Introduction of a de novo bioremediation ability, aryl reductive dechlorination, into anaerobic granular sludge by inoculation of sludge with Desulfomonile tiedjei

Relating Anaerobic Digestion Microbial Community and Process Function

Anaerobic digestion (AD) involves a consortium of microorganisms that convert substrates into biogas containing methane for renewable energy. The technology has suffered from the perception of being periodically unstable due to limited understanding of the relationship between microbial community structure and function. The emphasis of this review is to describe microbial communities in digesters and quantitative and qualitative relationships between community structure and digester function. Progress has been made in the past few decades to identify key microorganisms influencing AD. Yet, more work is required to realize robust, quantitative relationships between microbial community structure and functions such as methane production rate and resilience after perturbations. Other promising areas of research for improved AD may include methods to increase/control (1) hydrolysis rate, (2) direct interspecies electron transfer to methanogens, (3) community structure-function relationships of methanogens, (4) methanogenesis via acetate oxidation, and (5) bioaugmentation to study community-activity relationships or improve engineered bioprocesses.

Methane production from oleate: assessing the bioaugmentation potential of Syntrophomonas zehnderi

The potential for improving long-chain fatty acids (LCFA) conversion to methane was evaluated by bioaugmenting a non-acclimated anaerobic granular sludge with Syntrophomonas zehnderi. Batch bioaugmentation assays were performed with and without the solid microcarrier sepiolite, using 1 mM oleate as sole carbon and energy source. When S. zehnderi was added to the anaerobic sludge methane production from oleate was faster. High methane yields, i.e. 89 ± 5% and 72 ± 1%, were observed in bioaugmented assays in the absence and presence of sepiolite, respectively. Sepiolite stimulated a faster methane production from oleate and prevented the accumulation of acetate. Acetoclastic activity was affected by oleate in the absence of sepiolite, where methane production rate was 26% lower than in assays with microcarrier.

Bioaugmentation for improved recovery of anaerobic digesters after toxicant exposure

Bioaugmentation was investigated as a method to decrease the recovery period of anaerobic digesters exposed to a transient toxic event. Two sets of laboratory-scale digesters (SRT = 10 days, OLR = 2 g COD/L-day), started with inoculum from a digester stabilizing synthetic municipal wastewater solids (MW) and synthetic industrial wastewater (WW), respectively, were transiently exposed to the model toxicant, oxygen. Bioaugmented digesters received 1.2 g VSS/L-day of an H2-utilizing culture for which the archaeal community was analyzed. Soon after oxygen exposure, the bioaugmented digesters produced 25-60% more methane than non-bioaugmented controls (p < 0.05). One set of digesters produced lingering high propionate concentrations, and bioaugmentation resulted in significantly shorter recovery periods. The second set of digesters did not display lingering propionate, and bioaugmented digesters recovered at the same time as non-bioaugmented controls. The difference in the effect of bioaugmentation on recovery may be due to differences between microbial communities of the digester inocula originally employed. In conclusion, bioaugmentation with an H(2)-utilizing culture is a potential tool to decrease the recovery period, decrease propionate concentration, and increase biogas production of some anaerobic digesters after a toxic event. Digesters already containing rapidly adaptable microbial communities may not benefit from bioaugmentation, whereas other digesters with poorly adaptable microbial communities may benefit greatly.

The mineralization of 5-amino-2,4,6-triiodoisophthalic acid by a two-stage fixed-bed reactor

Iodinated X-ray contrast media have been detected in hospital effluent, sewage treatment plant effluent, rivers and groundwater aquifers. No process has been developed to remove triiodinated aromatic molecules. In this paper, we present a biological sequential process using an anaerobic fixed-bed reactor coupled in series with an aerobic fixed-bed reactor for degrading 5-amino-2,4,6-triiodoisophthalic acid (ATIA), the core structure of a X-ray contrast media family. The results obtained showed that the coupled reactor eliminated up to 870+/-44 mg of carbon L(-1) day(-1), with a molar ethanol/ATIA ratio of 4 in the feeding medium. The anaerobic reactor (ANR) undertook the majority of the deiodination of the aromatic nucleus and had a maximum deiodination rate of 23.4+/-0.06 mM day(-1). The aerobic reactor (AER) mineralized ATIA and was also able to eliminate its metabolites. This study suggests that the mineralization of ATIA can be achieved efficiently in a coupled anaerobic-aerobic bioreactor.

