co-metabolism.

Aerobic biodegradation of trichloroethylene and phenol co-contaminants in groundwater by a bacterial community using hydrogen peroxide as the sole oxygen source

Trichloroethylene (TCE) and phenol were often found together as co-contaminants in the groundwater of industrial contaminated sites. An effective method to remove TCE was aerobic biodegradation by co-metabolism using phenol as growth substrates. However, the aerobic biodegradation process was easily limited by low concentration of dissolved oxygen (DO) in groundwater, and DO was improved by air blast technique with difficulty. This study enriched a bacterial community using hydrogen peroxide (H2O2) as the sole oxygen source to aerobically degrade TCE by co-metabolism with phenol in groundwater. The enriched cultures were acclimatized to 2-8 mM H2O2 which induced catalase, superoxide dismutase and peroxidase to decompose H2O2 to release O2 and reduce the toxicity. The bacterial community could degrade 120 mg/L TCE within 12 days by using 8 mM H2O2 as the optimum concentration, and the TCE degradation efficiency reached up to 80.6%. 16S rRNA gene cloning and sequencing showed that Bordetella, Stenotrophomonas sp., Sinorhizobium sp., Variovorax sp. and Sphingobium sp. were the dominant species in the enrichments, which were clustered in three phyla: Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria. Polymerase chain reaction detection proved that phenol hydroxylase (Lph) gene was involved in the co-metabolic degradation of phenol and TCE, which indicated that hydroxylase might catalyse the epoxidation of TCE to form the unstable molecule TCE-epoxide. The findings are significant for understanding the mechanism of biodegradation of TCE and phenol co-contamination and helpful for the potential applications of an aerobic bioremediation in situ the contaminated sites.

Identification of biomarker genes to predict biodegradation of 1,4-dioxane

Bacterial multicomponent monooxygenase gene targets in Pseudonocardia dioxanivorans CB1190 were evaluated for their use as biomarkers to identify the potential for 1,4-dioxane biodegradation in pure cultures and environmental samples. Our studies using laboratory pure cultures and industrial activated sludge samples suggest that the presence of genes associated with dioxane monooxygenase, propane monooxygenase, alcohol dehydrogenase, and aldehyde dehydrogenase are promising indicators of 1,4-dioxane biotransformation; however, gene abundance was insufficient to predict actual biodegradation. A time course gene expression analysis of dioxane and propane monooxygenases in Pseudonocardia dioxanivorans CB1190 and mixed communities in wastewater samples revealed important associations with the rates of 1,4-dioxane removal. In addition, transcripts of alcohol dehydrogenase and aldehyde dehydrogenase genes were upregulated during biodegradation, although only the aldehyde dehydrogenase was significantly correlated with 1,4-dioxane concentrations. Expression of the propane monooxygenase demonstrated a time-dependent relationship with 1,4-dioxane biodegradation in P. dioxanivorans CB1190, with increased expression occurring after over 50% of the 1,4-dioxane had been removed. While the fraction of P. dioxanivorans CB1190-like bacteria among the total bacterial population significantly increased with decrease in 1,4-dioxane concentrations in wastewater treatment samples undergoing active biodegradation, the abundance and expression of monooxygenase-based biomarkers were better predictors of 1,4-dioxane degradation than taxonomic 16S rRNA genes. This study illustrates that specific bacterial monooxygenase and dehydrogenase gene targets together can serve as effective biomarkers for 1,4-dioxane biodegradation in the environment.

Bacterial community structure in the rhizosphere of a Cry1Ac Bt-brinjal crop and comparison to its non-transgenic counterpart in the tropical soil

To elucidate whether the transgenic crop alters the rhizospheric bacterial community structure, a 2-year study was performed with Cry1Ac gene-inserted brinjal crop (Bt) and their near isogenic non-transformed trait (non-Bt). The event of Bt crop (VRBT-8) was screened using an insect bioassay and enzyme-linked immunosorbent assay. Soil moisture, NH4 (+)-N, NO3 (-)-N, and PO4 (-)-P level had non-significant variation. Quantitative polymerase chain reaction revealed that abundance of bacterial 16S rRNA gene copies were lower in soils associated with Bt brinjal. Microbial biomass carbon (MBC) showed slight reduction in Bt brinjal soils. Higher MBC values in the non-Bt crop soil may be attributed to increased root activity and availability of readily metabolizable carbon compounds. The restriction fragment length polymorphism of PCR-amplified rRNA gene fragments detected 13 different bacterial groups with the exclusive presence of β-Proteobacteria, Chloroflexus, Planctomycetes, and Fusobacteria in non-Bt, and Cyanobacteria and Bacteroidetes in Bt soils, respectively, reflecting minor changes in the community structure. Despite the detection of Cry1Ac protein in the rhizospheric soil, the overall impact of Cry1Ac expressing Bt brinjal was less compared to that due to seasonal changes.

