Electricity-assisted biological hydrogen production from acetate by Geobacter sulfurreducens

Geobacter sulfurreducens is a well-known current-producing microorganism in microbial fuel cells, and is able to use acetate and hydrogen as electron donor. We studied the functionality of G. sulfurreducens as biocatalyst for hydrogen formation at the cathode of a microbial electrolysis cell (MEC). Geobacter sulfurreducens was grown in the bioelectrode compartment of a MFC with acetate as the substrate and reduction of complexed Fe(III) at the counter electrode. After depletion of the acetate the electrode potential of the bioelectrode was decreased stepwise to -1.0 V vs Ag/AgCl reference. Production of negative current was observed, which increased in time, indicating that the bioelectrode was now acting as biocathode. Headspace analyses carried out at electrode potentials ranging from -0.8 to -1.0 V showed that hydrogen was produced, with higher rates at more negative cathode potentials. Subsequently, the metabolic properties of G. sulfurreducens for acetate oxidation at the anode and hydrogen production at the cathode were combined in one-compartment membraneless MECs operated at applied voltages of 0.8 and 0.65 V. After two days, current densities were 0.44 A m(-2) at 0.8 V applied voltage and 0.22 A m(-2) at 0.65 V, using flat-surface carbon electrodes for both anode and cathode. The cathodic hydrogen recovery ranged from 23% at 0.5 V applied voltage to 43% at 0.9 V.     

Whole cell electrochemistry of electricity-producing microorganisms evidence an adaptation for optimal exocellular electron transport

The mechanism(s) by which electricity-producing microorganisms interact with an electrode is poorly understood. Outer membrane cytochromes and conductive pili are being considered as possible players, but the available information does not concur to a consensus mechanism yet. In this work we demonstrate that Geobacter sulfurreducens cells are able to change the way in which they exchange electrons with an electrode as a response to changes in the applied electrode potential. After several hours of polarization at 0.1 V Ag/AgCl-KCl (saturated), the voltammetric signature of the attached cells showed a single redox pair with a formal redox potential of about -0.08 V as calculated from chronopotentiometric analysis. A similar signal was obtained from cells adapted to 0.4 V. However, new redox couples were detected after conditioning at 0.6 V. A large oxidation process beyond 0.5 V transferring a higher current than that obtained at 0.1 V was found to be associated with two reduction waves at 0.23 and 0.50 V. The apparent equilibrium potential of these new processes was estimated to be at about 0.48 V from programmed current potentiometric results. Importantly, when polarization was lowered again to 0.1 V for 18 additional hours, the signals obtained at 0.6 V were found to greatly diminish in amplitude, whereas those previously found at the lower conditioning potential were recovered. Results clearly show the reversibility of cell adaptation to the electrode potential and pointto the polarization potential as a key variable to optimize energy production from an electricity producing population.     

Consolidated bioprocessing of AFEX-pretreated corn stover to ethanol and hydrogen in a microbial electrolysis cell

The consolidated bioprocessing (CBP) of corn stover pretreated via ammonia fiber expansion (AFEX-CS) into ethanol was investigated in a microbial electrolysis cell (MEC) driven by the exoelectrogen Geobacter sulfurreducens and the CBP bacterium Cellulomonas uda. C. uda was identified in a screening for its ethanologenic potential from AFEX-CS and for producing electron donors for G. sulfurreducens fermentatively. C. uda produced ethanol from AFEX-CS in MECs inoculated simultaneously or sequentially, with the concomitant conversion of the fermentation byproducts into electricity by G. sulfurreducens. The fermentation and electrical conversion efficiencies were high, but much of the AFEX-CS remained unhydrolyzed as nitrogen availability limited the growth of the CBP partner. Nitrogen supplementation stimulated the growth of C. uda, AFEX-CS hydrolysis and ethanologenesis. As a result, the synergistic activities of the CBP and exoelectrogen catalysts resulted in substantial energy recoveries from ethanologenesis alone (ca. 56%). The cogeneration of cathodic H(2) in the MEC further increased the energy recoveries to ca. 73%. This and the potential to optimize the activities of the microbial catalysts via culturing approaches and genetic engineering or adaptive evolution, make this platform attractive for the processing of agricultural wastes.     