The Desulfitobacterium genus

Desulfitobacterium spp. are strictly anaerobic bacteria that were first isolated from environments contaminated by halogenated organic compounds. They are very versatile microorganisms that can use a wide variety of electron acceptors, such as nitrate, sulfite, metals, humic acids, and man-made or naturally occurring halogenated organic compounds. Most of the Desulfitobacterium strains can dehalogenate halogenated organic compounds by mechanisms of reductive dehalogenation, although the substrate spectrum of halogenated organic compounds varies substantially from one strain to another, even with strains belonging to the same species. A number of reductive dehalogenases and their corresponding gene loci have been isolated from these strains. Some of these loci are flanked by transposition sequences, suggesting that they can be transmitted by horizontal transfer via a catabolic transposon. Desulfitobacterium spp. can use H2 as electron donor below the threshold concentration that would allow sulfate reduction and methanogenesis. Furthermore, there is some evidence that syntrophic relationships occur between Desulfitobacterium spp. and sulfate-reducing bacteria, from which the Desulfitobacterium cells acquire their electrons by interspecies hydrogen transfer, and it is believed that this relationship also occurs in a methanogenic consortium. Because of their versatility, desulfitobacteria can be excellent candidates for the development of anaerobic bioremediation processes. The release of the complete genome of Desulfitobacterium hafniense strain Y51 and information from the partial genome sequence of D. hafniense strain DCB-2 will certainly help in predicting how desulfitobacteria interact with their environments and other microorganisms, and the mechanisms of actions related to reductive dehalogenation.

Desulfitobacterium hafniense is present in a high proportion within the biofilms of a high-performance pentachlorophenol-degrading, methanogenic fixed-film reactor

We developed a pentachlorophenol (PCP)-degrading, methanogenic fixed-film reactor by using broken granular sludge from an upflow anaerobic sludge blanket reactor. This methanogenic consortium was acclimated with increasing concentrations of PCP. After 225 days of acclimation, the reactor was performing at a high level, with a PCP removal rate of 1,173 muM day(-1), a PCP removal efficiency of up to 99%, a degradation efficiency of approximately 60%, and 3-chlorophenol as the main chlorophenol residual intermediate. Analyses by PCR-denaturing gradient gel electrophoresis (DGGE) showed that Bacteria and Archaea in the reactor stabilized in the biofilms after 56 days of operation. Important modifications in the profiles of Bacteria between the original granular sludge and the reactor occurred, as less than one-third of the sludge DGGE bands were still present in the reactor. Fluorescence in situ hybridization experiments with probes for Archaea or Bacteria revealed that the biofilms were composed mostly of Bacteria, which accounted for 70% of the cells. With PCR species-specific primers, the presence of the halorespiring bacterium Desulfitobacterium hafniense in the biofilm was detected very early during the reactor acclimation period. D. hafniense cells were scattered in the biofilm and accounted for 19% of the community. These results suggest that the presence of PCP-dehalogenating D. hafniense in the biofilm was crucial for the performance of the reactor.