Changes in Actinomycetes community structure under the influence of Bt transgenic brinjal crop in a tropical agroecosystem

BACKGROUND

The global area under brinjal cultivation is expected to be 1.85 million hectare with total fruit production about 32 million metric tons (MTs). Brinjal cultivars are susceptible to a variety of stresses that significantly limit productivity. The most important biotic stress is caused by the Brinjal fruit and shoot Borer (FSB) forcing farmers to deploy high doses of insecticides; a matter of serious health concern. Therefore, to control the adverse effect of insecticides on the environment including the soil, transgenic technology has emerged as the effective alternative. However, the reports, regarding the nature of interaction of transgenic crops with the native microbial community are inconsistent. The effect of a Bt transgenic brinjal expressing the bio-insecticidal protein (Cry1Ac) on the rhizospheric community of actinomycetes has been assessed and compared with its non-transgenic counterpart.

RESULTS

Significant variation in the organic carbon observed between the crops (non-Bt and Bt brinjal) may be due to changes in root exudates quality and composition mediated by genetic attributes of Bt transgenic brinjal. Real time quantitative PCR indicated significant differences in the actinomycetes- specific 16S rRNA gene copy numbers between the non-Bt (5.62-27.86) × 1011 g-1 dws and Bt brinjal planted soil (5.62-24.04) × 1011 g-1 dws. Phylogenetic analysis indicated 14 and 11, actinomycetes related groups in soil with non-Bt and Bt brinjal crop, respectively. Micrococaceaea and Nocardiodaceae were the dominant groups in pre-vegetation, branching, flowering, maturation and post-harvest stage. However, Promicromonosporaceae, Streptosporangiaceae, Mycobacteriaceae, Geodermatophilaceae, Frankiaceae, Kineosporaceae, Actisymmetaceae and Streptomycetaceae were exclusively detected in a few stages in non-Bt brinjal rhizosphere soil while Nakamurellaceae, Corynebactericeae, Thermomonosporaceae and Pseudonocardiaceae in Bt brinjal counterpart.

CONCLUSION

Field trails envisage that cultivation of Bt transgenic brinjal had negative effect on organic carbon which might be attributed to genetic modifications in the plant. Changes in the organic carbon also affect the actinomycetes population size and diversity associated with rhizospheric soils of both the crops. Further long-term study is required by taking account the natural cultivar apart from the Bt brinjal and its near-isogenic non-Bt brinjal with particular reference to the effects induced by the Bt transgenic brinjal across different plant growth stages.

Effect of trichloroethylene and tetrachloroethylene on methane oxidation and community structure of methanotrophic consortium

The methane oxidation rate and community structure of a methanotrophic consortium were analyzed to determine the effects of trichloroethylene (TCE) and tetrachloroethylene (PCE) on methane oxidation. The maximum methane oxidation rate (Vmax ) of the consortium was 326.8 μmol·g-dry biomass(-1)·h(-1), and it had a half-saturation constant (Km ) of 143.8 μM. The addition of TCE or PCE resulted in decreased methane oxidation rates, which were decreased from 101.73 to 5.47-24.64 μmol·g-dry biomass(-1)·h(-1) with an increase in the TCE-to-methane ratio, and to 61.95-67.43 μmol·g-dry biomass(-1)·h(-1) with an increase in the PCE-to-methane ratio. TCE and PCE were non-competitive inhibitors for methane oxidation, and their inhibition constants (Ki ) were 33.4 and 132.0 μM, respectively. When the methanotrophic community was analyzed based on pmoA using quantitative real-time PCR (qRT-PCR), the pmoA gene copy numbers were shown to decrease from 7.3 ± 0.7 × 10(8) to 2.1-5.0 × 10(7) pmoA gene copy number · g-dry biomass(-1) with an increase in the TCE-to-methane ratio and to 2.5-7.0 × 10(7) pmoA gene copy number · g-dry biomass(-1) with an increase in the PCE-to-methane ratio. Community analysis by microarray demonstrated that Methylocystis (type II methanotrophs) were the most abundant in the methanotrophic community composition in the presence of TCE. These results suggest that toxic effects caused by TCE and PCE change not only methane oxidation rates but also the community structure of the methanotrophic consortium.

