Initial Reduction Rate Research Articles

Kinetic Analysis of Microbial Reduction of Fe(III) in Nontronite

Microbial reduction of structural Fe(III) in nontronite (NAu-2) was studied in batch cultures under non-growth condition using Shewanella putrefaciens strain CN32. The rate and extent of structural Fe(III) reduction was examined as a function of electron acceptor [Fe(III)] and bacterial concentration. Fe(ll) sorption onto NAu-2 and CN32 surfaces was independently measured and described by the Langmuir expression with the affinity constant (log K) of 3.21 and 3.30 for NAu-2 and bacteria, respectively. The Fe(II) sorption capacity of NAu-2 decreased with increasing NAu-2 concentration, suggesting a particle aggregation effect. An empirical equation for maximum sorption capacity was derived from the sorption isotherms as a function of NAu-2 concentration. The total reactive surface concentration of Fe(III) was proposed as a proxy for the "effective" or bioaccessible Fe(III) concentration. The initial rate of microbial reduction was first-order with respect to the effective Fe-(III) concentration. A kinetic biogeochemical model was assembled that incorporated the first-order rate expression with respect to the effective Fe(III) concentration, Fe(II) sorption to cell and NAu-2 surfaces, and the empirical equation for maximum sorption capacity. The model successfully described the experimental results with variable NAu-2 concentration. The initial rate of microbial reduction of Fe(III) in NAu-2 increased with increasing cell concentration from 10(2) up to approximately 10(8) cells/mL, and then leveled off with further increase. A saturation-type kinetics with respect to cell concentration was required to describe microbial reduction of Fe(III) in NAu-2 as a function of cell concentration. Overall, our results indicated that the kinetics of microbial reduction of Fe(III) in NAu-2 can be modeled at variable concentration of key variables (clay and cell concentration) by considering the surface saturation, Fe(II) production, and its sorption to NAu-2 and cell surfaces.

Reduction of pigment dispersions by Shewanella strain J18 143

In order to develop a biotechnological solution to the disposal of waste pigment dispersions, the ability of azo dye-reducing Shewanella strain J18 143 cells to reduce model pigment dispersions was assessed. Resting cells were able to couple the oxidation of formate to the reduction of the azo/ketohydrazone chromophore in virtually insoluble pigmentary species. Pigments from the azoacetoacetanilide range and the azonaphthol range were used to investigate the effects of process parameters such as dispersion quality, addition of biocides, temperature and use of exogenous extracellular redox mediators. The physical impact of bioreduction on the pigment structure was also assessed. Pigments present in industrially manufactured dispersions were more readily reduced than those in powder form and the presence of a biocide in the dispersion did not affect the activity of cells of Shewanella strain J18 143. The initial pigment reduction rate increased with temperature up to 50 °C, but the higher temperatures had a detrimental effect on the long term activity of Shewanella strain J18 143. The reduction of the pigment dispersions was stimulated by the addition of the soluble electron shuttle anthraquinone-2,6-disulphonate. Particle sizing and environmental scanning electron microscopy showed that Shewanella strain J18 143 was able to degrade large pigment aggregates to produce individual pigment particles. GemSperse Orange EX5 was the pigment dispersion reduced to the corresponding amines most efficiently by Shewanella strain J18 143.

Dynamics of smooth muscle cell deadhesion from thermosensitive hydroxybutyl chitosan