Enhancement of 2,4-dichlorophenol degradation in conventional activated sludge systems bioaugmented with mixed special culture

Efficient and reliable removal of recalcitrant compounds, which are present in wastewater intermittently, is critically important to prevent toxicity discharge and system upset, especially for those biotreatment systems with poor anti-shock loading capacity such as conventional activated sludge (CAS) systems. In this study, 2,4-dichlorophenol (2,4-DCP) was chosen as a model recalcitrant substance. 2,4-DCP degrading mixed culture was bioaugmented to a CAS system in terms of enhancing 2,4-DCP removal and maintaining system stability under shock loading conditions. The effects of bioaugmentation on the performance of CAS systems after single inoculation were investigated under long-term continuous operation, during which four times shock loading occurred. Results showed that the two bioaugmented CAS systems, which were inoculated with 5% and 15% 2,4-DCP degrading mixed culture, respectively, demonstrated and maintained stronger ability to degrade 2,4-DCP than the non-supplemented control one during the first three shock loading periods within the first month after the first inoculation. For the fourth 2,4-DCP shock loading which occurred 100 days after inoculation, the advantages demonstrated by the bioaugmented systems were greatly reduced compared to those of the previous three runs. In addition, ribosomal intergenic spacer analysis method was used to track the supplemented special culture and assay the effect of bioaugmentation on the changes of microbial community structure.

High-rate sulfate reduction at high salinity (up to 90 mS.cm?1) in mesophilic UASB reactors

Sulfate reduction in salt-rich wastewaters using unadapted granular sludge was investigated in 0.9 L UASB reactors (pH 7.0 +/- 0.2; hydraulic retention time from 8-14 h) fed with acetate, propionate, or ethanol at organic loading rates up to 10 gCOD x L(-1) x day(-1) and in excess sulfate (COD/SO(4) (2-) of 0.5). High-rate sulfate reduction rates (up to 3.7 gSO(4) (2-) x L(-1).day(-1)) were achieved at salinities exceeding 50 gNaCl.L(-1) and 1 gMgCl(2) x L(-1). Sulfate reduction proceeded at a salinity of up to 70 gNaCl x L(-1) and 1 gMgCl(2) x L(-1) (corresponding to a conductivity of about 85-90 mS x cm(-1)), although at lower rates compared to a conductivity of 60-70 mS x cm(-1). Ethanol as well as propionate were suitable substrates for sulfate reduction, with acetate and sulfide as the end products. The successful high-rate treatment was due to the proliferation of a halotolerant incomplete oxidizing SRB population present in the unadapted inoculum sludge. Bioaugmentation of this sludge with the acetate oxidizing halotolerant SRB Desulfobacter halotolerans was unsuccessful, as the strain washed out from the UASB reactor without colonizing the UASB granules.

Molecular ecology of anaerobic reactor systems

Anaerobic reactor systems are essential for the treatment of solid and liquid wastes and constitute a core facility in many waste treatment plants. Although much is known about the basic metabolism in different types of anaerobic reactors, little is known about the microbes responsible for these processes. Only a few percent of Bacteria and Archaea have so far been isolated, and almost nothing is known about the dynamics and interactions between these and other microorganisms. This lack of knowledge is most clearly exemplified by the sometimes unpredictable and unexplainable failures and malfunctions of anaerobic digesters occasionally experienced, leading to sub-optimal methane production and wastewater treatment. Using a variety of molecular techniques, we are able to determine which microorganisms are active, where they are active, and when they are active, but we still need to determine why and what they are doing. As genetic manipulations of anaerobes have been shown in only a few species permitting in-situ gene expression studies, the only way to elucidate the function of different microbes is to correlate the metabolic capabilities of isolated microbes in pure culture to the abundance of each microbe in anaerobic reactor systems by rRNA probing. This chapter focuses on various molecular techniques employed and problems encountered when elucidating the microbial ecology of anaerobic reactor systems. Methods such as quantitative dot blot/fluorescence in-situ probing using various specific nucleic acid probes are discussed and exemplified by studies of anaerobic granular sludge, biofilm and digester systems.

