Energy Generation Mechanism Research Articles

Comparative genomics analysis of completely sequenced microbial genomes reveals the ubiquity of N-linked glycosylation in prokaryotes

Glycosylation of proteins in prokaryotes has been known for the last few decades. Glycan structures and/or the glycosylation pathways have been experimentally characterized in only a small number of prokaryotes. Even this has become possible only during the last decade or so, primarily due to technological and methodological developments. Glycosylated proteins are diverse in their function and localization. Glycosylation has been shown to be associated with a wide range of biological phenomena. Characterization of the various types of glycans and the glycosylation machinery is critical to understand such processes. Such studies can help in the identification of novel targets for designing drugs, diagnostics, and engineering of therapeutic proteins. In view of this, the experimentally characterized pgl system of Campylobacter jejuni, responsible for N-linked glycosylation, has been used in this study to identify glycosylation loci in 865 prokaryotes whose genomes have been completely sequenced. Results from the present study show that only a small number of organisms have homologs for all the pgl enzymes and a few others have homologs for none of the pgl enzymes. Most of the organisms have homologs for only a subset of the pgl enzymes. There is no specific pattern for the presence or absence of pgl homologs vis-à-vis the 16S rRNA sequence-based phylogenetic tree. This may be due to differences in the glycan structures, high sequence divergence, horizontal gene transfer or non-orthologous gene displacement. Overall, the presence of homologs for pgl enzymes in a large number of organisms irrespective of their habitat, pathogenicity, energy generation mechanism, etc., hints towards the ubiquity of N-linked glycosylation in prokaryotes.

Quantitative proteomic analysis of the chemolithoautotrophic bacterium Nitrosomonas europaea: Comparison of growing- and energy-starved cells

Obligately aerobic ammonia-oxidizing bacteria (AOB) like Nitrosomonas europaea play a pivotal role in the global nitrogen cycle. Although starvation tolerance is a key environmental adaptation, little is known about this response in AOB. The goal of these studies was to compare the composition of the N. europaea proteome in growing- and energy-starved cells using ¹⁵N labeling and HPLC-ESI-MS/MS. More than 6500 peptides were sequenced with high confidence, and matched to 876 proteins (34% of the protein coding genes). Of these, 126 proteins had two or more peptide forms identified by 10 or more scans, and were used in quantitative analysis and 27 were found to be significantly different in abundance between growing and starved cells. Proteins showing greater abundance in growing cells are geared toward biosynthesis, particularly DNA replication. Energy-starved cells were shifted away from biosynthesis and toward survival functions that included: cell envelope modification, protein protection/degradation, detoxification, and implementation of alternative energy generation mechanisms. Most of these activities have not previously been reported as associated with energy-starvation stress in N. europaea. This study provides insights into the potential effects of fluctuating environmental conditions on the regulation of physiological networks in N. europaea.

Energy Generation Coupled to Azoreduction by Membranous Vesicles from Shewanella decolorationis S12

Previous studies have demonstrated that Shewanella decolorationis S12 can grow on the azo compound amaranth as the sole electron acceptor. Thus, to explore the mechanism of energy generation in this metabolism, membranous vesicles (MVs) were prepared and the mechanism of energy generation investigated. The membrane, which was fragmentized during preparation, automatically formed vesicles ranging from 37.5-112.5 nm in diameter under electron micrograph observation. Energy was conserved when coupling the azoreduction by the MVs of an azo compound or Fe(III) as the sole electron acceptor with H2, formate, or lactate as the electron donor. The amaranth reduction by the vesicles was found to be inhibited by specific respiratory inhibitors, including Cu(2+) ions, dicumarol, stigmatellin, and metyrapone, indicating that the azoreduction was indeed a respiration reaction. This finding was further confirmed by the fact that the ATP synthesis was repressed by the ATPase inhibitor N,N'-dicyclohexylcarbodiimide (DCCD). Therefore, this study offers solid evidence of a mechanism of microbial dissimilatory azoreduction on a subcell level.