Thermoresponsive polymer (TRP) enables the enzyme-free harvesting of cells through an acute increase in surface hydrophilicity of TRP across its lower critical solution temperature (LCST), rendering feasible the generation of polymer-free cell sheets for regenerative medicine applications. To date, the intricate mechanisms of cell deadhesion/detachment on TRP surface remain obscure. Elucidation of such biophysical responses would be valuable for the cell sheet technology. In this study, integrative biophysical techniques are applied to probe the thermal-induced deadhesion kinetics of smooth muscle cell (SMC) on thermoresponsive hydroxybutyl chitosan (HBC29) against different periods of pre-culture time at 37 °C. Atomic force microscopy demonstrates that both the surface topography and mechanical property of HBC29 film in water are acutely modulated across its LCST. Firstly, cells show negligible changes in adhesion contact area during low-temperature incubation on unmodified tissue culture polystyrene (TCPS). Secondly, the recession of adhesion contact and retraction of cell body for cells with different pre-culture times are triggered by HBC29 coating on TCPS. Interestingly, the initial rate of reduction in the normalized adhesion contact area of SMC is negatively correlated with the pre-culture time. Thirdly, the degree of cell deformation and average adhesion energy are reducing functions of time only for SMCs with the lowest pre-culture time. In contrast, adhesion energy per cell is a reducing function of time irrespective of the change of pre-culture time. Lastly, the temporal dynamics of cytoskeleton organization and β-actin/smoothelin-B mRNA expression for SMCs is strongly dependent on the pre-culture time. Overall, this study demonstrates that the thermal-induced deadhesion of SMC on TRP is characterized by the evolution of its contractile phenotypes.

Hexavalent chromium reduction by Acinetobacter haemolyticus isolated from heavy-metal contaminated wastewater

Possible application of a locally isolated environmental isolate, Acinetobacter haemolyticus to remediate Cr(VI) contamination in water system was demonstrated. Cr(VI) reduction by A. haemolyticus seems to favour the lower concentrations (10–30 mg/L). However, incomplete Cr(VI) reduction occurred at 70–100 mg/L Cr(VI). Initial specific reduction rate increased with Cr(VI) concentrations. Cr(VI) reduction was not affected by 1 or 10 mM sodium azide (metabolic inhibitor), 10 mM of PO 4 3−, SO 4 2−, SO 3 2−, NO 3 − or 30 mg/L of Pb(II), Zn(II), Cd(II) ions. However, heat treatment caused significant dropped in Cr(VI) reduction to less than 20% only. A. haemolyticus cells loses its shape and size after exposure to 10 and 50 mg Cr(VI)/L as revealed from TEM examination. The presence of electron-dense particles in the cytoplasmic region of the bacteria suggested deposition of chromium in the cells.

Method of Rapid Assessment of Photocatalytic Activities of Self-Cleaning Films

An ink, comprising the redox dye resazurin (Rz) and the sacrificial electron donor glycerol, is shown to be capable of the rapid assessment of the photocatalytic activities of self-cleaning films. In the key initial stage of photocatalysis the ink changes from blue to pink. Prolonged irradiation bleaches the ink and eventually mineralizes it. The kinetics of the initial photoinduced color change is studied as a function of UV irradiance, [glycerol], [Rz], and temperature. The results reveal an apparent approximate quantum yield of 3.5 x 10(-3) and an initial rate, r(i), which increases with [glycerol] and decreases with [Rz]. It is proposed that the reduction of Rz, dispersed throughout the thick (ca. 590 nm) indicator film, may take place either via the diffusion of the dye molecules in the ink film to the surface of the underlying semiconductor layer and their subsequent reaction with photogenerated electrons and/or via the diffusion of alpha-hydroxyalkyl radicals, produced by the oxidation of the glycerol by photogenerated holes, or hydroxy radicals, away from the surface of the semiconductor into the ink film and their subsequent reaction with the dye molecules therein. The decrease in r(i) with [Rz] appears to be due to dimer formation, with the latter impeding the reduction process. The activation energy for the initial color-change process is low, ca. 9.1 +/- 0.1 kJ mol(-1) and not unlike many other photocatalytic processes. The initial rate of dye reduction appears to be directly related to the rate of destruction of stearic acid. The ink can be applied by spin-coating, stamping, or writing, using a felt-tip pen. The efficacy of such an ink for assessing the photocatalytic activity of any photocatalytic film, including those employed on commercial self-cleaning glasses, tiles, and paving stones, is discussed briefly.