Molecular biology of stress genes in methanogens: potential for bioreactor technology

Many agents of physical, chemical, or biological nature, have the potential for causing cell stress. These agents are called stressors and their effects on cells are due to protein denaturation. Cells, microbes, for instance, perform their physiological functions and survive stress only if they have their proteins in the necessary concentrations and shapes. To be functional a protein shape must conform to a specific three-dimensional arrangement, named the native configuration. When a stressor (e.g., temperature elevation or heat shock, decrease in pH, hypersalinity, heavy metals) hits a microbe, it causes proteins to lose their native configuration, which is to say that stressors cause protein denaturation. The cell mounts an anti-stress response: house-keeping genes are down-regulated and stress genes are activated. Among the latter are the genes that produce the Hsp70(DnaK), Hsp60, and small heat protein (sHsp) families of stress proteins. Hsp70(DnaK) is part of the molecular chaperone machine together with Hsp40(DnaJ) and GrpE, and Hsp60 is a component of the chaperonin complex. Both the chaperone machine and the chaperonins play a crucial role in assisting microbial proteins to reach their native, functional configuration and to regain it when it is partially lost due to stress. Proteins that are denatured beyond repair are degraded by proteases so they do not accumulate and become a burden to the cell. All Archaea studied to date possess chaperonins but only some methanogens have the chaperone machine. A recent genome survey indicates that Archaea do not harbor well conserved equivalents of the co-chaperones trigger factor, Hip, Hop, BAG-1, and NAC, although the data suggest that Archaea have proteins related to Hop and to the NAC alpha subunit whose functions remain to be elucidated. Other anti-stress means involve osmolytes, ion traffic, and formation of multicellular structures. All cellular anti-stress mechanisms depend on genes whose products are directly involved in counteracting the effects of stressors, or are regulators. The latter proteins monitor and modulate gene activity. Biomethanation depends on the concerted action of at least three groups of microbes, the methanogens being one of them. Their anti-stress mechanisms are briefly discussed in this Chapter from the standpoint of their role in biomethanation with emphasis on their potential for optimizing bioreactor performance. Bioreactors usually contain stressors that come with the influent, or are produced during the digestion process. If the stressors reach levels above those that can be dealt with by the anti-stress mechanisms of the microbes in the bioreactor, the microbes will die or at least cease to function. The bioreactor will malfunction and crash. Manipulation of genes involved in the anti-stress response, particularly those pertinent to the synthesis and regulation of the Hsp70(DnaK) and Hsp60 molecular machines, is a promising avenue for improving the capacity of microbes to withstand stress, and thus to continue biomethanation even when the bioreactor is loaded with harsh waste. The engineering of methanogenic consortia with stress-resistant microbes, made on demand for efficient bioprocessing of stressor-containing effluents and wastes, is a tangible possibility for the near future. This promising biotechnological development will soon become a reality due to the advances in the study of the stress response and anti-stress mechanisms at the molecular and genetic levels.

Microstructure of anaerobic granules bioaugmented with Desulfitobacterium frappieri PCP-1

Oligonucleotide probes were used to study the structure of anaerobic granular biofilm originating from a pentachlorophenol-fed upflow anaerobic sludge bed reactor augmented with Desulfitobacterium frappieri PCP-1. Fluorescence in situ hybridization demonstrated successful colonization of anaerobic granules by strain PCP-1. Scattered microcolonies of strain PCP-1 were detected on the biofilm surface after 3 weeks of reactor operation, and a dense outer layer of strain PCP-1 was observed after 9 weeks. Hybridization with probes specific for Eubacteria and Archaea probes showed that Eubacteria predominantly colonized the outer layer, while Archaea were observed in the granule interior. Mathematical simulations showed a distribution similar to that observed experimentally when using a specific growth rate of 2.2 day(-1) and a low bacterial diffusion of 10(-7) dm(2) day(-1). Also, the simulations showed that strain PCP-1 proliferation in the outer biofilm layer provided excellent protection of the biofilm from pentachlorophenol toxicity.

