Biodegradation of methyl tert-butyl ether by a pure bacterial culture

Isolation and characterization of 2-butoxyethanol degrading bacterial strains

A total of 11 bacterial strains capable of completely degrading 2-butoxyethanol (2-BE) were isolated from forest soil, a biotrickling filter, a bioscrubber, and activated sludge, and identified by 16S rRNA gene sequence analysis. Eight of these strains belong to the genus Pseudomonas; the remaining three strains are Hydrogenophaga pseudoflava BOE3, Gordonia terrae BOE5, and Cupriavidus oxalaticus BOE300. In addition to 2-BE, all isolated strains were able to grow on 2-ethoxyethanol and 2-propoxyethanol, ethanol, n-hexanol, ethyl acetate, 2-butoxyacetic acid (2-BAA), glyoxylic acid, and n-butanol. Apart from the only gram-positive strain isolated, BOE5, none of the strains were able to grow on the nonpolar ethers diethyl ether, di-n-butyl ether, n-butyl vinyl ether, and dibenzyl ether, as well as on 1-butoxy-2-propanol. Strains H. pseudoflava BOE3 and two of the isolated pseudomonads, Pseudomonas putida BOE100 and P. vancouverensis BOE200, were studied in more detail. The maximum growth rates of strains BOE3, BOE100, and BOE200 at 30 °C were 0.204 h-1 at 4 mM, 0.645 h-1 at 5 mM, and 0.395 h-1 at 6 mM 2-BE, respectively. 2-BAA, n-butanol, and butanoic acid were detected as potential metabolites during the degradation of 2-BE. These findings indicate that the degradation of 2-BE by the isolated gram-negative strains proceeds via oxidation to 2-BAA with subsequent cleavage of the ether bond yielding glyoxylate and n-butanol. Since Gordonia terrae BOE5 was the only strain able to degrade nonpolar ethers like diethyl ether, the degradation pathway of 2-BE may be different for this strain.

Metabolites of the anaerobic degradation of diethyl ether by denitrifying betaproteobacterium strain HxN1

The constitutions of five metabolites formed during anaerobic degradation of diethyl ether by the denitrifying bacterium Aromatoleum sp. HxN1 were identified by comparison with synthesized standards using GC-MS.

Distinct Bacterial Consortia Established in ETBE-Degrading Enrichments from a Polluted Aquifer

Ethyl tert-butyl ether (ETBE) is a gasoline additive that became an important aquifer pollutant. The information about natural bacterial consortia with a capacity for complete ETBE degradation is limited. Here we assess the taxonomical composition of bacterial communities and diversity of the ethB gene (involved in ETBE biodegradation) in ETBE-enrichment cultures that were established from a gasoline-polluted aquifer, either from anoxic ETBE-polluted plume water (PW), or from an upstream non-polluted water (UW). We used a 16S rRNA microarray, and 16S rRNA and ethB gene sequencing. Despite the dissimilar initial chemical conditions and microbial composition, ETBE-degrading consortia were obtained from both PW and UW. The composition of ETBE-enrichment cultures was distinct from their initial water samples, reflecting the importance of the rare biosphere as a reservoir of potential ETBE degraders. No convergence was observed between the enrichment cultures originating from UW and PW, which were dominated by Mesorhizobium and Hydrogenophaga, respectively, indicating that distinct consortia with the same functional properties may be present at one site. Conserved ethB genes were evidenced in both PW and UW ETBE-enrichment cultures and in PW water. Our results suggest that the presence of ethB genes rather than the taxonomical composition of in situ bacterial communities indicate the potential for the ETBE degradation at a given site.

Emulsified polycolloid substrate biobarrier for benzene and petroleum-hydrocarbon plume containment and migration control - A field-scale study

The objective of this field-scale study was to assess the effectiveness of applying an emulsified polycolloid substrate (EPS; containing cane molasses, soybean oil, and surfactants) biobarrier in the control and remediation of a petroleum-hydrocarbon plume in natural waters. An abandoned petrochemical manufacturing facility site was contaminated by benzene and other petroleum products due to a leakage from a storage tank. Because benzene is a petroleum hydrocarbon with a high migration ability, it was used as the target compound in the field-scale study. Batch partition and sorption experiment results indicated that the EPS to water partition coefficient for benzene was 232 mg/mg at 25 °C. This suggests that benzene had a higher sorption affinity to EPS, which decreased the benzene concentrations in groundwater. The EPS solution was pressure-injected into three remediation wells (RWs; 150 L EPS in 800 L groundwater). Groundwater samples were collected from an upgradient background well, two downgradient monitor wells (MWs), and the three RWs for analyses. EPS injection increased total organic carbon (TOC) concentrations (up to 786 mg/L) in groundwater, which also resulted in the formation of anaerobic conditions. An abrupt drop in benzene concentration (from 6.9 to below 0.04 mg/L) was observed after EPS supplementation in the RWs due to both sorption and biodegradation mechanisms. Results show that the EPS supplement increased total viable bacteria and enhanced bioremediation efficiency, which accounted for the observed decrease in benzene concentration. The first-order decay rate in RW1 increased from 0.003 to 0.023 d^-1 after EPS application. Injection of EPS resulted in significant growth of indigenous bacteria, and 23 petroleum-hydrocarbon-degrading bacterial species were detected, which enhanced the in situ benzene biodegradation efficiency. Results demonstrate that the EPS biobarrier can effectively contain a petroleum-hydrocarbon plume and prevent its migration to downgradient areas, which reduces the immediate risk presented to downgradient receptors.

Application of an emulsified polycolloid substrate biobarrier to remediate petroleum-hydrocarbon contaminated groundwater

Emulsified polycolloid substrate (EPS) was developed and applied in situ to form a biobarrier for the containment and enhanced bioremediation of a petroleum-hydrocarbon plume. EPS had a negative zeta potential (-35.7 mv), which promoted its even distribution after injection. Batch and column experiments were performed to evaluate the effectiveness of EPS on toluene containment and biodegradation. The EPS-to-water partition coefficient for toluene (target compound) was 943. Thus, toluene had a significant sorption affinity to EPS, which caused reduced toluene concentration in water phase in the EPS/water system. Groundwater containing toluene (18 mg/L) was pumped into the three-column system at a flow rate of 0.28 mL/min, while EPS was injected into the second column to form a biobarrier. A significant reduction of toluene concentration to 0.1 mg/L was observed immediately after EPS injection. This indicates that EPS could effectively contain toluene plume and prevent its further migration to farther downgradient zone. Approximately 99% of toluene was removed after 296 PVs of operation via sorption, natural attenuation, and EPS-enhanced biodegradation. Increase in total organic carbon and bacteria were also observed after EPS supplement. Supplement of EPS resulted in a growth of petroleum-hydrocarbon degrading bacteria, which enhanced the toluene biodegradation.