Reduction of Fe(III) colloids by Shewanella putrefaciens: A kinetic model

A kinetic model for the microbial reduction of Fe(III) oxyhydroxide colloids in the presence of excess electron donor is presented. The model assumes a two-step mechanism: (1) attachment of Fe(III) colloids to the cell surface and (2) reduction of Fe(III) centers at the surface of attached colloids. The validity of the model is tested using Shewanella putrefaciens and nanohematite as model dissimilatory iron reducing bacteria and Fe(III) colloidal particles, respectively. Attachment of nanohematite to the bacteria is formally described by a Langmuir isotherm. Initial iron reduction rates are shown to correlate linearly with the relative coverage of the cell surface by nanohematite particles, hence supporting a direct electron transfer from membrane-bound reductases to mineral particles attached to the cells. Using internally consistent parameter values for the maximum attachment capacity of Fe(III) colloids to the cells, M max, the attachment constant, K P, and the first-order Fe(III) reduction rate constant, k, the model reproduces the initial reduction rates of a variety of fine-grained Fe(III) oxyhydroxides by S. putrefaciens. The model explains the observed dependency of the apparent Fe(III) half-saturation constant, K m ∗ , on the solid to cell ratio, and it predicts that initial iron reduction rates exhibit saturation with respect to both the cell density and the abundance of the Fe(III) oxyhydroxide substrate.

Geochemical and microbiological controls on dissimilatory iron reduction

Recent experimental studies permit development of conceptual and quantitative models of microbial Fe(III) oxide reduction at circumneutral pH that can be compared to and contrasted with established models of abiotic mineral dissolution. The findings collectively support a model for controls on enzymatic reduction that differs fundamentally from those applied to abiotic reductive dissolution as a result of two basic phenomena: (1) the relatively minor influence of oxide mineralogical and thermodynamic properties on rates of enzymatic reduction compared to abiotic reductive dissolution, and (2) the major limitation which sorption and/or surface precipitation of biogenic Fe(II) on residual oxide and Fe(III)-reducing bacterial cell surfaces poses to enzymatic electron transfer in the presence of excess electron donor. Parallel studies with two well-characterized Fe(III)-reducing organisms ( Shewanella putrefaciens and Geobacter sulfurreducens) lead to common conclusions regarding the importance of these phenomena in regulating the rate and long-term extent of Fe(III) oxide reduction. Models in which rates of enzymatic reduction are limited by Fe(III)-reducing bacterial cell density together with the abundance of ‘available’ oxide surface sites (as controlled by oxide surface area and the accumulation of surface-bound biogenic Fe(II)) provide an adequate macroscopic description of controls on the initial rate and long-term extent of oxide reduction. To cite this article: E.E. Roden, C. R. Geoscience 338 (2006).

Reduction of Cr(VI) by immobilized cells of Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776

Hexavalent chromium, a carcinogen and mutagen, can be reduced to Cr(III) by Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776. This study examined Cr(VI) reduction by immobilized cells of the two strains in a common solution matrix using various entrapment matrices. Chitosan and PVA-borate beads did not retain integrity and supported low or no reduction of Cr(VI) by the cells. A commercial preparation (Lentikats) was stable but also did not support Cr(VI) reduction. K-carrageenan beads were stable in batch suspensions but gel integrity was lost after only 5 h in a flow-through system in the presence of 100 microM Cr(VI). The best immobilization matrices were agar and agarose, where the initial rates of reduction of Cr(VI) (from 500 microM solution) for D. vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776 were 127 (agar) and 130 (agarose), and 15 (agar) and 12 (agarose) nmol h(-1) mg dry cell wt(-1), respectively. The higher removal of Cr(VI) by D. vulgaris was also seen in 14-mL packed-bed flow-through columns, where, at a flow rate of 2.4 mL h(-1), the percentage removal of Cr(VI) was approximately 95% and 60% for D. vulgaris and Microbacterium sp., respectively (agar-immobilized cells). The Cr(VI) reducing activities of D. vulgaris and Microbacterium sp. were lost after 159 and 140 h, respectively. Examination of the beads for structural integrity within the columns in situ using magnetic resonance imaging after 24 and 100 h of continuous operation against Cr(VI) (with negligible Cr retained within the columns) showed that agar beads were more stable with time. The most appropriate system for development of a continuous bioprocess is thus the use of D. vulgaris NCIMB 8303 immobilized in an agar gel matrix.