Halorespiring bacteria-molecular characterization and detection

Recently, a rapidly increasing number of bacteria has been isolated that is able to couple the reductive dehalogenation of various halogenated aromatic and aliphatic compounds like chlorophenols and tetrachloroethene to energy conservation by electron-transport-coupled phosphorylation. The potential of these halorespiring bacteria for innovative clean-up strategies of polluted anoxic environments has greatly stimulated efforts to unravel the molecular basis of the novel respiratory chains they possess. The thorough characterization of halorespiratory key components at the physiological, biochemical and molecular genetic level has revealed both structural and functional similarity of chloroaryl- and chloroalkyl-respiratory chains from different phylogenetically distinct microorganisms. The reductive dehalogenases from halorespiring bacteria were found to comprise a novel class of corrinoid-containing Fe/S-proteins. Sensitive molecular methods for monitoring both presence and fate of halorespiring bacteria have been developed, which will be instrumental for the design and maintenance of optimised in situ bioremediation processes.

Effects of bioaugmentation strategies in UASB reactors with a methanogenic consortium for removal of phenolic compounds

The removal of phenol, ortho- (o-) and para- (p-)cresol was studied with two series of UASB reactors using unacclimatized granular sludges bioaugmented with a consortium enriched against these substances. The parameters studied were the amount of inoculum added to the sludges and the method of immobilization of the inoculum. Two methods were used, adsorption to the biomass or encapsulation within calcium alginate beads. In the bioaugmentation by adsorption experiment, and with a 10% inoculum, complete phenol removal was obtained after 36 d, while 178 d were required in the control reactor. For p-cresol, 95% removal was obtained in the bioaugmented reactor on day 48 while 60 d were required to achieve 90% removal in the control reactor. For o-cresol, the removals were only marginally better with the bioaugmented reactors. Tests performed with the reactors biomass under non-limiting substrate concentrations showed that the specific activities of the bioaugmented biomasses were larger than the original biomass for phenol, and p-cresol even after 276 of operations, showing that the inoculum bacteria successfully colonized the sludge granules. Immobilization of the inoculum by encapsulation in calcium alginate beads, was performed with 10% of the inoculum. Results showed that the best activities were obtained when the consortium was encapsulated alone and the beads added to the sludges. This reactor presented excellent activity and the highest removal of the various phenolic compounds a few days after start-up. After 90 d, a high-phenolic compounds removal was still observed, demonstrating the effectiveness of the encapsulation technique for the start-up and maintenance of high-removal activities.

Immobilization patterns and dynamics of acetate-utilizing methanogens immobilized in sterile granular sludge in upflow anaerobic sludge blanket reactors

Sterile granular sludge was inoculated with either Methanosarcina mazeii S-6, Methanosaeta concilii GP-6, or both species in acetate-fed upflow anaerobic sludge blanket (UASB) reactors to investigate the immobilization patterns and dynamics of aceticlastic methanogens in granular sludge. After several months of reactor operation, the methanogens were immobilized, either separately or together. The fastest immobilization was observed in the reactor containing M. mazeii S-6. The highest effluent concentration of acetate was observed in the reactor with only M. mazeii S-6 immobilized, while the lowest effluent concentration of acetate was observed in the reactor where both types of methanogens were immobilized together. No changes were observed in the kinetic parameters (Ks and mumax) of immobilized M. concilii GP-6 or M. mazeii S-6 compared with suspended cultures, indicating that immobilization does not affect the growth kinetics of these methanogens. An enzyme-linked immunosorbent assay using polyclonal antibodies against either M. concilii GP-6 or M. mazeii S-6 showed significant variations in the two methanogenic populations in the different reactors. Polyclonal antibodies were further used to study the spatial distribution of the two methanogens. M. concilii GP-6 was immobilized only on existing support material without any specific pattern. M. mazeii S-6, however, showed a different immobilization pattern: large clumps were formed when the concentration of acetate was high, but where the acetate concentration was low this strain was immobilized on support material as single cells or small clumps. The data clearly show that the two aceticlastic methanogens immobilize differently in UASB systems, depending on the conditions found throughout the UASB reactor.

