Isolation from estuarine sediments of a Desulfovibrio strain which can grow on lactate coupled to the reductive dehalogenation of 2,4, 6-tribromophenol

Microbially influenced corrosion as a model system for the study of metal microbe interactions: a unifying electron transfer hypothesis

The general term biomineralisation refers to biologically induced mineralisation in which an organism modifies its local microenvironment creating conditions such that there is chemical precipitation of mineral phases extracellularly. Most usually this results from an oxidation or reduction carried out by some microbial species, with the formation of a recognised biomineralised product. These reactions play a major role in microbial physiology and ecology, and are of central importance to such engineering consequences as microbial mining and microbially influenced corrosion. This paper will examine metal microbe interactions, both in naturally occurring microbial ecosystems and in two particular cases of biocorrosion, with the objective of putting forward a unifying hypothesis relevant to the understanding of each of these apparently disparate processes.

Metabolic diversity in aromatic compound utilization by anaerobic microbes

A vast array of structurally diverse aromatic compounds is continually released into the environment due to the decomposition of green plants and as a consequence of human industrial activities. Increasing numbers of bacteria that utilize aromatic compounds in the absence of oxygen have been brought into pure culture in recent years. These include most major metabolic types of anaerobic heterotrophs and acetogenic bacteria. Diverse microbes utilize aromatic compounds for diverse purposes. Chlorinated aromatic compounds can serve as electron acceptors in dehalorespiration. Humic substances serve as electron shuttles to enable the use of inorganic electron acceptors, such as insoluble iron oxides, that are not always easily reduced by microbes. Substituents that are attached to aromatic rings may serve as carbon or energy sources for microbes. Examples include acyl side chains and methyl groups. Finally, aromatic compounds can be completely degraded to serve as carbon and energy sources. Routes by which various types of aromatic compounds, including toluene, ethylbenzene, phenol, benzoate, and dihydroxylated compounds, are degraded have been elucidated in recent years. Biochemical strategies employed by microbes to destabilize the aromatic ring in preparation for degradation have become apparent from this work.

Anaerobic Dehalogenation of Organohalide Contaminants in the Marine Environment

Microbially mediated dehalogenation processes contribute to the global cycling of both biogenic and anthropogenic halogenated organic compounds. Detailed information on biodegradation mechanisms for a variety of organohalides and on the microorganisms mediating these processes has greatly increased our understanding of the cycling and fate of these unique and widespread compounds in our environment. The marine environment appears to be a particularly rich source of dehalogenating microorganisms. It is well established by laboratory and field studies that anaerobic dehalogenation of sediment contaminants, such as PCBs, pesticides, and dioxins, occurs intrinsically and can be enhanced via various methods. Specific dehalogenating bacterial populations can be enriched on various organohalides. Biodehalogenation processes are likely to be significantly affected by the prevailing terminal electron-accepting condition, and thus, biotransformation of organohalide contaminants in marine and estuarine environments will vary as a function of the redox conditions within the sediment profile. Fundamental knowledge of the activities and interactions of dehalogenating microorganisms is providing a strong basis for development of new bioremediation technologies for removal of harmful halogenated compounds from our environment.

Dehalogenation of chlorinated ethenes and immobilization of nickel in anaerobic sediment columns under sulfidogenic conditions

A sediment column study was carried out to demonstrate the bioremediation of chloroethene- and nickel-contaminated sediment in a single anaerobic step under sulfate-reducing conditions. Four columns (one untreated control column and three experimental columns) with sediment from a chloroethene- and nickel-contaminated site were investigated for 1 year applying different treatments. By stimulating the activity of sulfate-reducing bacteria by the addition of sulfate as supplementary electron acceptor, complex anaerobic communities were maintained with lactate as electron donor (with or without methanol), which achieved complete dehalogenation of tetra- and trichloroethenes (PCE and TCE) to ethene and ethane. A few weeks after sulfate addition, production of sulfide increased, indicating an increasing activity of sulfate-reducing bacteria. The nickel concentration in the effluent of one nickel-spiked column was greatly reduced, likely due to the enhanced sulfide production, causing precipitation of nickel sulfide. At the end of the study, 94% of the initial amount of nickel added to that column was recovered in the sediment As compared to the untreated (nonspiked) control column, all chloroethene-spiked columns ladditions of PCE and TCE) showed a permanent release of small chloride ion quantities (approximately 0.5-0.7 mM chloride), which were detected in the effluents a few weeks after sulfide production was observed for the first time. The formation of ethene and ethane as final products after dechlorination of PCE and TCE was detected in some effluents and in some gas phases of the columns. Other metabolites or intermediates (such as DCE isomers) were only detected sporadically in negligible quantities. The results of this study demonstrated thatmicrobial activity stimulated under sulfate-reducing conditions can have a beneficial effect on both the precipitation of heavy metals and the complete dechlorination of organochlorines. The strongly negative redox potential created by the activity of sulfate-reducing bacteria may be one factor responsible for stimulating the activity of the dehalogenating bacteria in the test columns.