Gibbs free energy of formation of chlordecone and potential degradation products: implications for remediation strategies and environmental fate

Chlordecone (C(10)Cl(10)O; CAS number 143-50-0) has been used extensively as an organochlorine insecticide but is nowadays banned under The Stockholm Convention on Persistent Organic Pollutants (POPs). A search for chlordecone-respiring organisms and choosing between reductive versus oxidative remediation tools and strategies to clean up chlordecone-polluted environments would benefit from the availability of Gibbs free energy data of chlordecone and its potential dechlorination products. Presently such data are not available. Polycyclic "cage" molecules of which chlordecone is an example contain considerable strain energy. It is not a priori clear how this affects the thermodynamic properties of the chlorinated members of this unique class of compounds and to what extent redox potentials for the halogenated congeners are different from those of other aliphatic and aromatic organohalogens. We performed ab initio quantum chemical calculations to estimate Δ(f)H(m)° and Δ(f)G(m)° values of chlordecone and selected dechlorination products and used these data to calculate their Gibbs free energy and redox potential. With redox potentials in the range of 336-413 mV chlordecone has an E(o)' value similar to that of other organochlorines. The results indicate that there are no thermodynamic reasons why chlordecone-respiring or -fermenting organisms should not exist.     

Electron transfer from a solid-state electrode assisted by methyl viologen sustains efficient microbial reductive dechlorination of TCE

The ability to transfer electrons, via an extracellular path, to solid surfaces is typically exploited by microorganisms which use insoluble electron acceptors, such as iron-or manganese-oxides or inert electrodes in microbial fuel cells. The reverse process, i.e., the use of solid surfaces or electrodes as electron donors in microbial respirations, although largely unexplored, could potentially have important environmental applications, particularly for the removal of oxidized pollutants from contaminated groundwater or waste streams. Here we show, for the first time, that an electrochemical cell with a solid-state electrode polarized at -500 mV (vs standard hydrogen electrode), in combination with a low-potential redox mediator (methyl viologen), can efficiently transfer electrochemical reducing equivalents to microorganisms which respire using chlorinated solvents. By this approach, the reductive transformation of trichloroethene, a toxic yet common groundwater contaminant, to harmless end-products such as ethene and ethane could be performed. Furthermore, using a methyl-viologen-modified electrode we could even demonstrate that dechlorinating bacteria were able to accept reducing equivalents directly from the modified electrode surface. The innovative concept, based on the stimulation of dechlorination reactions through the use of solid-state electrodes (we propose for this process the acronym BEARD: Bio-Electrochemically Assisted Reductive Dechlorination), holds promise for in situ bioremediation of chlorinated-solvent-contaminated groundwater, and has several potential advantages over traditional approaches based on the subsurface injection of organic compounds. The results of this study raise the possibility that immobilization of selected redox mediators may be a general strategy for stimulating and controlling a range of microbial reactions using insoluble electrodes as electron donors.     

Cysteine-mediated reductive dissolution of poorly crystalline iron(III) oxides by Geobacter sulfurreducens

The reductive dissolution of poorly crystalline ferric oxides in the presence of cysteine was investigated to evaluate the potential of cysteine as a possible electron carrier to stimulate the reduction of iron(III) oxides by Geobacter sulfurreducens. The extent and rate of biotic and abiotic reduction of iron(III) oxides in the presence of cysteine at various concentrations were compared. Iron(III) oxides were reduced abiotically by cysteine. The initial rate and extent of iron(III) oxide reduction were correlated linearly with the cysteine concentration ranging from 0 to 6 mM. Also, addition of 0.5-2 mM cysteine significantly stimulated the rate and the extent of iron(III) oxide reduction in cultures of G. sulfurreducens. The cysteine concentration decreased in accordance with the increase of Fe(II) concentration and reached a nearly constant residual concentration. Cysteine depletion followed first-order kinetics and increased linearly with the cysteine concentration. An 8- to 11-fold increase in the extent of iron(III) oxide reduction relative to the abiotic system was observed. Comparison of sorbed and dissolved Fe(II) concentrations between cultures amended with cysteine and with other organic chelators showed that solubilization is not the main factor in cysteine-stimulated Fe(III) reduction. Addition of cystine could enhanced the extent of iron(III) oxide reduction, concomitant with the increase of the regenerated cysteine concentration and support the hypothesis that cysteine could serve as an electron carrier to transfer electrons from G. sulfurreducens to poorly crystalline iron(III) oxides.     