Analysis of in vitro activities and modes of action of synthetic antimicrobial peptides derived from an α-helical ‘sequence template’

Cationic antimicrobial peptides (AMPs) are indispensable components of innate immune systems and promising candidates for novel anti-infective strategies. We rationally designed a series of peptides based on a template derived from known alpha-helical AMPs, which were then analysed regarding efficacy against clinical isolates and antibiotic mechanisms. Efficacy tests included standard MIC and synergy assays. Whole cell assays with staphylococcal strains included killing kinetics, efflux experiments and determination of membrane depolarization. The transcriptional response of AMP-treated Staphylococcus aureus SG511 was analysed using a Scienion genomic microarray covering (approximately 90% of) the S. aureus N315 genome and AMP P16(6|E). The AMPs showed remarkable broad-spectrum activity against bacteria and fungi regardless of any pre-existing antibiotic resistance mechanism. Whole cell assays indicated that the AMPs target the cytoplasmic membrane; however, significant membrane leakage and depolarization was only observed with a standard laboratory test strain. Transcriptional profiling identified up-regulation of putative efflux pumps and of aerobic energy generation mechanisms as major counter activities. Important components of the staphylococcal cell wall stress stimulon were up-regulated and the lipid metabolism was also affected. The broad spectrum activity of amphiphilic helical AMPs is based on multiple stresses resulting from interactions with microbial membranes; however, rather than killing through formation of pores, the AMPs appear to interfere with the coordinated and highly dynamic functioning of membrane bound multienzyme complexes such as electron transport chains and cell wall or lipid biosynthesis machineries.

Intra-night optical variability of luminous radio-quiet QSOs

In the present paper we report the detection of intra-night variability in some of the RQQSOs and one LDQ. To study intra-night variability, we carried out photometric monitoring of seven RQQSOs and one LDQ in Johnson V-passband using 1.2 meter optical/IR telescope at Gurushikhar, Mount Abu, India. Observations were made in nine nights during the first half of the year 2000; seven RQQSOs: 0748+291, 0945+438, 1017+280, 1029+329, 1101+319, 1225+317, 1252+020 and one LDQ: 1103-006 were observed. RQQSOs 0748+291, 1225+317 and LDQ 1103-006 have shown existence of intra-night variations. In the case of 1017+280 (RQQSO) there is indication of intra-night variation in one night where as the observations in another night do not show convincingly the existence of intra-night variability. RQQSOs 0945+438, 1029+329, 1101+319 and 1252+020 have not shown any intra-night variations. We compiled intra-night variability data for radio-loud and radio-quiet AGNs from the literature for statistical analysis. It is found that a good fraction of radio-quiet AGNs show intra-night variations with the maximum amplitude of variation being about 10%. On the other hand blazars show at times intra-night flux variability up to 100%. In case of radio-loud AGNs (excluding blazars), maximum amplitude of intra-night variation lies between the variability amplitude of radio-quiet AGNs and blazars i.e. the flux variation is close to 50%. The results indicate that the energy generation mechanism and the environment around the central engine in different classes of AGNs may be similar, if not identical. The standard model for radio-loud AGNs, where shocks are propagating down relativistic jet or models based on disturbances in accretion disks can also explain the micro-variability in RQQSOs.

Behaviour of matter close to the event horizon

ABSTRACT Investigation of the behaviour of accreting matter close to the black hole event horizonis of fundamental importance in relativistic and high energy astrophysics because theyprovide the key features of the diagnostic spectra of the stellar mass and super-massiveblack holes. In this paper, we examine the terminal behaviour of general relativis-tic matter in multi-transonic, advective black hole accretion discs. We compute, forthe first time we believe, the values of various dynamical and thermodynamic multi-transonic flow variables extremely close (≤0.01 r g ) to the event horizon and study thedependence of these variables on fundamental accretion parameters. Our calculationis useful for a better understanding of Hawking radiation from acoustic black holes.Key words: Accretion, accretion discs — black hole physics — general relativity —hydrodynamics — Hawking Radiation 1 INTRODUCTIONGravitational capture of surrounding fluid by massive astrophysical objects is known as accretion. There remains a major dif-ference between black hole (BH) accretion and accretion onto other cosmic objects including neutron stars and white dwarfs.For celestial bodies other than black holes, infall of matter terminates either by a direct collision with the hard surface of theaccretor or with the outer boundary of the magneto-sphere, resulting the luminosity through energy release from the surface.Whereas for black hole accretion, matter ultimately dives through the event horizon from where radiation is prohibited toescape according to the rule of classical general relativity (GR) and the emergence of luminosity occurs on the way towardsthe black hole event horizon. The efficiency of accretion process may be thought as a measure of the fractional conversion ofgravitational binding energy of matter to the emergent radiation and is considerably high for black hole accretion comparedto accretion onto any other astrophysical objects. Hence accretion onto classical astrophysical black holes has been recognizedas a fundamental phenomena of increasing importance in relativistic and high energy astrophysics (Frank, King & Raine1992, hereafter FKR, Shapiro & Teukolsky 1983). The extraction of gravitational energy from the black hole accretion isbelieved to power the energy generation mechanism of X-ray binaries and of the most luminous objects of the Universe, theQuasars and active galactic nuclei (AGN). The BH accretion is, thus, the most appealing way through which the all pervadingpower of gravity is explicitly manifested. If the infalling matter does not possess intrinsic angular momentum, accretion flowremains spherically symmetric. However, Inter stellar / intergalactic fluid is always likely to posses non-vanishing rotationalenergy, sufficient to dynamically break the spherical symmetry, and in almost all real physical situations, accreting matter isthrown into circular orbits around the central accretor, leading to the formation of the accretion disc around the galactic andextra-galactic black holes.If the instantaneous dynamical velocity and local acoustic velocity of the accreting fluid, moving along a space curve parame-terized by r, are u(r) and a(r) respectively, then the local Mach number M(r) of the fluid can be defined as M(r) =