Tetrachloroethene dehalorespiration and growth of Desulfitobacterium frappieri TCE1 in strict dependence on the activity of Desulfovibrio fructosivorans

Tetrachloroethene (PCE) dehalorespiration was investigated in a continuous coculture of the sulfate-reducing bacterium Desulfovibrio fructosivorans and the dehalorespiring Desulfitobacterium frappieri TCE1 at different sulfate concentrations and in the absence of sulfate. Fructose (2.5 mM) was the single electron donor, which could be used only by the sulfate reducer. With 2.5 mM sulfate, the dehalogenating strain was outnumbered by the sulfate-reducing bacterium, sulfate reduction was the dominating process, and only trace amounts of PCE were dehalogenated by strain TCE1. With 1 mM sulfate in the medium, complete sulfate reduction and complete PCE dehalogenation to cis-dichloroethene (cis-DCE) occurred. In the absence of sulfate, PCE was also completely dehalogenated to cis-DCE, and the population size of strain TCE1 increased significantly. The results presented here describe for the first time dehalogenation of PCE by a dehalorespiring anaerobe in strict dependence on the activity of a sulfate-reducing bacterium with a substrate that is exclusively used by the sulfate reducer. This interaction was studied under strictly controlled and quantifiable conditions in continuous culture and shown to depend on interspecies hydrogen transfer under sulfate-depleted conditions. Interspecies hydrogen transfer was demonstrated by direct H(2) measurements of the gas phase and by the production of methane after the addition of a third organism, Methanobacterium formicicum.

Phytoremediation to increase the degradation of PCBs and PCDD/Fs. Potential and limitations

Phytoremediation is already regarded as an efficient technique to remove or degrade various pollutants in soils, water and sediments. However, hydrophobic organic molecules such as PAHs, PCBs and PCDD/Fs are much less responsive to bioremediation strategies than, for example, BTEX or LAS. PCDD/Fs and PCBs represent 3 prominent groups of persistent organic pollutants that share common chemical, toxicological and environmental properties. Their widespread presence in the environment may be explained by their chemical and biological stability. This review considers their fate and dissipation mechanisms. It is then possible to identify major sinks and to understand biological activities useful for remediation. Public health and economic priorities lead to the conclusion that alternative techniques to physical treatments are required. This review focuses on particular problems encountered in biodegradation and bioavailability of PCDD/Fs and PCBs. It highlights the potential and limitations of plants and micro-organisms as bioremediation agents and summarises how plants can be used to augment bacterial activity. Phytoremediation is shown to provide some new possibilities in reducing risks associated with dioxins and PCBs.

Successional changes in an evolving anaerobic chlorophenol-degrading community used to infer relationships between population structure and system-level processes

The response of a complex methanogenic sediment community to 2-chlorophenol (2-CP) was evaluated by monitoring the concentrations of this model contaminant and important metabolic intermediates and products and by using rRNA-targeted probes to track several microbial populations. Key relationships between the evolving population structure, formation of metabolic intermediates, and contaminant mineralization were identified. The nature of these relationships was intrinsically linked to the metabolism of benzoate, an intermediate that transiently accumulated during the mineralization of 2-CP. Before the onset of benzoate fermentation, reductive dehalogenation of 2-CP competed with methanogenesis for endogenous reducing equivalents. This suppressed H(2) levels, methane production, and archaeal small-subunit (SSU)-rRNA concentrations in the sediment community. The concentrations of bacterial SSU rRNA, including SSU rRNA derived from "Desulfovibrionaceae" populations, tracked with 2-CP levels, presumably reflecting changes in the activity of dehalogenating organisms. After the onset of benzoate fermentation, the abundance of Syntrophus-like SSU rRNA increased, presumably because these syntrophic organisms fermented benzoate to methanogenic substrates. Consequently, although the parent substrate 2-CP served as an electron acceptor, cleavage of its aromatic nucleus also influenced the sediment community by releasing the electron donors H(2) and acetate. Increased methane production and archaeal SSU-rRNA levels, which tracked with the Syntrophus-like SSU-rRNA concentrations, revealed that methanogenic populations in particular benefited from the input of reducing equivalents derived from 2-CP.