Kinetics of microbial and chemical reduction of humic substances: implications for electron shuttling

Humic substances (HS) are redox-active natural organic compounds and serve as electron shuttles between microorganisms and iron(III) minerals. Here we demonstrate that electron shuttling is possible only at concentrations of dissolved HS of at least 5-10 mg C/L. Although such concentrations can be found in many rivers, lakes, and even in some aquifers there are also many marine and freshwater systems with DOC < 5 mg C/L where consequently electron shuttling is not expected to happen. We found that in the case of HS concentrations which do not limit electron shuttling, Geobacter sulfurreducens transfers electrons to HS at least 27 times faster than to Fe(III)hydroxide. Microbially reduced HS transfer electrons to ferrihydrite at least 7 times faster than cells thereby first demonstrating that microbial mineral reduction via HS significantly accelerates Fe(III) mineral reduction and second that electron transfer from reduced HS to Fe(III) minerals represents the rate-limiting step in microbial Fe(III) mineral reduction via HS. Microbial reduction of HS transfers as many electrons to HS as chemical reduction with H2 indicating that all redox-active functional groups that can be reduced at a redox potential of -418 mV (Eh(0) of H2/H+ redox couple at pH 7) can also be reduced by microorganisms.     

Remediation and recovery of uranium from contaminated subsurface environments with electrodes

Previous studies have demonstrated that Geobacter species can effectively remove uranium from contaminated groundwater by reducing soluble U(VI) to the relatively insoluble U(IV) with organic compounds serving as the electron donor. Studies were conducted to determine whether electrodes might serve as an alternative electron donor for U(VI) reduction by a pure culture of Geobacter sulfurreducens and microorganisms in uranium-contaminated sediments. Electrodes poised at -500 mV (vs a Ag/AgCl reference) rapidly removed U(VI) from solution in the absence of cells. However, when the poise at the electrode was removed, all of the U(VI) returned to solution, demonstrating that the electrode did not reduce U(VI). If G. sulfurreducens was present on the electrode, U(VI) did not return to solution until the electrode was exposed to dissolved oxygen. This suggeststhat G. sulfurreducens on the electrode reduced U(VI) to U(IV) which was stably precipitated until reoxidized in the presence of oxygen. When an electrode was placed in uranium-contaminated subsurface sediments, U(VI) was removed and recovered from groundwater using poised electrodes. Electrodes emplaced in flow-through columns of uranium-contaminated sediments readily removed U(VI) from the groundwater, and 87% of the uranium that had been removed was recovered from the electrode surface after the electrode was pulled from the sediments. These results suggest that microorganisms can use electrons derived from electrodes to reduce U(VI) and that it may be possible to remove and recover uranium from contaminated groundwater with poised electrodes.     

Microbial reduction of U(VI) at the solid-water interface

Microbial (Geobacter sulfurreducens) reduction of 0.1 mM U(VI) in the presence of synthetic Fe(III) oxides and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids, so that U(VI) reduction was governed by reactions at the solid-water interface. The rate and extent of reduction of U(VI) associated with surfaces of synthetic Fe(III) oxides (hydrous ferric oxide, goethite, and hematite) was comparable to that observed during reduction of aqueous U(VI). In contrast, microbial reduction of U(VI) sorbed to several different natural Fe(III) oxide-containing solids was slower and less extensive compared to synthetic Fe(III) oxide systems. Addition of the electron shuttling agent anthraquinone-2,6-disulfonate (AQDS; 0.1 mM) enhanced the rate and extent of both Fe(III) and U(VI) reduction. These findings suggest that AQDS facilitated electron transfer from G. sulfurreducens to U(VI) associated with surface sites atwhich direct enzymatic reduction was kinetically limited. Our results demonstrate that association of U(VI) with diverse surface sites in natural soils and sediments has the potential to limit the rate and extent of microbial U(VI) reduction and thereby modulate the effectiveness of in situ U(VI) bioremediation.     