UV and X-ray observations of mass outflows in AGN

More than half of all low-redshift AGN exhibit UV and X-ray absorption by highly ionized gas. The observed UV and X-ray absorption lines are almost aiways blue-shifted at velocities of hundreds of km s−1, indicating that the absorbing gas is outflowing from the active nucleus. In some cases the inferred mass flux rivals the Eddington limit of the central black hole, an indication that these outflows are intimately related to the mass accretion and energy generation mechanism in AGN. The ejected material can also have an affect on the interstellar medium of the host galaxy and the surrounding intergalactic medium. Over the past several years, coordinated UV and X-ray observations of several bright AGN at high spectral resolution using HST, FUSE, Chandra, and XMM-Newton have contributed greatly to our understanding of these outflows. I will give an overview of these recent observations, summarize our FUSE survey of low-redshift AGN, and interpret the results in the context of models of winds from accretion disks and thermally driven winds from the obscuring torus.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Energy conservation in acetogenic bacteria.

The anaerobic way of life comprises a great metabolic diversity and, correspondingly, diversity of energy-generating mechanisms ([52][1]). The facultative anaerobes have a respiratory system that uses preferentially oxygen, but in the absence of oxygen and presence of alternative electron acceptors

Microbial genome analyses: comparative transport capabilities in eighteen prokaryotes

Here, we present a comprehensive analysis of solute transport systems encoded within the completely sequenced genomes of 18 prokaryotic organisms. These organisms include four Gram-positive bacteria, seven Gram-negative bacteria, two spirochetes, one cyanobacterium and four archaea. Membrane proteins are analyzed in terms of putative membrane topology, and the recognized transport systems are classified into 76 families, including four families of channel proteins, four families of primary carriers, 54 families of secondary carriers, six families of group translocators, and eight unclassified families. These families are analyzed in terms of the paralogous and orthologous relationships of their protein members, the substrate specificities of their constituent transporters and their distributions in each of the 18 organisms studied. The families vary from large superfamilies with hundreds of represented members, to small families with only one or a few members. The mode of transport generally correlates with the primary mechanism of energy generation, and the numbers of secondary transporters relative to primary transporters are roughly proportional to the total numbers of primary H+ and Na+ pumps in the cell. The phosphotransferase system is less prevalent in the analyzed bacteria than previously thought (only six of 14 bacteria transport sugars via this system) and is completely lacking in archaea and eukaryotes. Escherichia coli is shown to be exceptionally broad in its transport capabilities and therefore, at a membrane transport level, does not appear representative of the bacteria thus far sequenced. Archaea and spirochetes exhibit fewer proteins with multiple transmembrane segments and fewer net transporters than most bacteria. These results provide insight into the relevance of transport to the overall physiology of prokaryotes.

High Resolution Radio Observations of Luminous IR Galaxies

High resolution radio observations provide a unique tool for investigation ofultraluminous infrared galaxies, due to the combination of high angularresolution and source transparency available at centimeter wavelengths. Thetypical angular size of the IR emitting region in the most luminous systems isof order 0.1 arcsec, while dust absorption appears to obscure this region fromdirect view at wavelengths shorter than ∼ 100 microns. Radio observationswith VLBI and MERLIN are revealing compact radio structures indicative of bothstarburst and AGN activity in these systems. Additional observations arerequired to clarify relationships between radio structures and the energygeneration mechanisms, and to realize the full potential of radio interferometryfor ULIRG studies.