People, planet and profit: Unintended consequences of legacy building materials

Although an explosion of new building materials are being introduced into today's market, adequate up-front research into their chemical and physical properties as well as their potential health and environmental consequences is lacking. History has provided us with several examples where building materials were broadly deployed into society only to find that health and environmental problems resulted in unintended sustainability consequences. In the following paper, we use lead and asbestos as legacy building materials to show their similar historical trends and sustainability consequences. Our research findings show unintended consequences such as: increased remediation and litigation costs; adverse health effects; offshoring of related industries; and impediments to urban revitalization. As numerous new building materials enter today's market, another building material may have already been deployed, representing the next "asbestos." This paper also proposes an alternative methodology that can be applied in a cost-effective way into existing and upcoming building materials, to minimize and prevent potential unintended consequences and create a pathway for sustainable communities. For instance, our findings show that this proposed methodology could have prevented the unintended incurred sustainability costs of approximately $272-$359 billion by investing roughly $24 million in constant 2014 U.S. dollars on up-front research into lead and asbestos.

Stimulation effect of electric current density (ECD) on microbial community of a three dimensional particle electrode coupled with biological aerated filter reactor (TDE-BAF)

Improving the stimulation effect of electric current density (ECD) on microbial community is critical in designing and operating TDE-BAF. This study investigated the effect of ECD at 0.00, 4.08, 6.12, 12.20, 14.25, 16.30 and 20.20A·m^-2 on the removal performance, diversity and structure of microbial community in TDE-BAF. Results indicated that the ECD of 14.25A·m^-2 exhibited the highest COD, TOC and NH₄⁺-N average removal rates with 93.33%, 91.26% and 93.87%, respectively; Under high ECD, especially exceeding 14.25A·m^-2, the inhibition of growth and activity because of plasmatorrhexis was in agreement with the sharp biomass decline; there was no significant relation between community richness and diversity and removal efficiency below optimum ECD, while above optimal ECD, it was just the opposite; Microbial communities mainly including Hydrogenophaga, Saprospiraceae_uncultured, Delftia, Enterobacter, Pseudomonas, Pseudoxanthomonas, and Nitrosospira and physicochemical properties well explained the excellent removal performance at the optimum ECD.

Toxicity and biofilm-based selection for methyl tert-butyl ether bioremediation technology

Extractive membrane biofilm reactor (EMBFR) technology offers productive solutions for volatile and semi-volatile compound removal from water bodies. In this study, the bacterial strains Paenibacillus etheri SH7^T (CECT 8558), Agrobacterium sp. MS2 (CECT 8557) and Rhodococcus ruber strains A5 (CECT 8556), EE6 (CECT 8612) and EE1 (CECT 8555), previously isolated from fuel-contaminated sites, were tested for adherence on tubular semipermeable membranes in laboratory-scale systems designed for methyl tert-butyl ether (MTBE) bioremediation. Biofilm formation on the membrane surface was evaluated through observation by field-emission scanning electron microscope (FESEM) as well as the acute toxicity (as EC₅₀) of the bacterial growth media. Moreover, extracellular polymeric substance (EPS) production for each strain under different MTBE concentrations was measured. Strains A5 and MS2 were biofilm producers and their adherence increased when the MTBE flowed through the inner tubular semipermeable membrane. No biofilm was formed by Paenibacillus etheri SH7^T, nevertheless, the latter and strain MS2 exhibited the lowest toxicity after growth on the EMBFR. The results obtained from FESEM and toxicity analysis demonstrate that bacterial strains R. ruber EE6, A5, P. etheri SH7^T and Agrobacterium sp. MS2 could be excellent candidates to be used as selective inocula in EMBFR technology for MTBE bioremediation.

Production of unburned calcium silicon filter material (UCSFM) from oyster shell and its performance investigation in an A/O integrated biological aerated filter reactor (A/O-BAF)

A considerable amount of the oyster shells as a waste product of mariculture is produced every year, which leads to a major disposal problem with coastal regions of China.

Hydrocarbon degrading microbial communities in bench scale aerobic biobarriers for gasoline contaminated groundwater treatment

BTEX compounds (benzene, toluene, ethylbenzene and xylenes) and methyl tert-butyl ether (MTBE) are some of the main constituents of gasoline and can be accidentally released in the environment. In this work the effect of bioaugmentation on the microbial communities in a bench scale aerobic biobarrier for gasoline contaminated water treatment was studied by 16S rRNA gene sequencing. Catabolic genes (tmoA and xylM) were quantified by qPCR, in order to estimate the biodegradation potential, and the abundance of total bacteria was estimated by the quantification of the number of copies of the 16S rRNA gene. Hydrocarbon concentration was monitored over time and no difference in the removal efficiency for the tested conditions was observed, either with or without the microbial inoculum. In the column without the inoculum the most abundant genera were Acidovorax, Bdellovibrio, Hydrogenophaga, Pseudoxanthomonas and Serpens at the beginning of the column, while at the end of the column Thauera became dominant. In the inoculated test the microbial inoculum, composed by Rhodococcus sp. CE461, Rhodococcus sp. CT451 and Methylibium petroleiphilum LMG 22953, was outcompeted. Quantitative PCR results showed an increasing in xylM copy number, indicating that hydrocarbon degrading bacteria were selected during the treatment, although only a low increase of the total biomass was observed. However, the bioaugmentation did not lead to an increase in the degradative potential of the microbial communities.

Can meta-omics help to establish causality between contaminant biotransformations and genes or gene products?

There is increasing interest in using meta-omics association studies to investigate contaminant biotransformations. The general strategy is to characterize the complete set of genes, transcripts, or enzymes from in situ environmental communities and use the abundances of particular genes, transcripts, or enzymes to establish associations with the communities' potential to biotransform one or more contaminants. The associations can then be used to generate hypotheses about the underlying biological causes of particular biotransformations. While meta-omics association studies are undoubtedly powerful, they have a tendency to generate large numbers of non-causal associations, making it potentially difficult to identify the genes, transcripts, or enzymes that cause or promote a particular biotransformation. In this perspective, we describe general scenarios that could lead to pervasive non-causal associations or conceal causal associations. We next explore our own published data for evidence of pervasive non-causal associations. Finally, we evaluate whether causal associations could be identified despite the discussed limitations. Analysis of our own published data suggests that, despite their limitations, meta-omics association studies might still be useful for improving our understanding and predicting the contaminant biotransformation capacities of microbial communities.

