The mineral-respiring bacterium Shewanella oneidensis uses a protein complex, MtrCAB, composed of two decaheme cytochromes, MtrC and MtrA, brought together inside a transmembrane porin, MtrB, to transport electrons across the outer membrane to a variety of mineral-based electron acceptors. A proteoliposome system containing a pool of internalized electron carriers was used to investigate how the topology of the MtrCAB complex relates to its ability to transport electrons across a lipid bilayer to externally located Fe(III) oxides. With MtrA facing the interior and MtrC exposed on the outer surface of the phospholipid bilayer, the established in vivo orientation, electron transfer from the interior electron carrier pool through MtrCAB to solid-phase Fe(III) oxides was demonstrated. The rates were 10 3 times higher than those reported for reduction of goethite, hematite, and lepidocrocite by S. oneidensis, and the order of the reaction rates was consistent with those observed in S. oneidensis cultures. In contrast, established rates for single turnover reactions between purified MtrC and Fe(III) oxides were 10 3 times lower. By providing a continuous flow of electrons, the proteoliposome experiments demonstrate that conduction through MtrCAB directly to Fe(III) oxides is sufficient to support in vivo, anaerobic, solid-phase iron respiration.mineral respiration | multiheme cytochromes | proteoliposome

The multi-heme, outer membrane c-type cytochrome (c-Cyt) OmcB of Geobacter sulfurreducens was previously proposed to mediate electron transfer across the outer membrane. However, the underlying mechanism has remained uncharacterized. In G. sulfurreducens, the omcB gene is part of two tandem four-gene clusters, each is predicted to encode a transcriptional factor (OrfR/OrfS), a porin-like outer membrane protein (OmbB/OmbC), a periplasmic c-type cytochrome (OmaB/OmaC) and an outer membrane c-Cyt (OmcB/OmcC) respectively. Here, we showed that OmbB/OmbC, OmaB/OmaC and OmcB/OmcC of G. sulfurreducens PCA formed the porin-cytochrome (Pcc) protein complexes, which were involved in transferring electrons across the outer membrane. The isolated Pcc protein complexes reconstituted in proteoliposomes transferred electrons from reduced methyl viologen across the lipid bilayer of liposomes to Fe(III)-citrate and ferrihydrite. The pcc clusters were found in all eight sequenced Geobacter and 11 other bacterial genomes from six different phyla, demonstrating a widespread distribution of Pcc protein complexes in phylogenetically diverse bacteria. Deletion of ombB-omaB-omcB-orfS-ombC-omaC-omcC gene clusters had no impact on the growth of G. sulfurreducens PCA with fumarate but diminished the ability of G. sulfurreducens PCA to reduce Fe(III)-citrate and ferrihydrite. Complementation with the ombB-omaB-omcB gene cluster restored the ability of G. sulfurreducens PCA to reduce Fe(III)-citrate and ferrihydrite.

U(VI) doped hematite was synthesized and exposed to two different organic reductants with E(0) of 0.23 and 0.70 V. A combination of HAADF-TEM and EXAFS provided evidence that uranium was incorporated in hematite in uranate, likely octahedral coordination. XPS indicated that structurally incorporated U(VI) was reduced to U(V), whereas non-incorporated U(VI) was reduced to U(IV). Specifically, the experiments indicate that U(V) was the dominant oxidation state of uranium in hematite around Eh -0.24 to -0.28 V and pH 7.7-8.6 for at least up to 5 weeks of reaction time. U(V), but not U(IV), was also detected in hematite at Eh +0.21 V (pH 7.1-7.3). The results support the hypothesis, based on previous experimental and theoretical work, that the stability field of U(V) is widened relative to U(IV) and U(VI) in uranate coordination environments where the coordination number of U is less than 8.

Flavins are secreted by the dissimilatory iron-reducing bacterium Shewanella and can function as endogenous electron transfer mediators. To assess the potential importance of flavins in Fe(III) bioreduction, we investigated the redox reaction kinetics of reduced flavin mononucleotide (i.e., FMNH(2)) and reduced riboflavin (i.e., RBFH(2)) with ferrihydrite and lepidocrocite. The organic reductants rapidly reduced and dissolved ferrihydrite and lepidocrocite in the pH range 4-8. The rate constant k for 2-line ferrihydrite reductive dissolution by FMNH(2) was 87.5 ± 3.5 M(-1)·s(-1) at pH 7.0 in batch reactors, and k was similar for RBFH(2). For lepidocrocite, k was 500 ± 61 M(-1)·s(-1) for FMNH(2) and 236 ± 22 M(-1)·s(-1) for RBFH(2). The surface area normalized initial reaction rates (r(a)) were between 0.08 and 77 μmol·m(-2)·s(-1) for various conditions in stopped-flow experiments. Initial rates (r(o)) were first-order with respect to iron(III) oxide concentration, and r(a) increased with decreasing pH. Poorly crystalline 2-line ferrihydrite yielded the highest r(a), followed by more crystalline 6-line ferrihydrite and crystalline lepidocrocite. Compared to a previous whole-cell study with Shewanella oneidensis strain MR-1, our findings suggest that the reduction of electron transfer mediators by the Mtr (i.e., metal-reducing) pathway coupled to lactate oxidation is rate limiting, rather than heterogeneous electron transfer to the iron(III) oxide.