Electricity production from cellulose in a microbial fuel cell using a defined binary culture

Microbial fuel cells (MFCs) convert biodegradable materials into electricity, potentially contributing to an array of renewable energy production strategies tailored for specific applications. Since there are no known microorganisms that can both metabolize cellulose and transfer electrons to solid extracellular substrates, the conversion of cellulosic biomass to electricity requires a syntrophic microbial community that uses an insoluble electron donor (cellulose) and electron acceptor (anode). Electricity was generated from cellulose in an MFC using a defined coculture of the cellulolytic fermenter Clostridium cellulolyticum and the electrochemically active Geobacter sulfurreducens. In fed-batch tests using two-chamber MFCs with ferricyanide as the catholyte, the coculture achieved maximum power densities of 143 mW/ m2 (anode area) and 59.2 mW/m2 from 1 g/L carboxymethyl cellulose (CMC) and MN301 cellulose, respectively. Neither pure culture alone produced electricity from these substrates. The coculture increased CMC degradation from 42% to 64% compared to a pure C. cellulolyticum culture. COD removal using CMC and MN301 was 38 and 27%, respectively, with corresponding Coulombic efficiencies of 47 and 39%. Hydrogen, acetate, and ethanol were the main residual metabolites of the binary culture. Cellulose conversion to electricity was also demonstrated using an uncharacterized mixed culture from activated sludge with an aerobic aqueous cathode.     

Experimentally determined uranium isotope fractionation during reduction of hexavalent U by bacteria and zero valent iron

Variations in stable isotope ratios of redox sensitive elements are often used to understand redox processes occurring near the Earth's surface. Presented here are measurements of mass-dependent U isotope fractionation induced by U(VI) reduction by zerovalent iron (Fe0) and bacteria under controlled pH and HCO3- conditions. In abiotic experiments, Fe0 reduced U(VI), but the reaction failed to induce an analytically significant isotopic fractionation. Bacterial reduction experiments using Geobacter sulfurreducens and Anaeromyxobacter dehalogenans reduced dissolved U(VI) and caused enrichment of 238U relative to 235U in the remaining U(VI). Enrichmentfactors (epsilon) calculated using a Rayleigh distillation model are -0.31% per hundred and -0.34% per hundred for G. sulfurreducens and A. dehalogenans, respectively, under identical experimental conditions. Further studies are required to determine the range of possible values for 238U/235U fractionation factors under a variety of experimental conditions before broad application of these results is possible. However, the measurable variations in delta(5238)U show promise as indicators of reduction for future studies of groundwater contamination, geochronology, U ore deposit formation, and U biogeochemical cycling.     

Biotic and abiotic reduction and solubilization of Pu(IV)O₂•xH₂O(am) as affected by anthraquinone-2,6-disulfonate (AQDS) and ethylenediaminetetraacetate (EDTA)

This study measured reductive solubilization of plutonium(IV) hydrous oxide (Pu(IV)O(2)·xH(2)O((am))) with hydrogen (H(2)) as electron donor, in the presence or absence of dissimilatory metal-reducing bacteria (DMRB), anthraquinone-2,6-disulfonate (AQDS), and ethylenediaminetetraacetate (EDTA). In PIPES buffer at pH 7 with excess H(2), Shewanella oneidensis and Geobacter sulfurreducens both solubilized <0.001% of 0.5 mM Pu(IV)O(2)·xH(2)O((am)) over 8 days, with or without AQDS. However, Pu((aq)) increased by an order of magnitude in some treatments, and increases in solubility were associated with production of Pu(III)((aq)). The solid phase of these treatments contained Pu(III)(OH)(3(am)), with more in the DMRB treatments compared with abiotic controls. In the presence of EDTA and AQDS, PuO(2)·xH(2)O((am)) was completely solubilized by S. oneidensis and G. sulfurreducens in ～24 h. Without AQDS, bioreductive solubilization was slower (～22 days) and less extensive (～83-94%). In the absence of DMRB, EDTA facilitated reductive solubilization of 89% (without AQDS) to 98% (with AQDS) of the added PuO(2)·xH(2)O((am)) over 418 days. An in vitro assay demonstrated electron transfer to PuO(2)·xH(2)O((am)) from the S. oneidensis outer-membrane c-type cytochrome MtrC. Our results (1) suggest that PuO(2)·xH(2)O((am)) reductive solubilization may be important in reducing environments, especially in the presence of complexing ligands and electron shuttles, (2) highlight the environmental importance of polynuclear, colloidal Pu, (3) provide additional evidence that Pu(III)-EDTA is a more likely mobile form of Pu than Pu(IV)-EDTA, and (4) provide another example of outer-membrane cytochromes and electron-shuttling compounds facilitating bioreduction of insoluble electron acceptors in geologic environments.     