Lab-scale tests and numerical simulations for in situ treatment of polluted groundwater

Methyl tert-butyl ether (MTBE) is used at significant percentages as an additive of unleaded gasoline. The physical-chemical properties of the substance (water solubility, soil organic carbon-water partition coefficient) cause high mobility and high concentrations in groundwater. Laboratory scale batch and column tests and mathematical modeling were performed to study the feasibility of a biobarrier (BB), that is an in situ permeable biological barrier with or without inoculation, for the remediation of MTBE and other gasoline-derived pollutants (benzene, toluene, ethylbenzene, o-xylene and m+p-xylenes, BTEXs) polluted groundwater and to estimate kinetic constants. The experimental results showed simultaneous biodegradation of MTBE and BTEXs, with similar removals in the uninoculated and the inoculated systems. Ranges for the first order kinetic removal were obtained for MTBE ((0.18±0.02)/(0.28±0.11d(-1))), B ((0.39±0.12)/(0.56±0.12d(-1))), T ((0.51±0.03)/(0.78±0.15d(-1))), E ((0.46±0.18)/(1.57±0.21d(-1))), o-X ((0.24±0.08)/(0.64±0.09d(-1))) and m+p-X ((0.20±0.04)/(1.21±0.04d(-1))). The results of the laboratory tests allowed to improve mathematical modeling in order to design a full-scale BB at a gasoline-contaminated site.

Gene mdpC plays a regulatory role in the methyl-tert-butyl ether degradation pathway of Methylibium petroleiphilum strain PM1

Among the few bacteria known to utilize methyl tert-butyl ether (MTBE) as a sole carbon source, Methylibium petroleiphilum PM1 is a well-characterized organism with a sequenced genome; however, knowledge of the genetic regulation of its MTBE degradation pathway is limited. We investigated the role of a putative transcriptional activator gene, mdpC, in the induction of MTBE-degradation genes mdpA (encoding MTBE monooxygenase) and mdpJ (encoding tert-butyl alcohol hydroxylase) of strain PM1 in a gene-knockout mutant mdpC(-). We also utilized quantitative reverse transcriptase PCR assays targeting genes mdpA, mdpJ and mdpC to determine the effects of the mutation on transcription of these genes. Our results indicate that gene mdpC is involved in the induction of both mdpA and mdpJ in response to MTBE and tert-butyl alcohol (TBA) exposure in PM1. An additional independent mechanism may be involved in the induction of mdpJ in the presence of TBA.

Ethyl tert-butyl ether (ETBE)-degrading microbial communities in enrichments from polluted environments

The ethyl tert-butyl ether (ETBE) degradation capacity and phylogenetic composition of five aerobic enrichment cultures with ETBE as the sole carbon and energy source were studied. In all cases, ETBE was entirely degraded to biomass and CO2. Clone libraries of the 16S rRNA gene were prepared from each enrichment. The analyses of the DNA sequences obtained showed different taxonomic compositions with a majority of Proteobacteria in three cases. The two other enrichments have different microbiota with an abundance of Acidobacteria in one case, whereas the microbiota in the second was more diverse (majority of Actinobacteria, Chlorobi and Gemmatimonadetes). Actinobacteria were detected in all five enrichments. Several bacterial strains were isolated from the enrichments and five were capable of degrading ETBE and/or tert-butyl alcohol (TBA), a degradation intermediate. The five included three Rhodococcus sp. (IFP 2040, IFP 2041, IFP 2043), one Betaproteobacteria (IFP 2047) belonging to the Rubrivivax/Leptothrix/Ideonella branch, and one Pseudonocardia sp. (IFP 2050). Quantification of these five strains and two other strains, Rhodococcus sp. IFP 2042 and Bradyrhizobium sp. IFP2049, which had been previously isolated from one of the enrichments was carried out on the different enrichments based on quantitative PCR with specific 16S rRNA gene primers and the results were consistent with the hypothesized role of Actinobacteria and Betaproteobacteria in the degradation of ETBE and the possible role of Bradyrhizobium strains in the degradation of TBA.

Overview of technologies for removal of methyl tert-butyl ether (MTBE) from water

Wide use of methyl tert-butyl ether (MTBE) as fuel oxygenates leads to worldwide environment contamination with this compound basically due to fuel leaks from storage or pipelines. Presence of MTBE in drinking water is of high environmental and social concern. Existing methods for MTBE removal from water have a number of limitations which can be possibly overcome in the future with use of emerging technologies. This work aims to provide an updated overview of recent developments in technologies for MTBE removal from water.

Using DNA-Stable Isotope Probing to Identify MTBE- and TBA-Degrading Microorganisms in Contaminated Groundwater

Although the anaerobic biodegradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) has been documented in the laboratory and the field, knowledge of the microorganisms and mechanisms involved is still lacking. In this study, DNA-stable isotope probing (SIP) was used to identify microorganisms involved in anaerobic fuel oxygenate biodegradation in a sulfate-reducing MTBE and TBA plume. Microorganisms were collected in the field using Bio-Sep® beads amended with ¹³C₅-MTBE, ¹³C₁-MTBE (only methoxy carbon labeled), or¹³C₄-TBA. ¹³C-DNA and ¹²C-DNA extracted from the Bio-Sep beads were cloned and 16S rRNA gene sequences were used to identify the indigenous microorganisms involved in degrading the methoxy group of MTBE and the tert-butyl group of MTBE and TBA. Results indicated that microorganisms were actively degrading ¹³C-labeled MTBE and TBA in situ and the ¹³C was incorporated into their DNA. Several sequences related to known MTBE- and TBA-degraders in the Burkholderiales and the Sphingomonadales orders were detected in all three¹³C clone libraries and were likely to be primary degraders at the site. Sequences related to sulfate-reducing bacteria and iron-reducers, such as Geobacter and Geothrix, were only detected in the clone libraries where MTBE and TBA were fully labeled with ¹³C, suggesting that they were involved in processing carbon from the tert-butyl group. Sequences similar to the Pseudomonas genus predominated in the clone library where only the methoxy carbon of MTBE was labeled with ¹³C. It is likely that members of this genus were secondary degraders cross-feeding on ¹³C-labeled metabolites such as acetate.