Molecular characterization of sulfate-reducing bacteria in anaerobic hydrocarbon-degrading consortia and pure cultures using the dissimilatory sulfite reductase (dsrAB) genes

The characterization of sulfate-reducing bacteria (SRBs) is presented using the dissimilatory sulfite reductase (dsrAB) gene from various samples capable of mineralizing petroleum components. These samples include several novel, sulfidogenic pure cultures which degrade alkanes, toluene, and tribromophenol. Additionally, we have sulfidogenic consortia which re-mineralize benzene, naphthalene, 2-methylnaphthalene, and phenanthrene as a sole carbon source. In this study, 22 new dsrAB genes were cloned and sequenced. The dsrAB genes from our pollutant-degrading cultures or consortia were distributed among known SRBs and previously described dsrAB environmental clones, suggesting that many biodegradative SRBs are phylogenetically distinct and geographically wide spread. Specifically, the same dsrAB gene was discovered in independently established consortia capable of benzene, phenanthrene, and methylnaphthalene degradation, indicating that this particular SRB may be a key player in anaerobic degradation of hydrocarbons in the environment.

Coexistence of a sulphate-reducing Desulfovibrio species and the dehalorespiring Desulfitobacterium frappieri TCE1 in defined chemostat cultures grown with various combinations of sulfate and tetrachloroethene

A two-member co-culture consisting of the dehalorespiring Desulfitobacterium frappieri TCE1 and the sulphate-reducing Desulfovibrio sp. strain SULF1 was obtained via anaerobic enrichment from soil contaminated with tetrachloroethene (PCE). In this co-culture, PCE dechlorination to cis-dichloroethene was due to the activity of the dehalorespiring bacterium only. Chemostat experiments with lactate as the primary electron donor for both strains along with varying sulphate and PCE concentrations showed that the sulphate-reducing strain outnumbered the dehalogenating strain at relatively high ratios of sulphate/PCE. Stable co-cultures with both organisms present at similar cell densities were observed when both electron acceptors were supplied in the reservoir medium in nearly equimolar amounts. In the presence of low sulphate/PCE ratios, the Desulfitobacterium sp. became the numerically dominant strain within the chemostat co-culture. Surprisingly, in the absence of sulphate, strain SULF1 did not disappear completely from the co-culture despite the fact that there was no electron acceptor provided with the medium to be used by this sulphate reducer. Therefore, we propose a syntrophic association between the sulphate-reducing and the dehalorespiring bacteria via interspecies hydrogen transfer. The sulphate reducer was able to sustain growth in the chemostat co-culture by fermenting lactate and using the dehalogenating bacterium as a 'biological electron acceptor'. This is the first report describing growth of a sulphate-reducing bacterium in a defined two-member continuous culture by syntrophically coupling the electron and hydrogen transfer to a dehalorespiring bacterium.

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.

Isolation and characterization of Desulfovibrio dechloracetivorans sp. nov., a marine dechlorinating bacterium growing by coupling the oxidation of acetate to the reductive dechlorination of 2-chlorophenol

Strain SF3, a gram-negative, anaerobic, motile, short curved rod that grows by coupling the reductive dechlorination of 2-chlorophenol (2-CP) to the oxidation of acetate, was isolated from San Francisco Bay sediment. Strain SF3 grew at concentrations of NaCl ranging from 0.16 to 2.5%, but concentrations of KCl above 0. 32% inhibited growth. The isolate used acetate, fumarate, lactate, propionate, pyruvate, alanine, and ethanol as electron donors for growth coupled to reductive dechlorination. Among the halogenated aromatic compounds tested, only the ortho position of chlorophenols was reductively dechlorinated, and additional chlorines at other positions blocked ortho dechlorination. Sulfate, sulfite, thiosulfate, and nitrate were also used as electron acceptors for growth. The optimal temperature for growth was 30 degrees C, and no growth or dechlorination activity was observed at 37 degrees C. Growth by reductive dechlorination was revealed by a growth yield of about 1 g of protein per mol of 2-CP dechlorinated, and about 2.7 g of protein per mole of 2,6-dichlorophenol dechlorinated. The physiological features and 16S ribosomal DNA sequence suggest that the organism is a novel species of the genus Desulfovibrio and which we have designated Desulfovibrio dechloracetivorans. The unusual physiological feature of this strain is that it uses acetate as an electron donor and carbon source for growth with 2-CP but not with sulfate.