Current trends in trichloroethylene biodegradation: a review

Over the past few years biodegradation of trichloroethylene (TCE) using different microorganisms has been investigated by several researchers. In this review article, an attempt has been made to present a critical summary of the recent results related to two major processes--reductive dechlorination and aerobic co-metabolism used for TCE biodegradation. It has been shown that mainly Clostridium sp. DC-1, KYT-1, Dehalobacter, Dehalococcoides, Desulfuromonas, Desulfitobacterium, Propionibacterium sp. HK-1, and Sulfurospirillum bacterial communities are responsible for the reductive dechlorination of TCE. Efficacy of bacterial communities like Nitrosomonas, Pseudomonas, Rhodococcus, and Xanthobacter sp. etc. for TCE biodegradation under aerobic conditions has also been examined. Mixed cultures of diazotrophs and methanotrophs have been used for TCE degradation in batch and continuous cultures (biofilter) under aerobic conditions. In addition, some fungi (Trametes versicolor, Phanerochaete chrysosporium ME-446) and Actinomycetes have also been used for aerobic biodegradation of TCE. The available information on kinetics of biofiltration of TCE and its degradation end-products such as CO2 are discussed along with the available results on the diversity of bacterial community obtained using molecular biological approaches. It has emerged that there is a need to use metabolic engineering and molecular biological tools more intensively to improve the robustness of TCE degrading microbial species and assess their diversity.

Removal of gaseous trichloroethylene (TCE) in a composite membrane biofilm reactor

A membrane biofilm reactor (MBfR) was investigated for the degradation of trichloroethylene (TCE) vapors inoculated by Burkholderia vietnamiensis G4. Toluene (TOL) was used as the primary substrate. The MBfR was loaded sequentially with TOL, TCE (or both) during 110 days. In this study, a maximum steady-state TCE removal efficiency of 23% and a maximum volumetric elimination capacity (EC) of 2.1 g m(-3) h(-1) was achieved. A surface area based maximum elimination capacity (EC(m)) of 4.2 × 10(-3) g m(-2) h(-1) was observed, which is 2-10 times higher than reported in other gas phase biological treatment studies. However, further research is needed to optimize the TCE feeding cycle and to evaluate the inhibiting effects of TCE and its intermediates on TOL biodegradation.

Use of specific gene analysis to assess the effectiveness of surfactant-enhanced trichloroethylene cometabolism

The objective of this study was to evaluate the effectiveness of in situ bioremediation of trichloroethylene (TCE)-contaminated groundwater using specific gene analyses under the following conditions: (1) pretreatment with biodegradable surfactants [Simple Green™ (SG) and soya lecithin (SL)] to enhance TCE desorption and dissolution, and (2) supplementation with SG, SL, and cane molasses as primary substrates to enhance the aerobic cometabolism of TCE. Polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), and nucleotide sequence analysis were applied to monitor the variations in specific activity-dependent enzymes and dominant microorganisms. Results show that TCE-degrading enzymes, including toluene monooxygenase, toluene dioxygenase, and phenol monooxygenase, were identified from sediment samples collected from a TCE-spill site. Results from the microcosm study show that addition of SG, SL, or cane molasses can enhance the aerobic cometabolism of TCE. The TCE degradation rates were highest in microcosms with added SL, the second highest in microcosms containing SG, and lowest in microcosms containing cane molasses. This indicates that SG and SL can serve as TCE dissolution agents and act as primary substrates for indigenous microorganisms. Four dominant microorganisms (Rhodobacter sp., Methyloversatilis sp., Beta proteobacterium sp., and Hydrogenophaga pseudoflava) observed in microcosms might be able to produce TCE-degrading enzymes for TCE cometabolic processes.

Effect of anthraquinone-2,6-disulfonate on the trichloroethene degradation by Dehalococcoides-containing consortium

This paper reports the findings of an examination on the influence of anthraquinone-2,6-disulfonate (AQDS) on the degradation of trichloroethene (TCE) by Dehalococcoides-containing consortium (designated UC-1). Compared with the control, the results indicated that (i) in 100 μmol/L AQDS, TCE was rapidly degraded. More ethene was produced, while less vinyl chloride (VC) was accumulated. AQDS might improve the activity of organisms in dechlorinating populations which resulted in more ethene being accumulated in the medium; (ii) in 500 μmol/L AQDS, TCE was incompletely degraded. Presumably, 500 μmol/L AQDS might have an inhibition effect on methanogens in the UC-1. The inhibition effect might influence the interactions among methanogens, Dehalococcoides species and other organisms in the UC-1.