A novel multicomponent regulatory system mediates H2 sensing in Alcaligenes eutrophus.

Oxidation of molecular hydrogen catalyzed by [NiFe] hydrogenases is a widespread mechanism of energy generation among prokaryotes. Biosynthesis of the H2-oxidizing enzymes is a complex process subject to positive control by H2 and negative control by organic energy sources. In this report we describe a novel signal transduction system regulating hydrogenase gene (hox) expression in the proteobacterium Alcaligenes eutrophus. This multicomponent system consists of the proteins HoxB, HoxC, HoxJ*, and HoxA. HoxB and HoxC share characteristic features of dimeric [NiFe] hydrogenases and form the putative H2 receptor that interacts directly or indirectly with the histidine protein kinase HoxJ*. A single amino acid substitution (HoxJ*G422S) in a conserved C-terminal glycine-rich motif of HoxJ* resulted in a loss of H2-dependent signal transduction and a concomitant block in autophosphorylating activity, suggesting that autokinase activity is essential for the response to H2. Whereas deletions in hoxB or hoxC abolished hydrogenase synthesis almost completely, the autokinase-deficient strain maintained high-level hox gene expression, indicating that the active sensor kinase exerts a negative effect on hox gene expression in the absence of H2. Substitutions of the conserved phosphoryl acceptor residue Asp55 in the response regulator HoxA (HoxAD55E and HoxAD55N) disrupted the H2 signal-transduction chain. Unlike other NtrC-like regulators, the altered HoxA proteins still allowed high-level transcriptional activation. The data presented here suggest a model in which the nonphosphorylated form of HoxA stimulates transcription in concert with a yet unknown global energy-responsive factor.

In vitro reassembly of the malolactic fermentation pathway of Leuconostoc oenos (Oenococcus oeni).

The mechanism of metabolic energy generation by malolactic fermentation was studied with artificial membrane vesicles of Leuconostoc oenos (Oenococcus oeni). (Note that although L. oenos was recently reclassified as O. oeni [L. M. T. Dicks, F. Dellaglio, and M. D. Collins, Int. J. Syst. Bacteriol. 45:395-397, 1995], the old designation was kept in the present work.) Purified malolactic enzyme was entrapped in artificial membrane vesicles prepared from L. oenos cells able to transport L-malate. We show that the in vitro reconstituted system, including an electrogenic L-malate carrier and the decarboxylating malolactic enzyme, generated a proton motive force that was able to drive intravesicular accumulation of leucine.

High-Resolution Mid-Infrared Imaging of Infrared-luminous Galaxies

view Abstract Citations (42) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution Mid-Infrared Imaging of Infrared-luminous Galaxies Miles, J. W. ; Houck, J. R. ; Hayward, T. L. ; Ashby, M. L. N. Abstract We present high-resolution mid-infrared (~10 micron) imaging of a flux-limited sample of galaxies with far-infrared luminosity L_FIR_ > 10^11^ L_sun_. This work was done to investigate whether the energy generation mechanism powering these infrared-luminous galaxies comes from massive bursts of star formation or from active galactic nuclei (AGNs). Our images trace the spatial extent of the warm (~>200 K) dust, which must lie close to the heating source. The majority of the sample galaxies are extended, which implies a starburst as the source of at least part of the energy. However, most of the galaxies also contain an unresolved nucleus, so the presence of an AGN cannot be ruled out. We argue that the observations are consistent with an evolutionary model in which the infrared-luminous galaxies are seen as dusty cocoons for the birth of quasars. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177412 Bibcode: 1996ApJ...465..191M Keywords: INFRARED: GENERAL; GALAXIES: STARBURST; GALAXIES: NUCLEI; GALAXIES: ACTIVE full text sources ADS | data products SIMBAD (15) NED (10)

Radio Luminosity Functions

Luminosity functions give the space densities of source populations as functions of spectral luminosity. They constrain source physics, clustering, cosmological evolution, and contributions to the radiation background. Extragalactic radio sources span eight decades of luminosity, so dividing them into luminosity ranges effectively separates them by energy generation mechanism and other fundamental properties. Multivariate luminosity functions divide the source population in additional ways (e.g., by optical morphology, optical absolute magnitude, or radio spectral index) selected to emphasize particular source characteristics.