Pb(IV)O2(s, plattnerite) nanoparticle aggregrates in aqueous suspension are readily reduced by Mn2+(aq) and Fe2+(aq) between pH 3.0 and 8.5, yielding pb2+(aq) and adsorbed Pb". Fe2+(aq) oxidation generates Fe(III) (hydr)oxides that impede Pb(IV) reduction, especially at pH > or =5. Under acidic conditions, production of dissolved Fe(III) may also be significant. Mn2+(aq) oxidation generates mixed Mn(III)/Mn(IV) (hydr)oxides that are less of an impediment to Pb(IV) reduction. Adding both Fe2+(aq) and Mn2+(aq) can set in motion a Mn redox cycle that catalyzes PbO2(s) reduction by Fe2+(aq). Reaction with Fe2+(aq) and Mn2+(aq) can affect subsequent reactions with natural organic matter. Hydroquinone was employed as a representative organic reductant. Both hydroquinone and its two-electron oxidation product p-benzoquinone are readily quantified by HPLC. Adding Fe2+(aq) before hydroquinone greatly diminishes p-benzoquinone production. Adding Mn2+(aq) before hydroquinone has little effect on p-benzoquinone production. Free chlorine residual variations in premise plumbing can setthe stage for the reactions documented here.

Natural organic matter (NOM) is a diverse collection of molecules, each possessing its own reductant, complexant, and adsorption properties. Here, we are interested in the ability of NOM to bring about the reductive dissolution of Pb(IV)O2(s). Adding the coagulants FeCl3 or Al2(SO4)3 followed by membrane filtration is one way to remove a subset of NOM molecules from surface water samples. Another is to pass water samples through a granular activated carbon (GAC) column. Results from applying these treatments to Great Dismal Swamp water (DSW) and Nequasset Bog Water (NBW) can best be explained as follows: (i) GAC column treatment is more efficient at removing the NOM fraction most responsible for reductive dissolution. (ii) Coagulation/filtration, with either coagulant, is most efficient at removing a second, inhibitory fraction. Inhibition may arise from (i) adsorption at the mineral/water interface, which blocks approach of reductant molecules and (ii) a micelle-like aggregate nature, which provides hydrophobic pockets that capture reductantmolecules, again keeping them away from the mineral/water interface. Hypotheses regarding reductant and inhibitory fractions are further evaluated using representative low-molecular-weight compounds. Substituted hydroquinones are used as mimics of the reductant fraction, and malonic acid, quinic acid, trehalose, alginic acid, and polygalacturonic acid are used as mimics of the inhibitory fraction.

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Reductive dissolution of hematite in porous media was investigated using a micromodel (8.1 × 4.5 × 0.028 mm) with realistic pore network structures that include distinctive advection domain, macropores and micropores created in silicon substrate. The micromodel pore surface was sputter deposited with a thin layer (230 nm) of hematite. The hematite in the micromodel was reduced by injecting pH-varying solutions (pH 5.0, 6.0, 7.0) containing a reduced form of flavin mononucleotide (FMNH2, 100 μM), a biogenic soluble electron transfer mediator produced by Shewanella species. The reduction kinetics was determined by measuring effluent Fe(II) (aq) concentration and by spectroscopically monitoring the hematite dissolution front in the micromodel. Batch experiment was also performed to estimate the hematite reduction rate under the well-mixed condition. Results showed significant spatial variation in local redox reaction rate that was controlled by the coupled transport and reaction. The overall rate of the redox reaction in the micromodel required a three-domain numerical model to effectively describe reaction kinetics either with distinctive apparent rate parameters or mass transfer coefficients in different pore domains. Results from this study demonstrated the feasibility of a domain-based modeling approach for scaling reaction rates from batch to porous media systems where reactions may be significantly limited by transport.