An electrochemical approach to the studies of biological redox reactions and their applications to biosensors, bioreactors, and biofuel cells

Enzymatic redox reactions for the oxidation (or reduction) of the substrates use organic dyes or metal complexes as electron acceptors (or donors), which can be regenerated by electrochemical reactions of the compounds at an electrode surface. This type of coupling of the enzymatic reactions with the electrochemical reactions is called bioelectrocatalysis and allows us to measure the enzymatic reactions by an electrochemical method. The enzyme-electrochemical method provides a new technique for investigating a variety of biological redox reactions and for applying the reactions to biosensors, bioreactors, and biofuel cells. The bioelectrocatalysis-based research works are described here. First, a new method of protein redox potential measurements and a novel electrochemical kinetic analysis of oxidoreductase reactions are described. Second, a new methodology of characterizing microbial catalytic activities is presented. In the last, applications to the constructions of biosensors, bioreactors, and biofuel cells are mentioned.     

Bioreduction of uranium: environmental implications of a pentavalent intermediate

The release of uranium and other transuranics into the environment, and their subsequent mobility, are subjects of intense public concern. Uranium dominates the inventory of most medium- and low-level radioactive waste sites and under oxic conditions is highly mobile as U(VI), the soluble uranyl dioxocation (UO2)2+. Specialist anaerobic bacteria are, however, able to reduce U(VI)to insoluble U(IV), offering a strategy for the bioremediation of uranium-contaminated groundwater and a potential mechanism for the biodeposition of uranium ores. Despite the environmental importance of U(VI) bioreduction, there is little information on the mechanism of this transformation. In the course of this study we used X-ray absorption spectroscopy (XAS) to show that the subsurface metal-reducing bacterium Geobacter sulfurreducens reduces U(VI) by a one-electron reduction, forming an unstable (UO2)+ species. The final, insoluble U(IV) product could be formed either through further reduction of U(V) or through its disproportionation. When G. sulfurreducens was challenged with the chemically analogous (NpO2)+, which is stable with respect to disproportionation, it was not reduced, suggesting that it is disproportionation of U(V) which leads to the U(IV) product. This surprising discrimination between U and Np illustrates the need for mechanistic understanding and care in devising in situ bioremediation strategies for complex wastes containing other redox-active actinides, including plutonium.     

Novel reduction of mercury (II) by mercury-sensitive dissimilatory metal reducing bacteria

The dissimilatory metal reducing bacterium (DMRB) Shewanella oneidensis MR-1 reduces ionic mercury (Hg[II]) to elemental mercury (Hg[0]) by an activity not related to the MerA mercuric reductase. In S. oneidensis, this activity is constitutive and effective at Hg(II) concentrations too low to induce mer operon functions. Reduction of Hg(II) by MR-1 required the presence of electron donors and electron acceptors. Reduction occurred with oxygen or fumarate, but had the highest rate when ferric oxyhydroxide was used as a terminal electron acceptor. Geobacter sulfurreducens PCA and Geobacter metallireducens GS-15 reduced Hg(II) to Hg(0) with activity comparable to MR-1; however, neither the DMRB Anaeromyxobacter dehalogenans 2CP-C nor the nitrate reducer Pseudomonas stutzeri OX-1 reduced Hg(II) during growth. This discovery of constitutive mercury reduction among anaerobes has implications to the mobilization of mercury and production of methylmercury in anoxic environments.     

Enhanced reduction of Cr(VI) by direct electric current in a contaminated clay

The probable relation between diffuse double-layer processes and redox reactions that enhance degradation or conversion of contaminants under an applied electric field were examined in a clay medium. Kaolinite clay, precontaminated with hexavalent chromium, was the test soil medium. Analyte, containing ferrous iron, was transported through the kaolinite clay using direct electric current. The Cr(VI) reduction to Cr(III) was followed by measuring the soil redox potential and pH at discrete locations in the clay bed. The post-test distribution of Cr showed significantly more Cr(III) than Cr(VI) at low to slightly acidic pH distribution (2 < pH < 6) in clay. The stoichiometric analyses of measured chromium and iron species concentrations versus the measured redox potentials were compared to Nernst equation predictions of an equivalent aqueous system. An average of +0.37 V shift was measured from the linear Nernstian prediction of cell potential. The applied electric field appeared to provide additional "cathodic current" to drive forth the redox reactions. The redox potential shift was explained by possible overpotential development at the clay-water interfaces due to double-layer polarization under the applied field.     