The Nuclear Energy Sources Powering Bright Infrared-selected Galaxies

view Abstract Citations (21) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nuclear Energy Sources Powering Bright Infrared-selected Galaxies Ashby, M. L. N. ; Houck, J. R. ; Matthews, K. Abstract We present the results of a multiwavelength investigation of energy generation mechanisms in 33 objects from the IRAS Bright Galaxy Sample. This study is based on long-slit optical spectroscopy and near-infrared broadband imaging at J, H, and K. We used established emission-line criteria, involving [O III] λ5007/Hβ, [N II λ6583/Hα, [S II] (λ6716 + λ6731)/Hα, and [O I] λ6300/Hα intensity ratios, to classify the optical spectra of all 36 sample nuclei (three sources were binary systems). The line ratios indicate that our sample contains seven active galactic nuclei (AGNs) and 24 starburst galaxies. We also find that five galaxies (straddlers) cannot be straightforwardly classified by this simple scheme. Follow-up near-infrared imaging of the straddler galaxies yields near-infrared colors consistent with emission from relatively weak AGNs. Considered together with the fact that their narrow optical emission-line ratios resemble those of NGC 7469, which is known to contain an unresolved AGN inside an H II region-like galaxy disk or torus, the evidence seems to indicate that NGC 7469 may be typical of a pattern exhibited by infrared luminous galaxies, i.e., that low-luminosity AGNs and starbursts often coexist in infrared-selected galaxies. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175898 Bibcode: 1995ApJ...447..545A Keywords: GALAXIES: ACTIVE; GALAXIES: NUCLEI; GALAXIES: STARBURST; INFRARED: GALAXIES full text sources ADS | data products SIMBAD (54) NED (34)

THE ENERGY GENERATION MECHANISMS POWERING INFRARED-SELECTED GALAXIES

ABSTRACT I have applied newly available optical emission line diagnostics to two samples of infrared-luminous galaxies in a program to discriminate between possible energy generation mechanisms, i.e., active nuclei and starbursts. The first sample was composed of very faint (F(60 microns) < 180 mJy, z < 0.3) galaxies drawn from coadded IRAS scans of the North Ecliptic Pole Region (NEPR). The second sample consisted of 37 much brighter sources drawn from the IRAS Bright Galaxy Sample (BGS). Analysis of the redshift distribution of the NEPR sample revealed clustering is present on scales ~10 Mpc. In addition, a very strong cluster was identified at z=0.088 that has a surface density roughly three times greater than the Great Wall. Out of the 76 galaxies observed from the NEPR sample 17 had strong spectra suitable for the analysis, which was based on the relative intensities of forbidden and permitted emission lines. While 3 sources could be classified as AGNs and another 7 as starburst galaxies, the remaining 7 eluded classification. Longslit optical emission-line spectra, which effectively separated nuclear and disk contributions to the emission, indicated the BGS galaxies contained 7 AGNs and 24 starbursts. In a minority of 6 cases, however, the line diagnostics failed to unambiguously classify the luminosity source. Follow-up near-infrared imaging showed that the 'straddlers' have redder near-infrared colors than the other BGS galaxies. The optical spectra and near-infrared colors taken together with morphological peculiarities imply that the straddlers are composite objects harboring low-luminosity active nuclei residing within starburst disks. Finally, the optical spectrum of a high redshift IRAS galaxy was analysed. IRAS 18003+6652 is the third most distant (z=0.088) galaxy known to have been observed by IRAS and has an unusually strong emission line spectrum in combination with low-ionization absorption features.