Thinking about bacterial populations as multicellular organisms

It has been a decade since multicellularity was proposed as a general bacterial trait. Intercellular communication and multicellular coordination are now known to be widespread among prokaryotes and to affect multiple phenotypes. Many different classes of signaling molecules have been identified in both Gram-positive and Gram-negative species. Bacteria have sophisticated signal transduction networks for integrating intercellular signals with other information to make decisions about gene expression and cellular differentiation. Coordinated multicellular behavior can be observed in a variety of situations, including development of E. coli and B. subtilis colonies, swarming by Proteus and Serratia, and spatially organized interspecific metabolic cooperation in anaerobic bioreactor granules. Bacteria benefit from multicellular cooperation by using cellular division of labor, accessing resources that cannot effectively be utilized by single cells, collectively defending against antagonists, and optimizing population survival by differentiating into distinct cell types.

Improved Dechlorinating Performance of Upflow Anaerobic Sludge Blanket Reactors by Incorporation of Dehalospirillum multivorans into Granular Sludge

Dechlorination of tetrachloroethene, also known as perchloroethylene (PCE), was investigated in an upflow anaerobic sludge blanket (UASB) reactor after incorporation of the strictly anaerobic, reductively dechlorinating bacterium Dehalospirillum multivorans into granular sludge. This reactor was compared to the reference 1 (R1) reactor, where the granules were autoclaved to remove all dechlorinating abilities before inoculation, and to the reference 2 (R2) reactor, containing only living granular sludge. All three reactors were fed mineral medium containing 3 to 57 µM PCE, 2 mM formate, and 0.5 mM acetate and were operated under sterile conditions. In the test reactor, an average of 93% (mole/mole) of the effluent chloroethenes was dichloroethene (DCE), compared to 99% (mole/mole) in the R1 reactor. The R2 reactor, with no inoculation, produced only trichloroethene (TCE), averaging 43% (mole/mole) of the effluent chloroethenes. No dechlorination of PCE was observed in an abiotic control consisting of sterile granules without inoculum. During continuous operation with stepwise-reduced hydraulic retention times (HRTs), both the test reactor and the R1 reactor showed conversion of PCE to DCE, even at HRTs much lower than the reciprocal maximum specific growth rate of D. multivorans, indicating that this bacterium was immobilized in the living and autoclaved granular sludge. In contrast, the R2 reactor, with no inoculation of D. multivorans, only converted PCE to TCE under the same conditions. Immobilization could be confirmed by using fluorescein-labeled antibody probes raised against D. multivorans. In granules obtained from the R1 reactor, D. multivorans grew mainly in microcolonies located in the centers of the granules, while in the test reactor, the bacterium mainly covered the surfaces of granules.

Introduction of anaerobic dechlorinating bacteria into soil slurry microcosms and nested-PCR monitoring

Desulfomonile tiedjei and Desulfitobacterium dehalogenans were chosen as model bacteria to demonstrate the introduction of an anaerobic microbia reductive dechlorination activity into nonsterile soil slurry microcosms by inoculation. De novo 3-chlorobenzoate dechlorination activity was established with the bacterium D. tiedjei in microcosms normally devoid of this dechlorination capacity. The addition of D. tiedjei to microcosms supplemented with 20 mM pyruvate as the cosubstrate resulted in total biotransformation of 1.5 mM 3-chlorobenzoate within 7 days. The introduction of the bacterium Desulfitobacterium dehalogenans into nonsterile microcosms resulted in a shortening of the period required for dechlorination activity to be established. In microcosms inoculated with Desulfitobacterium dehalogenans, total degradation of 6 mM 3-chloro-4-hydroxy phenoxyacetic acid (3-Cl-4-OHPA) was observed after 4 days in contrast to the result in noninoculated microcosms, where the total degradation of 3-Cl-4-OHPA by indigenous microorganisms was observed after 11 days. Both externally introduced bacterial strains were detected in soil slurry microcosms by a nested-PCR methodology.