Biodegradation of gasoline ether oxygenates

Ether oxygenates such as methyl tertiary butyl ether (MTBE) are added to gasoline to improve fuel combustion and decrease exhaust emissions. Ether oxygenates and their tertiary alcohol metabolites are now an important group of groundwater pollutants. This review highlights recent advances in our understanding of the microorganisms, enzymes and pathways involved in both the aerobic and anaerobic biodegradation of these compounds. This review also aims to illustrate how these microbiological and biochemical studies have guided, and have helped refine, molecular and stable isotope-based analytical approaches that are increasingly being used to detect and quantify biodegradation of these compounds in contaminated environments.

Synthesis of short-chain diols and unsaturated alcohols from secondary alcohol substrates by the Rieske nonheme mononuclear iron oxygenase MdpJ

The Rieske nonheme mononuclear iron oxygenase MdpJ of the fuel oxygenate-degrading bacterial strain Aquincola tertiaricarbonis L108 has been described to attack short-chain tertiary alcohols via hydroxylation and desaturation reactions. Here, we demonstrate that also short-chain secondary alcohols can be transformed by MdpJ. Wild-type cells of strain L108 converted 2-propanol and 2-butanol to 1,2-propanediol and 3-buten-2-ol, respectively, whereas an mdpJ knockout mutant did not show such activity. In addition, wild-type cells converted 3-methyl-2-butanol and 3-pentanol to the corresponding desaturation products 3-methyl-3-buten-2-ol and 1-penten-3-ol, respectively. The enzymatic hydroxylation of 2-propanol resulted in an enantiomeric excess of about 70% for the (R)-enantiomer, indicating that this reaction was favored. Likewise, desaturation of (R)-2-butanol to 3-buten-2-ol was about 2.3-fold faster than conversion of the (S)-enantiomer. The biotechnological potential of MdpJ for the synthesis of enantiopure short-chain alcohols and diols as building block chemicals is discussed.

Biodegradation of 4-aminobenzenesulfonate by Ralstonia sp. PBA and Hydrogenophaga sp. PBC isolated from textile wastewater treatment plant

A co-culture consisting of Hydrogenophaga sp. PBC and Ralstonia sp. PBA, isolated from textile wastewater treatment plant could tolerate up to 100 mM 4-aminobenzenesulfonate (4-ABS) and utilize it as sole carbon, nitrogen and sulfur source under aerobic condition. The biodegradation of 4-ABS resulted in the release of nitrogen and sulfur in the form of ammonium and sulfate respectively. Ninety-eight percent removal of chemical oxygen demand attributed to 20 mM of 4-ABS in cell-free supernatant could be achieved after 118 h. Effective biodegradation of 4-ABS occurred at pH ranging from 6 to 8. During batch culture with 4-ABS as sole carbon and nitrogen source, the ratio of strain PBA to PBC was dynamic and a critical concentration of strain PBA has to be reached in order to enable effective biodegradation of 4-ABS. Haldane inhibition model was used to fit the degradation rate at different initial concentrations and the parameters μ(max), K(s) and K(i) were determined to be 0.13 h⁻¹, 1.3 mM and 42 mM respectively. HPLC analyses revealed traced accumulation of 4-sulfocatechol and at least four unidentified metabolites during biodegradation. This is the first study to report on the characterization of 4-ABS-degrading bacterial consortium that was isolated from textile wastewater treatment plant.

Aerobic biodegradation of iso-butanol and ethanol and their relative effects on BTEX biodegradation in aquifer materials

The aerobic biodegradability of iso-butanol, a new biofuel, and its impact on benzene, toluene, ethylbenzene and xylenes (BTEX) degradation was investigated in aerobic microcosms consisting of groundwater and sediment from a California site with a history of gasoline contamination. To the best of our knowledge this is the first study directly examining the effects of iso-butanol on BTEX degradation. Microcosms that received either low (68 μM) or high (3400 μM) concentrations of iso-butanol showed complete biodegradation of iso-butanol within 7 and 23 d, respectively, of incubation at 15°C under aerobic conditions. A maximum utilization rate coefficient of 2.3±0.1×10⁻⁷ μmol cell⁻¹ h⁻¹ and a half saturation constant of 610±54 μM were regressed from the iso-butanol data. Iso-butanol biodegradation resulted in transient formation of the degradation intermediate products iso-butylaldehyde and iso-butyric acid, and both compounds were subsequently degraded within the timeframe of the experiments. Ethanol was biodegraded more slowly than iso-butanol. Ethanol also exhibited greater adverse impacts on BTEX biodegradation than iso-butanol. Results of the study suggest that iso-butanol added to fuels will be readily biodegraded in the environment under aerobic conditions without the accumulation of major intermediate products (iso-butylaldehyde and iso-butyric acid), and that it will pose less impacts on BTEX biodegradation than ethanol.

Anaerobic biodegradation of iso-butanol and ethanol and their relative effects on BTEX biodegradation in aquifer materials

Biologically produced iso-butanol is currently being considered as an additive in gasoline blends. To evaluate its potential environmental fate in groundwater aquifers, a laboratory microcosm study was performed to evaluate iso-butanol biodegradation under various anaerobic conditions (nitrate-reducing, sulfate-reducing and methanogenic). The impacts of iso-butanol on benzene, toluene, ethylbenzene, and total xylenes (BTEX) biodegradation were also assessed, and microcosms prepared using ethanol instead of iso-butanol were evaluated to provide a basis for comparison. Iso-butanol was biodegraded under all conditions studied, with an observed apparent first-order rate constant ranging from approximately 0.2 d⁻¹ (nitrate-reducing) to approximately 0.02 d⁻¹ (sulfate-reducing). Iso-butanol typically was degraded in a time frame that was shorter than or similar to BTEX compounds. Iso-butyric acid and trace levels of iso-butylaldehyde were identified as transient intermediates, and both of these compounds were subsequently degraded within the time frame of the experiments. Iso-butanol and ethanol were biodegraded in similar time frames under methanogenic conditions. Under sulfate-reducing conditions, iso-butanol biodegradation initially proceeded more slowly than ethanol, and then increased to a rate greater than that observed for ethanol; this observation likely was due to the growth of iso-butanol degrading bacteria. Iso-butanol generally exhibited less adverse impacts on BTEX biodegradations than ethanol under the anaerobic conditions studied. In some cases, addition of iso-butanol enhanced the rate of TEX biodegradation.