Ceramic Micro/Nanoparticle Size Evolution in Wet Grinding in Stirred Ball Mill

Micro- and nanostructured ceramic materials have received increasing attention in light of the attainable mechanical properties of the resulting components, parts, and products. Stirred ball mill grinding is an important process in reducing the size of ceramic micro- and/or nanoparticles to a desirable range to be used as a constituent for micro- and nanostructured materials. In this study, the time change of particle size of titanium dioxide TiO2 micro- and nanoparticles in the stirred ball mill grinding process is characterized with a fracture mechanics analysis combined with a population balance model. The approach provides both the mean and the statistical distribution of particle sizes produced by ball grinding. It also yields an estimate for the amount of time necessary to achieve a desired particle size. The model examines the effects of process parameters, including the grinding speed, the viscosity of the suspending fluid, and the concentration of the feed as input variables. Experiments performed with TiO2 suspended in ethylene glycol are used for comparison to model predictions for validation. The results show that the initial particle-size reduction rate is relatively high, however, as the particle size decreases, the time required for further reduction increases significantly. Good agreement exists between the model predictions and the experimental results in the context of micro- and nanoparticle-size reduction trends.

Dissolution of phosphate in a phosphorus-enriched ultisol as affected by microbial reduction.

Phosphorus dissolution often increases as soils become more reduced, but the mechanisms are not fully understood. The objectives of this research were to determine rates and mechanisms of P dissolution during microbial reduction of a surface soil from the North Carolina Coastal Plain. Duplicate suspensions of silt + clay fractions from a Cape Fear sandy clay loam (fine, mixed, semiactive, thermic Typic Umbraquult) were reduced in a continuously stirred redox reactor for 40 d. We studied the effects of three treatments on P dissolution: (i) 2 g dextrose kg(-1) solids added as a microbial carbon source at time 0 d; (ii) 2 g dextrose kg(-1) solids split into three additions at 0, 12, and 26 d; and (iii) no added dextrose. After 40 d of reduction, concentrations of dissolved reactive phosphorus (DRP) were similar for all treatments and increased up to sevenfold from 1.5 to 10 mg L(-1). The initial rate of reduction and dissolution of DRP was significantly greater for the 0-d treatment. A linear relationship (R(2) = 0.79) was found between DRP and dissolved organic carbon (DOC). Dissolved Fe and Al and pH increased, suggesting the formation of aqueous Fe- and Al-organic matter complexes. Separate batch experiments were performed to study the effects of increasing pH and citrate additions on PO(4) dissolution under aerobic conditions. Increasing additions of citrate increased concentrations of DRP, Fe, and Al, while increasing pH had no effect. Results indicated that increased dissolved organic matter (DOM) during soil reduction contributed to the increase in DRP, perhaps by competitive adsorption or formation of aqueous ternary DOM-Fe-PO(4) or DOM-Al-PO(4) complexes.

Highly efficient dendrimer-based mimic of glutathione peroxidase.

In this communication, we report the synthesis of the three generations of Fréchet-type poly(aryl ether) dendrimers with a diselenide core that demonstrate generation-dependent glutathione peroxidase (GPx) activity with initial reduction rates as high as 2431.20 muM min-1 for the third-generation product, around 1400 times faster than Ebselen. It represents a successful example of using a dendrimer as a model for a GPx mimic.

The rate of the CD8-dependent initial reduction in tumor volume is not limited by contact-dependent perforin, Fas ligand, or TNF-mediated cytolysis.