Introduction of a de novo bioremediation activity into anaerobic granular sludge using the dechlorinating bacterium DCB-2

The strictly anaerobic, pentachlorophenol (PCP) degrading bacterium DCB-2 was immobilized in an Upflow Anaerobic Sludge Blanket (UASB) reactor containing sterile granules. PCP and lactate were fed to the reactor and the concentration of chlorophenols in the effluent were monitored for 641 days. PCP was found to be degraded and transformed into 3,4,5-trichlorophenol in the reactor where DCB-2 introduced PPCP was still transformed to 3,4,5-trichlorophenol when the hydraulic retention time was decreased to six hours which was much lower than the generation time of DCB-2 insuring no free living cells in the reactor. This indicated that DCB-2 was immobilized in the granular layer. A control reactor that contained only sterile granules did not dechlorinate PCP indicating that the performance in the inoculated reactor was only due to the introduced bacteria. Immobilization of DCB-2 in the granules was further demonstrated by adding an antibody raised against DCB-2 to sliced granules. Bacteria thus visualized formed a net structure inside the granules. No DCB-2 bacteria could be found in granules from the control reactor. When lactate was omitted from the influent, the reactor still dechlorinated PCP in accordance with our findings that lactate was not used by DCB-2. This suggested that the reducing equivalents for reductive dechlorination were derived from the granules themselves. The reactor performance was 120 mumol.l reactor-1.day-1, comparable to the best described performance of a UASB-reactor and to aerobic reactors. Our study demonstrates that granules can be constructed which possess specific abilities such as a dechlorinating activity and at the same time be high performing. This result have implications for eco-engineering of granules for anaerobic treatment of contaminated waters.

Purification and characterization of a novel 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of Desulfomonile tiedjei DCB-1

Although reductive dehalogenation by anaerobic microorganisms offers great potential for the degradation of halocarbons, little is known about the biochemical mechanisms involved. It has previously been demonstrated that the dehalogenase activity involved in 3-chlorobenzoate dehalogenation by Desulfomonile tiedjei DCB-1 is present in the membrane fraction of the cell extracts. We report herein the purification of a 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of D. tiedjei DCB-1. The dehalogenase activity was monitored by the conversion of 3-chlorobenzoate to benzoate with reduced methyl viologen as a reducing agent. The membrane fraction of the cell extracts was obtained by ultracentrifugation, and the membrane proteins were solubilized with either the detergent CHAPS (3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate) or Triton X-100 in the presence of glycerol. The solubilized dehalogenase was purified by ammonium sulfate fractionation and a combination of anion exchange, hydroxyapatite, and hydrophobic interaction chromatographies. This procedure yielded about 7% of the total dehalogenase activity with a 120-fold increase in specific activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified dehalogenase consisted of two subunits with molecular weights of 64,000 and 37,000. The enzyme converted 3-chlorobenzoate to benzoate at its highest specific activity in 10 mM potassium phosphate buffer (pH 7.2) at 38 degrees C. The enzyme was yellow and probably a heme protein. The enzyme had an adsorbance peak at 408 nm. The dithionite-reduced enzyme displayed absorbance peaks at 416, 522, and 550 nm. The dithionite-reduced enzyme was able to complex with carbon monoxide. The nature of the heme chromophore is currently unknown.

Physiological meaning and potential for application of reductive dechlorination by anaerobic bacteria

The physiological meaning of reductive dechlorination reactions catalyzed by anaerobic bacteria can be explained as a co-metabolic activity or as a novel type of respiration. Co-metabolic activities have been found mainly with alkyl halides. They are non-specific reactions catalyzed by various enzyme systems of facultative as well as obligate anaerobic bacteria. In contrast, the reductive dechlorinations involved in metabolic respiration processes are very specific reactions. Only a limited number of alkyl and aryl chlorinated compounds is presently known to function as a terminal electron acceptor in a few, recently isolated bacteria. Metabolic dechlorination rates are in general several orders of magnitude higher than co-metabolic ones. Both reaction types are suitable for the anaerobic treatment of waste streams.