Biodegradation of 2-chloroaniline, 3-chloroaniline, and 4-chloroaniline by a novel strain Delftia tsuruhatensis H1

A new strain Delftia tsuruhatensis H1 able to degrade several chloroanilines (CAs) as individual compounds or a mixture was isolated from a CA-degrading mixed bacterial culture. The isolated strain could completely degrade 3-CA and 4-CA as growth substrates, while concurrently metabolize 2-CA by growing on other CA compounds. The strain could also efficiently degrade all the three CA components when presented as a mixture. Following CA consumption, stoichiometric amounts of chloride were released and small amount of soluble metabolites accumulated in the medium, indicating that the loss of CA was mainly via mineralization and incorporation into cell material. The additions of yeast extract, citrate or succinate appeared to accelerate CA degradation. In contrast, aniline strongly inhibited the CA degradation. The strain H1 could also decompose other substituted aniline compounds such as 3,4-dichloroaniline, 4-methylaniline, 2,3-dichloroaniline and 2,4-dichloroaniline. The elimination of these CA compounds seemed to occur via an ortho-cleavage pathway.

Elucidating MTBE degradation in a mixed consortium using a multidisciplinary approach

The structure and function of a microbial community capable of biodegrading methyl-tert-butyl ether (MTBE) was characterized using compound-specific stable isotope analysis (CSIA), clone libraries and stable isotope probing of proteins (Protein-SIP). The enrichment culture (US3-M), which originated from a gasoline-impacted site in the United States, has been enriched on MTBE as the sole carbon source. The slope of isotopic enrichment factors (epsilon(C) of -2.29+/-0.03 per thousand; epsilon(H) of -58+/-6 per thousand) for carbon and hydrogen discrimination (Deltadelta(2)H/Deltadelta(13)C) was on average equal to Lambda=24+/-2, a value closely related to the reaction mechanism of MTBE degradation in Methylibium petroleiphilum PM1. 16S rRNA gene libraries revealed sequences belonging to M. petroleiphilum PM1, Hydrogenophaga sp., Thiothrix unzii, Rhodobacter sp., Nocardiodes sp. and different Sphingomonadaceae bacteria. Protein-SIP analysis of the culture grown on (13)C-MTBE as the only carbon source revealed that proteins related to members of the Comamonadaceae family, such as Delftia acidovorans, Acidovorax sp. or Comamonas sp., were not (13)C-enriched, whereas proteins related to M. petroleiphilum PM1 showed an average incorporation of 94.5 atom%(13)C. These results indicate a key role for this species in the degradation of MTBE within the US3-M consortia. The combination of CSIA, molecular biology and Protein-SIP facilitated the analysis of an MTBE-degrading mixed culture from a functional and phylogenetic point of view.

Degradation of hexane and other recalcitrant hydrocarbons by a novel isolate, Rhodococcus sp. EH831

BACKGROUND, AIM, AND SCOPE

Hexane, a representative VOC, is used as a solvent for extraction and as an ingredient in gasoline. The degradation of hexane by bacteria is relatively slow due to its low solubility. Moreover, the biodegradation pathway of hexane under aerobic conditions remains to be investigated; therefore, a study relating to aerobic biodegradation mechanisms is required. Consequently, in this study, an effective hexane degrader was isolated and the biodegradation pathway examined for the first time. In addition, the degradation characteristics of a variety of recalcitrant hydrocarbons were qualitatively and quantitatively investigated using the isolate.

MATERIALS AND METHODS

A hexane-degrading bacterium was isolated from an enrichment culture using petroleum-contaminated soil as an inoculum with hexane as the sole carbon and energy source. The bacterium was also identified using the partial 16S rRNA gene sequence. To test the hexane-degrading capacity of the isolate, 10 ml of an EH831 cell suspension was inoculated into a 600-ml serum bottle with hexane (7.6-75.8 micromol) injected as the sole carbon source. The rates of hexane degradation were determined by analyzing the concentrations of hexane using headspace gas chromatography. In addition, the hexane biodegradation pathway under aerobic conditions was investigated by identifying the metabolites using gas chromatography-mass spectrometry with solid-phase microextraction. 14C-hexane was used to check if EH831 could mineralize hexane in the same experimental system. The degradabilities of other hydrocarbons were examined using EH831 with methanol, ethanol, acetone, cyclohexane, methyl tert-butyl ether (MTBE), dichloromethane (DCM), trichloroethylene, tetrachloroethylene, benzene, toluene, ethylbenzene, xylene (BTEX), pyrene, diesel, lubricant oil, and crude oil as sole carbon sources.

RESULTS

A bacterium, EH831, was isolated from the enriched hexane-degrading consortium, which was able to degrade hexane and various hydrocarbons, including alcohols, chlorinated hydrocarbons, cyclic alkanes, ethers, ketones, monoaromatic and polyaromatic hydrocarbons, and petroleum hydrocarbons. The maximum hexane degradation rate (V max) of EH831 was 290 micromol g dry cell weight(-1) h(-1), and the saturation constant (K s) was 15 mM. Using 14C-hexane, EH831 was confirmed to mineralize approximately 49% of the hexane into CO2 and, converted approximately, 46% into biomass; the rest (1.7%) remained as extracellular metabolites in the liquid phase. The degradation pathway was assessed through the qualitative analysis of the hexane intermediates due to EH831, which were 2-hexanol, 2-hexanone, 5-hexen-2-one and 2,5-hexanedione, in that order, followed by 4-methyl-2-pentanone, 3-methyl-1-butanol, 3-methyl-1-butanone and butanal, and finally, CO2. EH831 could degrade methanol, ethanol, acetone, cyclohexane, MTBE, DCM, BTEX, pyrene, diesel, and lubricant oil.

DISCUSSION

EH831 was able to degrade many recalcitrant hydrocarbons at higher degradation rates compared with previous well-known degraders. Furthermore, this study primarily suggested the aerobic biodegradation pathway, which may provide valuable information for researchers and engineers working in the field of environmental engineering.

CONCLUSIONS

Rhodococcus sp. EH831 is a promising bioresource for removing hexane and other recalcitrant hydrocarbons from a variety of environments. Moreover, the aerobic biodegradation pathway is reported for the first time in this study, which offers valuable information for understanding the microbial degradation of hexane.

RECOMMENDATIONS AND PERSPECTIVES

The utility of the strain isolated in this study needs to be proved by its application to biological process systems, such as biofilters and bioreactors, etc., for the degradation of hexane and many other recalcitrant hydrocarbons. Detailed investigations will also be needed to clarify the enzymatic characteristics relating the degradation of both recalcitrant hydrocarbons and hexane.