Simultaneous utilization of acetate and hydrogen by Geobacter sulfurreducens and implications for use of hydrogen as an indicator of redox conditions

Dissolved hydrogen concentrations, in conjunction with other geochemical indicators, are becoming an accepted means to determine terminal electron acceptor processes (TEAPs) in groundwater aquifers. Aqueous hydrogen concentrations have been found to fall within specific ranges under methanogenic, sulfate-reducing, iron-reducing, and denitrification conditions. Although hydrogen is gaining in acceptance for determining subsurface TEAPs, there is a dearth of data with regards to the kinetic coefficients for hydrogen utilization in the presence or absence of an additional electron donor under different TEAPs. This study expands the kinetic data for hydrogen utilization through a series of batch experiments, which were conducted to study the utilization of acetate and hydrogen by Geobacter sulfurreducens under iron-reducing conditions. The results of these experiments indicate that the kinetic coefficients (cell yield and first-order degradation rate) describing the rate of hydrogen utilization by G. sulfurreducens under iron-reducing conditions correlate energetically with the coefficients found in previous experiments under methanogenic and sulfate-reducing conditions. In addition, with acetate and hydrogen as simultaneous electron donors, there is slight inhibition between the two electron donors for G. sulfurreducens, and this can be modeled through competitive inhibition terms in the classic Monod formulation. Finally, a key result of this study is that the TEAP-dependent hydrogen concentration in aquifers is not related solely to the microbial kinetics of the hydrogen-consuming organisms as previously suggested but is affected by the multi-substrate kinetics of hydrogen being consumed simultaneously with other electron donors as well as the availability of the electron acceptor.     

图有特殊[a,b]-因子的邻域条件

设G是一个n阶图，设1≤a≤b是整数。设H1和H2是G的任意两个边不交子图，它们分别具有m1和m2条边，以及δ（G）表示最小度。证明：若δ（G）≥a m2，n≥2(a b-m1)(a b-m1-1)/(b-m2)，a≤b-(m1 m2)，并且|NG(x)∪NG(y)|≥an/(a b-m1) 2m2对任意两个不相邻的顶点x和y成立，那么G有[a,b]-因子F使得E(H1)真包含于E(F)和E(H2)∩E(F)=φ。     

Electrochemical analysis of proton and electron transfer equilibria of the reducible moieties in humic acids

Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric E(h) measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials E(h) of HA with increasing pH. This was confirmed by potentiometric E(h)-pH titrations of HA at different redox states. E(h) measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from E(h)(0)* = +0.15 to -0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing E(h). The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions.     

The potential of biodetoxification activity as a probiotic property of Lactobacillus reuteri

Previous work on the metabolism of Lactobacillus reuteri ATCC 55730 anticipated a variability in the use of organic electron acceptors as a means to relieve metabolic redox problems. Therefore, investigations focusing on this unique metabolism of L. reuteri may reveal a basis for new probiotic properties. For instance, L. reuteri may use reactive aldehydes and ketones as electron acceptors to balance their redox metabolism, which opens the possibility to exploit this bacterium for in vivo bioreduction of deleterious compounds in the gastrointestinal tract (GIT). Herein we demonstrate that L. reuteri ATCC 55730 cultures on glucose are able to use furfural (1 g/L), and hydroxymethylfurfural (HMF) (0.5 g/L), as electron acceptors. The former enhances the growth rate by about 25% and biomass yield by 15%, whereas the latter is inhibitory. Furfural is stoichiometrically reduced to furfuryl alcohol by the culture. The conversion of furfural had no effect on the flux distribution between the simultaneously operating phosphoketolase and Embden-Meyerhof pathways, but initiated a flux to acetate production. In addition to furfural and HMF, cellular extracts showed potential to reoxidize NADH and/or NADPH with acrolein, crotonaldehyde, and diacetyl, indicating that conversion reactions take place intracellularly, however, utilization mechanisms for the latter compounds may not be present in this strain. The strain did not reduce other GIT-related reactive compounds, including acrylamide, glyoxal, and furan.