Multiwavelength monitoring of active galactic nuclei

Recent multiwavelength monitoring of active galactic nuclei (AGN), particularly with the IUE satellite, has produced extraordinary advances in our understanding of the energy-generation mechanism(s) in the central engine and of the structure of the surrounding material. Examples discussed here include both ordinary AGN and blazars (the collective name for highly variable, radio-loud AGN like BL Lac objects and Optically Violently Variable quasars). In the last decade, efforts to obtain single-epoch multiwavelength spectra led to fundamentally new models for the structure of AGN, involving accretion disks for AGN and relativistic jets for blazars. Recent extensions of multiwavelength spectroscopy into the temporal domain have shown that while these general pictures may be correct, the details were probably wrong. Campaigns to monitor Seyfert 1 galaxies like NGC 4151, NGC 5548 and Fairall 9 at infrared, optical, ultraviolet and X-ray wavelengths indicate that broad-emission line regions are stratified by ionization, density, and velocity; argue against a standard thin accretion disk model; and suggest that X-rays represent primary rather than reprocessed radiation. For blazars, years of radio monitoring indicated emission from an inhomogeneous synchrotron-emitting plasma, which could also produce at least some of the shorter-wavelength emission. The recent month-long campaign to observe the BL Lac object PKS 2155-304 has revealed remarkably rapid variability that extends from the infrared through the X-ray with similar amplitude and little or no discernible lag. This lends strong support to relativistic jet models and rules out the proposed accretion disk model for the ultraviolet-X-ray continuum.

Gravitational Waves and gamma -Ray Bursts

Coalescing binaries in distant galaxies are one of the most promising sources of gravitational waves detectable by the LIGO project.$^{[1-5]}$ They are also a copious source of neutrinos,$^{[1]}$ however these neutrino pulses are far too weak to be detected on earth. Several years ago Eichler \etal$\,$$^{[6]}$ suggested that they are also sources of $\gamma$-ray bursts (GRBs). Recently it was found$^{[7]}$ that GRBs are likely to be cosmological in origin, and coalescing binary systems$^{[6,8-11]}$ are probably the most promising cosmological sources. The current estimates of the burst and LIGO rates from a cosmologically distributed population are based on the systems observed in our galaxy.$^{[12-14]}$ These estimates are based on only three binarys so there are large statistical uncertainties in the coalescence rate. If we accept the cosmological/coalescing binary hypothesis for GRBs, then we get a more accurate estimate of the rate at which binaries coalesce and hence of the predicted LIGO signal. The association between GRBs and binaries can significantly improve the performance of LIGO. The detection of gravitational radiation from a GRB source not only confirms the coalescing binary model, but it will also provide information on the geometry and energy generation mechanism of the burst.

Generation of a proton motive force by histidine decarboxylation and electrogenic histidine/histamine antiport in Lactobacillus buchneri

Lactobacillus buchneri ST2A vigorously decarboxylates histidine to the biogenic amine histamine, which is excreted into the medium. Cells grown in the presence of histidine generate both a transmembrane pH gradient, inside alkaline, and an electrical potential (delta psi), inside negative, upon addition of histidine. Studies of the mechanism of histidine uptake and histamine excretion in membrane vesicles and proteoliposomes devoid of cytosolic histidine decarboxylase activity demonstrate that histidine uptake, histamine efflux, and histidine/histamine exchange are electrogenic processes. Histidine/histamine exchange is much faster than the unidirectional fluxes of these substrates, is inhibited by an inside-negative delta psi and is stimulated by an inside positive delta psi. These data suggest that the generation of metabolic energy from histidine decarboxylation results from an electrogenic histidine/histamine exchange and indirect proton extrusion due to the combined action of the decarboxylase and carrier-mediated exchange. The abundance of amino acid decarboxylation reactions among bacteria suggests that this mechanism of metabolic energy generation and/or pH regulation is widespread.

Heterotrophic sulfur reduction by Thermotoga sp. strain FjSS3.B1

Thermotoga sp. strain FjSS3.B1 was able to reduce sulfur to sulfide when grown on a mineral medium with glucose as the sole carbon and energy source. There was no increase in specific growth yield coupled to sulfur reduction, but the specific growth rate, final growth yield, and tolerance of H 2 were all increased in the presence of sulfur. At dissolved H 2 concentrations, of 550 to 600 μmol/1 (at 77°C) growth was not possible unless sulfur was added. Glucose was fermented via the Embden-Meyerhof-Parnas pathway to lactate, acetate, H 2 and CO 2 (and other unidentified minor products). The thermodynamic problems associated with the relatively high redox potential electrons from the 1,3-bisphosphoglycerate/glyceraldehyde 3-phosphate couple ( E′ 0 = − 350 mV) are overcome by reducing sulfur to sulfide ( E′ 0 = − 270 mV) rather than the energetically unfavourable production of H 2 ( E′ 0 = − 414 mV). Under high hydrogen partial pressures there was increased production of lactate as an alternative electron sink. The results indicate that sulfur reduction operates primarily as an electron sink rather than as a detoxification reaction or energy-generating mechanism.