Enhanced bioremediation of methyl tert-butyl ether (MTBE) by microbial consortia obtained from contaminated aquifer material

A microcosm study was carried out to evaluate the potential for biodegradation of methyl tert-butyl ether (MTBE) impacting groundwater at a former oil refinery site located in Naples (SW Italy). A screening of aerobic, anaerobic and co-metabolic aerobic conditions was carried out by triplicate batch reactors, using contaminated soil and groundwater from the study site. All microcosms were amended with ammonium and phosphate salts and, if aerobic, they were supplied with excess oxygen throughout the static incubation period of 6 months. Propane, pentane and n-hexane were selected as the primary substrates for co-metabolic treatments. After the initial lag phase (40-60d), quantitative MTBE decay was repeatedly observed in the aerobic set amended only with nitrogen and phosphorus and further fed with MTBE, thus suggesting that the indigenous soil bacteria have the ability to degrade MTBE. All other treatments, i.e., anaerobic and co-metabolic aerobic, resulted unsuccessful after incubation extending up to 190d. Bacterial consortia in the active microcosms were later enriched and further studied through second and third generation batch reactors with no soil, operated under continuous mixing for 5-7 months. MTBE degradation rate progressively increased with reactor operating time, following a zero order kinetics in the concentration range 1-10mgL(-1) and leading to a residual concentration of less than 10microgL(-1). The calculated maximum biodegradation rate was 20mg(MTBE)g(VSS)(-1)h(-1). An accumulation of nitrite ions also occurred after long operating times, thus inhibiting MTBE degradation. This effect was minimized by replacing ammonium with nitrate. Identified degradation intermediates were tert-butyl alcohol and tert-butyl formate. Fluorescent in situ hybridization was applied for a preliminary microbiological screening of the consortia, suggesting that the detected cocci (about 0.5 and 1.5microm diameter, respectively) and long bacilli with a narrow diameter might be as yet undescribed species.

Biodegradation potential of MTBE in a fractured chalk aquifer under aerobic conditions in long-term uncontaminated and contaminated aquifer microcosms

The potential for aerobic biodegradation of MTBE in a fractured chalk aquifer is assessed in microcosm experiments over 450 days, under in situ conditions for a groundwater temperature of 10 degrees C, MTBE concentration between 0.1 and 1.0 mg/L and dissolved O2 concentration between 2 and 10 mg/L. Following a lag period of up to 120 days, MTBE was biodegraded in uncontaminated aquifer microcosms at concentrations up to 1.2 mg/L, demonstrating that the aquifer has an intrinsic potential to biodegrade MTBE aerobically. The MTBE biodegradation rate increased three-fold from a mean of 6.6+/-1.6 microg/L/day in uncontaminated aquifer microcosms for subsequent additions of MTBE, suggesting an increasing biodegradation capability, due to microbial cell growth and increased biomass after repeated exposure to MTBE. In contaminated aquifer microcosms which also contained TAME, MTBE biodegradation occurred after a shorter lag of 15 or 33 days and MTBE biodegradation rates were higher (max. 27.5 microg/L/day), probably resulting from an acclimated microbial population due to previous exposure to MTBE in situ. The initial MTBE concentration did not affect the lag period but the biodegradation rate increased with the initial MTBE concentration, indicating that there was no inhibition of MTBE biodegradation related to MTBE concentration up to 1.2 mg/L. No minimum substrate concentration for MTBE biodegradation was observed, indicating that in the presence of dissolved O2 (and absence of inhibitory factors) MTBE biodegradation would occur in the aquifer at MTBE concentrations (ca. 0.1 mg/L) found at the front of the ether oxygenate plume. MTBE biodegradation occurred with concomitant O2 consumption but no other electron acceptor utilisation, indicating biodegradation by aerobic processes only. However, O2 consumption was less than the stoichiometric requirement for complete MTBE mineralization, suggesting that only partial biodegradation of MTBE to intermediate organic metabolites occurred. The availability of dissolved O2 did not affect MTBE biodegradation significantly, with similar MTBE biodegradation behaviour and rates down to ca. 0.7 mg/L dissolved O2 concentration. The results indicate that aerobic MTBE biodegradation could be significant in the plume fringe, during mixing of the contaminant plume and uncontaminated groundwater and that, relative to the plume migration, aerobic biodegradation is important for MTBE attenuation. Moreover, should the groundwater dissolved O2 concentration fall to zero such that MTBE biodegradation was inhibited, an engineered approach to enhance in situ bioremediation could supply O2 at relatively low levels (e.g. 2-3 mg/L) to effectively stimulate MTBE biodegradation, which has significant practical advantages. The study shows that aerobic MTBE biodegradation can occur at environmentally significant rates in this aquifer, and that long-term microcosm experiments (100s days) may be necessary to correctly interpret contaminant biodegradation potential in aquifers to support site management decisions.

Biodegradation of methyl tert-butyl ether using bacterial strains

Prospective methyl tert-butyl ether (MTBE) degrading bacterial strains and/or consortia were identified. The potential for aerobic degradation of MTBE was examined using bacterial isolates from contaminated soils and groundwater. Using the 16S rDNA protocol, two isolates capable of degrading MTBE (Rhodococcus pyridinivorans 4A and Achromobacter xylosoxidans 6A) were identified. The most efficient consortium of microorganisms was acquired from contaminated groundwater. The growth of both strains and the consortium on MTBE was supported by various organic substrates, and monitored using Bioscreen. The biochemical oxygen demand of the cultures was measured using OxiTop, and their MTBE concentrations were estimated by gas chromatography. After 3 weeks of aerobic cultivation using n-alkanes as cosubstrate, the concentration of MTBE in R. pyridinivorans 4A was reduced to 62.4 % of its initial amount (50 ppm).

Isolation and characterization of a new benzene, toluene, and ethylbenzene degrading bacterium, Acinetobacter sp. B113

A bacterium designated strain B113, able to degrade benzene, toluene, and ethylbenzene compounds (BTE), was isolated from gasoline-contaminated sediment at a gas station in Geoje, Korea. Phylogenetic analysis based on 16S rRNA gene sequences showed that the isolate belonged to the genus Acinetobacter. The biodegradation rates of benzene, toluene, and ethylbenzene were relatively low in MSB broth, but the addition of yeast extract had a substantial impact on the biodegradation of BTE compounds, which suggested that yeast extract might provide a factor that was necessary for its growth or BTE biodegradation activity. However, interestingly, the biodegradation of BTE compounds occurred very quickly in slurry systems amended with sterile soil. Moreover, if soil was combusted first to remove organic matters, the enhancement effect on BTE biodegradation was lost, indicating that some insoluble organic compounds were probably beneficial for BTE degradation in contaminated sediment. This study suggests that strain B113 may play an important role for biodegradation of BTE in the contaminated site.