Influence of humic acid on the trichloroethene degradation by Dehalococcoides-containing consortium

By taking an anaerobic Dehalococcoides-containing consortium (designated UC-1) as the research object, the influence of humic acid on the degradation of TCE by UC-1 was examined. The results indicated that (i) TCE was more rapidly degraded in the presence of humic acid compared with the control and the TCE removal efficiencies increased with the increase of concentrations of humic acid; and (ii) at the end of experiments, in the presence of humic acid, much more ethene was produced compared with the control, whereas less VC was accumulated in the medium. Presumably, humic acid improves the activity of organisms in dechlorinating populations resulting in more ethene accumulated in the medium, and (iii) the degradation of TCE stimulated by humic acid by UC-1 might be a biotic process or an abiotic process. Thus, humic acid could influence the degradation of TCE by UC-1 directly via enhancing electron transfer between UC-1 and TCE. This work is a preliminary step for accelerating the degradation of TCE in the groundwater environment using a kind of natural organic matter - humic acid.

Fluroxypyr biodegradation in soils by multiple factors

Fluroxypyr (4-amino-3,5-dichloro-6-fluoro-2-pyridinyl-1-methylheptyl ester) is a widely used herbicide for controlling weeds, fungi, and insects. However, extensive use of the herbicide has led to its high accumulation in ecosystems and contamination to soils and crops. Environmental behaviors and fate of herbicides are dependent on many physiochemical and biological factors. Whether fluroxypyr is significantly affected and how it is degraded under the environmental conditions is largely unknown. The present study investigated the effects of soil microbe, soil type, dissolved organic matter (DOM), temperature, soil moisture, and surfactant on fluroxypyr degradation in soils. Application of DOM derived from sludge and straw to fluroxypyr-contaminated soils increased degradation of fluroxypyr. Environmental factors such as temperature, moisture, soil microbe and soil type could affect the rate of fluroxypyr dissipation. Also, the microorganism affected the degradation of fluroxypyr. Analysis by gas chromatography-mass revealed that the reaction in soils might include the removal of 1-methylheptyl ester to generate fluroxypyr acid (4-amino-3,5-dichloro-6-fluoro-2-pyridiny). Our results provided initial data that a set of biological and physiochemical factors coordinately regulates the decay of fluroxypyr in soils.

Substrate inhibition during bio-filtration of TCE using diazotrophic bacterial community

The kinetics of biodegradation of TCE in the biofilter packed with wood charcoal and inoculated with diazotrophic bacterial community had been investigated. Use of Michaelis-Menten type model showed that substrate inhibition was present in the system. The kinetic model proposed by Edwards (1970) was used to calculate kinetic parameters-maximum elimination capacity (EC(max)), substrate constant (K(s)), and inhibition constant (K(I)). The model fitted well with the experimental data and the EC(max) was found to be in the range of 10.8-6.1 g/m(3) h. The K(s) values depended upon substrate concentration and ranged from 0.024 to 0.043 g/m(3) indicating the high affinity of diazotrophs for TCE. The K(I) values were low and nearly constant (0.011-0.015 g/m(3)) indicating a moderate substrate inhibition.

Effect of substrate interaction on oxidation of methane and benzene in enriched microbial consortia from landfill cover soil

The interaction of methane and benzene during oxidation in enriched methane-oxidizing consortium (MOC) and in benzene-oxidizing consortium (BOC) from landfill cover soil was characterized. Oxidation of both methane and benzene occurred in the MOC due to the coexistence of bacteria responsible for benzene oxidation, as well as methanotrophs, whereas in the BOC, only benzene was oxidized, not methane. Methane oxidation rates in the MOC were decreased with increasing benzene/methane ratio (mol/mol), indicating its methane oxidation was inhibited by the benzene coexistence. Benzene oxidation rates in the MOC, however, were increased with increasing benzene/methane ratio. The benzene oxidation in the BOC was not affected by the coexistence of methane or by the ratio of methane/benzene ratio (mol/mol). No effect of methane or benzene was found on the dynamics of functional genes, such as particulate methane monooxygenase and toluene monooxygenase, in association with oxidation of methane and benzene in the MOC and BOC.

Biological removal of the xenobiotic trichloroethylene (TCE) through cometabolism in nitrifying systems

In the present study, cometabolic TCE degradation was evaluated using NH(4)-N as the growth-substrate. At initial TCE concentrations up to 845 microg/L, TCE degradation followed first-order kinetics. The increase in ammonium utilization rate favored the degradation of TCE. This ensured that biological transformation of TCE in nitrifying systems is accomplished through a cometabolic pathway by the catalysis of non-specific ammonia oxygenase enzyme of nitrifiers. The transformation yield (T(y)) of TCE, the amount of TCE degraded per unit mass of NH(4)-N, strongly depended on the initial NH(4)-N and TCE concentrations. In order to allow a rough estimation of TCE removal and nitrification at different influent TCE and NH(4)-N concentrations, a linear relationship was developed between 1/T(y) and the initial NH(4)-N/TCE ratio. The estimated T(y) values lead to the conclusion that nitrifying systems are promising candidates for biological removal of TCE through cometabolism.