Anaerobic-aerobic process for microbial degradation of tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is a flame retardant that is used as an additive during manufacturing of plastic polymers and electronic circuit boards. Little is known about the fate of this compound in the environment. In the current study we investigated biodegradation of TBBPA, as well as 2,4,6-tribromophenol (TBP), in slurry of anaerobic sediment from a wet ephemeral desert stream bed contaminated with chemical industry waste. Anaerobic incubation of the sediment with TBBPA and peptone-tryptone-glucose-yeast extract medium resulted in a 80% decrease in the TBBPA concentration and accumulation of a single metabolite. This metabolite was identified by gas chromatography-mass spectrometry (GC-MS) as nonbrominated bisphenol A (BPA). On the other hand, TBP was reductively dehalogenated to phenol, which was further metabolized under anaerobic conditions. BPA persisted in the anaerobic slurry but was degraded aerobically. A gram-negative bacterium (strain WH1) was isolated from the contaminated soil, and under aerobic conditions this organism could use BPA as a sole carbon and energy source. During degradation of BPA two metabolites were detected in the culture medium, and these metabolites were identified by GC-MS and high-performance liquid chromatography as 4-hydroxybenzoic acid and 4-hydroxyacetophenone. Both of those compounds were utilized by WH1 as carbon and energy sources. Our findings demonstrate that it may be possible to use a sequential anaerobic-aerobic process to completely degrade TBBPA in contaminated soils.

Microbial reductive dehalogenation of polychlorinated biphenyls

Under anaerobic conditions, microbial reductive dechlorination of polychlorinated biphenyls (PCBs) occurs in soils and aquatic sediments. In contrast to dechlorination of supplemented single congeners for which frequently ortho dechlorination has been observed, reductive dechlorination mainly attacks meta and/or para chlorines of PCB mixtures in contaminated sediments, although in a few instances ortho dechlorination of PCBs has been observed. Different microorganisms appear to be responsible for different dechlorination activities and the occurrence of various dehalogenation routes. No axenic cultures of an anaerobic microorganism have been obtained so far. Most probable number determinations indicate that the addition of PCB congeners, as potential electron acceptors, stimulates the growth of PCB-dechlorinating microorganisms. A few PCB-dechlorinating enrichment cultures have been obtained and partially characterized. Temperature, pH, availability of naturally occurring or of supplemented carbon sources, and the presence or absence of H(2) or other electron donors and competing electron acceptors influence the dechlorination rate, extent and route of PCB dechlorination. We conclude from the sum of the experimental data that these factors influence apparently the composition of the active microbial community and thus the routes, the rates and the extent of the dehalogenation. The observed effects are due to the specificity of the dehalogenating bacteria which become active as well as changing interactions between the dehalogenating and non-dehalogenating bacteria. Important interactions include the induced changes in the formation and utilization of H(2) by non-dechlorinating and dechlorinating bacteria, competition for substrates and other electron donors and acceptors, and changes in the formation of acidic fermentation products by heterotrophic and autotrophic acidogenic bacteria leading to changes in the pH of the sediments.

Influence of different electron donors and acceptors on dehalorespiration of tetrachloroethene by Desulfitobacterium frappieri TCE1

Strain TCE1, a strictly anaerobic bacterium that can grow by reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE), was isolated by selective enrichment from a PCE-dechlorinating chemostat mixed culture. Strain TCE1 is a gram-positive, motile, curved rod-shaped organism that is 2 to 4 by 0.6 to 0.8 microm and has approximately six lateral flagella. The pH and temperature optima for growth are 7.2 and 35 degrees C, respectively. On the basis of a comparative 16S rRNA sequence analysis, this bacterium was identified as a new strain of Desulfitobacterium frappieri, because it exhibited 99.7% relatedness to the D. frappieri type strain, strain PCP-1. Growth with H(2), formate, L-lactate, butyrate, crotonate, or ethanol as the electron donor depends on the availability of an external electron acceptor. Pyruvate and serine can also be used fermentatively. Electron donors (except formate and H(2)) are oxidized to acetate and CO(2). When L-lactate is the growth substrate, strain TCE1 can use the following electron acceptors: PCE and TCE (to produce cis-1,2-dichloroethene), sulfite and thiosulfate (to produce sulfide), nitrate (to produce nitrite), and fumarate (to produce succinate). Strain TCE1 is not able to reductively dechlorinate 3-chloro-4-hydroxyphenylacetate. The growth yields of the newly isolated bacterium when PCE is the electron acceptor are similar to those obtained for other dehalorespiring anaerobes (e.g., Desulfitobacterium sp. strain PCE1 and Desulfitobacterium hafniense) and the maximum specific reductive dechlorination rates are 4 to 16 times higher (up to 1.4 micromol of chloride released. min(-1). mg of protein(-1)). Dechlorination of PCE and TCE is an inducible process. In PCE-limited chemostat cultures of strain TCE1, dechlorination is strongly inhibited by sulfite but not by other alternative electron acceptors, such as fumarate or nitrate.