Influence of soil components on the biodegradation of benzene, toluene, ethylbenzene, and o-, m-, and p-xylenes by the newly isolated bacterium Pseudoxanthomonas spadix BD-a59

A bacterium designated strain BD-a59, able to degrade all six benzene, toluene, ethylbenzene, and o-, m-, and p-xylene (BTEX) compounds, was isolated by plating gasoline-contaminated sediment from a gasoline station in Geoje, Republic of Korea, without enrichment, on minimal salts basal (MSB) agar containing 0.01% yeast extract, with BTEX as the sole carbon and energy source. Taxonomic analyses showed that the isolate belonged to Pseudoxanthomonas spadix, and until now, the genus Pseudoxanthomonas has not included any known BTEX degraders. The BTEX biodegradation rate was very low in MSB broth, but adding a small amount of yeast extract greatly enhanced the biodegradation. Interestingly, degradation occurred very quickly in slurry systems amended with sterile soil solids but not with aqueous soil extract. Moreover, if soil was combusted first to remove organic matter, the enhancement effect on BTEX biodegradation was lost, indicating that some components of insoluble organic compounds are nutritionally beneficial for BTEX degradation. Reverse transcriptase PCR-based analysis of field-fixed mRNA revealed expression of the tmoA gene, whose sequence was closely related to that carried by strain BD-a59. This study suggests that strain BD-a59 has the potential to assist in BTEX biodegradation at contaminated sites.

Mineralization of methyl tert-butyl ether and other gasoline oxygenates by Pseudomonads using short n-alkanes as growth source

Biodegradation of methyl tert-butyl ether (MTBE) by cometabolism has shown to produce recalcitrant metabolic intermediates that often accumulate. In this work, a consortium containing Pseudomonads was studied for its ability to fully degrade oxygenates by cometabolism. This consortium mineralized MTBE and TBA with C3-C7 n-alkanes. The highest degradation rates for MTBE (75 +/- 5 mg g(protein) (-1) h(-1)) and TBA (86.9 +/- 7.3 mg g(protein) (-1) h(-1)) were obtained with n-pentane and n-propane, respectively. When incubated with radiolabeled MTBE and n-pentane, it converted more than 96% of the added MTBE to (14)C-CO(2). Furthermore, the consortium degraded tert-amyl methyl ether, tert-butyl alcohol (TBA), tert-amyl alcohol, ethyl tert-butyl ether (ETBE) when n-pentane was used as growth source. Three Pseudomonads were isolated but only two showed independent MTBE degradation activity. The maximum degradation rates were 101 and 182 mg g(protein) (-1) h(-1) for Pseudomonas aeruginosa and Pseudomonas citronellolis, respectively. The highest specific affinity (a degrees (MTBE)) value of 4.39 l g(protein) (-1) h(-1) was obtained for Pseudomonas aeruginosa and complete mineralization was attained with a MTBE: n-pentane ratio (w/w) of 0.7. This is the first time that Pseudomonads have been reported to fully mineralize MTBE by cometabolic degradation.

Involvement of a novel enzyme, MdpA, in methyl tert-butyl ether degradation in Methylibium petroleiphilum PM1

Methylibium petroleiphilum PM1 is a well-characterized environmental strain capable of complete metabolism of the fuel oxygenate methyl tert-butyl ether (MTBE). Using a molecular genetic system which we established to study MTBE metabolism by PM1, we demonstrated that the enzyme MdpA is involved in MTBE removal, based on insertional inactivation and complementation studies. MdpA is constitutively expressed at low levels but is strongly induced by MTBE. MdpA is also involved in the regulation of tert-butyl alcohol (TBA) removal under certain conditions but is not directly responsible for TBA degradation. Phylogenetic comparison of MdpA to related enzymes indicates close homology to the short-chain hydrolyzing alkane hydroxylases (AH1), a group that appears to be a distinct subfamily of the AHs. The unique, substrate-size-determining residue Thr(59) distinguishes MdpA from the AH1 subfamily as well as from AlkB enzymes linked to MTBE degradation in Mycobacterium austroafricanum.

Degradation of fuel oxygenates and their main intermediates by Aquincola tertiaricarbonis L108

Growth of Aquincola tertiaricarbonis L108 on the fuel oxygenates methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME), as well as on their main metabolites tert-butyl alcohol (TBA), tert-amyl alcohol (TAA) and 2-hydroxyisobutyrate (2-HIBA) was systematically investigated to characterize the range and rates of oxygenate degradation by this strain. The effective maximum growth rates for MTBE, ETBE and TAME at pH 7 and 30 degrees C were 0.045 h(-1), 0.06 h(-1) and 0.055 h(-1), respectively, whereas TAA, TBA and 2-HIBA permitted growth at rates up to 0.08 h(-1), 0.1 h(-1) and 0.17 h(-1), respectively. The experimental growth yields with all these substrates were high. Yields of 0.55 g dry mass (dm) (g MTBE)(-1), 0.53 g dm (g ETBE)(-1), 0.81 g dm (g TAME)(-1), 0.48 g dm (g TBA)(-1), 0.76 g dm (g TAA)(-1) and 0.54 g dm (g 2-HIBA)(-1) were obtained. Maximum specific degradation rates were 0.92 mmol MTBE h(-1) (g dm)(-1), 1.11 mmol ETBE h(-1) g(-1), 0.66 mmol TAME h(-1) g(-1), 1.19 mmol TAA h(-1) g(-1), 2.82 mmol TBA h(-1) g(-1), and 3.27 mmol 2-HIBA h(-1) g(-1). The relatively high rates with TBA, TAA and 2-HIBA indicate that the transformations of these metabolites did not limit the metabolism of MTBE and the related ether compounds. Despite the fact that these metabolites still carry a tertiary carbon atom that is commonly suspected to confer recalcitrance to the ether oxygenates, the transformation rates were in the same range as those with succinate and fructose. With MTBE, strain L108 grew at pHs between 5.5 and 8.0 at near-maximal rate, whereas no growth was found below pH 5.0 and above pH 9.0. The optimum growth temperature was 30 degrees C, but at 5 degrees C still about 15 % of the maximum rate remained, whereas no growth occurred at 42 degrees C. This indicates that MTBE metabolites are valuable substrates and that A. tertiaricarbonis L108 is a good candidate for bioremediation purposes. The possible origin of its exceptional metabolic capability is discussed in terms of the evolution of enzymic activities involved in the conversion of compounds carrying tertiary butyl groups.

Degradation of methyl tert-butyl ether by gel immobilized Methylibium petroleiphilum PM1

Cells of Methylibium petroleiphilum PM1 were immobilized in gel beads to degrade methyl tert-butyl ether (MTBE). Calcium alginate, agar, polyacrylamide and polyvinvyl alcohol were screened as suitable immobilization matrices, with calcium alginate demonstrating the fastest MTBE-degradation rate. The rate was accelerated by 1.8-fold when the beads had been treated in physiological saline for 24h at 28 degrees C. MTBE degradation in mineral salts medium (MSM) was accompanied by the increase of biomass. The half-life of MTBE-degradation activity for the encapsulated cells stored at 28 degrees C was about 120 h, which was obviously longer than that of free cells (approximately 36 h). Efficient reusability of the beads up to 30 batches was achieved in poor nutrition solution as compared to only 6 batches in MSM. The immobilized cells could be operated in a packed-bed reactor for degradation of 10 mg L(-1) MTBE in groundwater with more than 99% removal efficiency at hydraulic retention time of 20 min. These results suggested that immobilized cells of PM1 in bioreactor might be applicable to a groundwater treatment system for the removal of MTBE.

Modeling the competitive effect of ammonium oxidizers and heterotrophs on the degradation of MTBE in a packed bed reactor

A mathematical model was used to study effects on the degradation of methyl tert-butyl ether (MTBE) in a packed bed reactor due to the presence of contaminants such as ammonium, and the mix of benzene, toluene, ethylbenzene and xylenes (BTEX). It was shown that competition between the slower growing MTBE degraders and the co-contaminant oxidizers prevented MTBE's degradation when oxygen was limited. In this event, the co-contaminant oxidizers out-competed the MTBE degraders in the reactor's biofilm. However, if the oxygen supply was sufficient, MTBE would be fully degraded after the zone where the co-contaminants were oxidized. The results of the model further indicate that contradicting findings in the literature about the effects of BTEX on the degradation of MTBE are mainly due to differences in the study methodologies. Effects such as short-term toxicity of BTEX and the lack of steady-state conditions may also add to contradictions among reports.

Biodegradation of methyl tert-butyl ether by Methylibium petroleiphilum PM1 in poor nutrition solution

In this study, degradation of methyl tert-butyl ether (MTBE) by resting cells of Methylibium petroleiphilum PM1 was performed in poor nutrition solution, major component of which was MTBE. It was found that the biomass was hard to increase in poor nutrition, and the MTBE degradation activity was enhanced by 4.65-fold when 1 mM of Ba(2+) was added into deionized water. It was also found that the MTBE degradation could be significantly improved by the dissolved oxygen level. All of 50 mg L(-1) MTBE could be degraded under aerobic condition, while only 5% was degraded under anaerobic condition by resting cells in poor nutrition solution after 12 h. In the above solution, the degradation of tert-butyl alcohol (TBA) was found to be faster than that of MTBE, which suggested that TBA degradation might not be the limiting step in MTBE metabolism. While in the poor nutrition solution with the mixture of MTBE and TBA, the addition of TBA did not affect MTBE degradation while MTBE inhibited TBA degradation weakly, which suggested that different and independent enzymes were responsible for degrading such compounds. The success of MTBE degradation by PM1 cells in real contaminated groundwater demonstrated its feasibility to biodegrade MTBE under poor environment, and it also indicated the great potential of MTBE bioremediation by entrapped cells in future application.

Influences of metals on kinetics of methyl tert-butyl ether biodegradation by Ochrobactrum cytisi

The influence of zinc, manganese, and nickel on the degradation of MTBE (methyl tert-butyl ether), by an aerobic MTBE-degrading strain, Ochrobactrum cytisi, were investigated. The result showed that unlike previous findings, O. cytisi was able to degrade MTBE through direct metabolism when MTBE was present as the only carbon source. The degradation rate of MTBE was rapid, completed within 80 h. MTBE biodegradation by this strain was stimulated at low concentrations of Zn(2+) (1-5 mg l(-1)) and Mn(2+) (1-5 mg l(-1)) but inhibited at high concentrations of Zn(2+) (20 mg l(-1)) and Mn(2+) (20 mg l(-1)), and at low concentration of Ni(2+) (1-4 mg l(-1)). Kinetic parameters for MTBE degradation in the presence or absence of metals were obtained through nonlinear regression and a least-square minimization procedure. In all cases, a good agreement was achieved between kinetic simulations and experimental results.

Aerobic MTBE biodegradation in the presence of BTEX by two consortia under batch and semi-batch conditions

This study explores the effect of microbial consortium composition and reactor configuration on methyl tert-butyl ether (MTBE) biodegradation in the presence of benzene, toluene, ethylbenzene and p-xylenes(BTEX). MTBE biodegradation was monitored in the presence and absence of BTEX in duplicate batch reactors inoculated with distinct enrichment cultures: MTBE only (MO-originally enriched on MTBE) and/or MTBE BTEX (MB-originally enriched on MTBE and BTEX). The MO culture was also applied in a semi-batch reactor which received both MTBE and BTEX periodically in fresh medium after allowing cells to settle. The composition of the microbial consortia was explored using a combination of 16S rRNA gene cloning and quantitative polymerase chain reaction targeting the known MTBE-degrading strain PM1T. MTBE biodegradation was completely inhibited by BTEX in the batch reactors inoculated with the MB culture, and severely retarded in those inoculated with the MO culture (0.18+/-0.04 mg/L-day). In the semi-batch reactor, however, the MTBE biodegradation rate in the presence of BTEX was almost three times as high as in the batch reactors (0.48+/-0.2 mg/L-day), but still slower than MTBE biodegradation in the absence of BTEX in the MO-inoculated batch reactors (1.47+/-0.47 mg/L-day). A long lag phase in MTBE biodegradation was observed in batch reactors inoculated with the MB culture (20 days), but the ultimate rate was comparable to the MO culture (0.95+/-0.44 mg/L-day). Analysis of the cultures revealed that strain PM1T concentrations were lower in cultures that successfully biodegraded MTBE in the presence of BTEX. Also, other MTBE degraders, such as Leptothrix sp. and Hydrogenophaga sp. were found in these cultures. These results demonstrate that MTBE bioremediation in the presence of BTEX is feasible, and that culture composition and reactor configuration are key factors.