Established EG7 tumors expressing OVA and growing at an intradermal site become rapidly reduced in size following adoptive therapy with in vitro-generated type I CD8 T cell (Tc1) effectors generated from naive CD8 T cells from transgenic TCR OVA-specific mice. Tc1 effectors kill EG7 target cells in vitro by a perforin-dependent mechanism. However, we show that there is no quantitative diminution of the initial phase of antitumor activity in vivo, whether the Tc1 effectors are derived from perforin-, Fas ligand-, or TNF-deficient transgenic TCR mice or whether the recipients are perforin deficient. Tumors are also equally well controlled whether the Tc1 effectors come from mice deficient in perforin plus Fas ligand or perforin plus TNF. Control of tumor growth is diminished when Tc1 effectors generated from IFN-gamma-deficient mice are used. We conclude that control of tumor growth is not in any way affected by loss of contact-mediated lytic mechanisms, and conclude that the CD8 effectors must act by recruiting host effector mechanisms to control tumor growth.

Manganese oxide reactivity in North Sea sediments

Bulk Mn oxide dissolution rates were determined in dilute acid (HCl, pH 3, proton-assisted dissolution) and in saturating ascorbic acid (pH 3, proton-assisted plus reductive dissolution) for North Sea sediments from three sites characterised by different energy regimes, one at the Frisian Front, one in the German Bight, and one in the Skagerrak. Profiles of extractable manganese (0.1 N hydrochloric acid, 1 N hydrochloric acid, citrate-dithionite-bicarbonate buffer and hydroxylamine hydrochloride in acetic acid) are presented for the three sites. The assemblage of sedimentary Mn oxides was described as a reactive continuum with a gamma distribution of Mn oxide reactivities. Initial reduction rates were highest for the oxidised sediment samples. The reactivity of the Mn assemblages during reductive dissolution in ascorbic acid decreased by up to ∼9 orders of magnitude. The largest heterogeneity in Mn reactivity of the oxidised sediment samples was found at the German Bight site, while the most homogeneous Mn assemblage was found at the Skagerrak site. The large range in sedimentary Mn oxide reactivity may explain the great variability in values for kinetic coefficients used to describe Mn reduction. Considering the broad spectrum of Mn oxide reactivities, overlap of the Mn reduction zone with that of Fe oxides is expected.

Dehydroascorbate influences the plant cell cycle through a glutathione-independent reduction mechanism.

Glutathione is generally accepted as the principal electron donor for dehydroascorbate (DHA) reduction. Moreover, both glutathione and DHA affect cell cycle progression in plant cells. But other mechanisms for DHA reduction have been proposed. To investigate the connection between DHA and glutathione, we have evaluated cellular ascorbate and glutathione concentrations and their redox status after addition of dehydroascorbate to medium of tobacco (Nicotiana tabacum) L. cv Bright Yellow-2 (BY-2) cells. Addition of 1 mm DHA did not change the endogenous glutathione concentration. Total glutathione depletion of BY-2 cells was achieved after 24-h incubation with 1 mm of the glutathione biosynthesis inhibitor l-buthionine sulfoximine. Even in these cells devoid of glutathione, complete uptake and internal reduction of 1 mm DHA was observed within 6 h, although the initial reduction rate was slower. Addition of DHA to a synchronized BY-2 culture, or depleting its glutathione content, had a synergistic effect on cell cycle progression. Moreover, increased intracellular glutathione concentrations did not prevent exogenous DHA from inducing a cell cycle shift. It is therefore concluded that, together with a glutathione-driven DHA reduction, a glutathione-independent pathway for DHA reduction exists in vivo, and that both compounds act independently in growth control.

Periplasmic cytochrome c3 of Desulfovibrio vulgaris is directly involved in H2-mediated metal but not sulfate reduction.

Kinetic parameters and the role of cytochrome c(3) in sulfate, Fe(III), and U(VI) reduction were investigated in Desulfovibrio vulgaris Hildenborough. While sulfate reduction followed Michaelis-Menten kinetics (K(m) = 220 micro M), loss of Fe(III) and U(VI) was first-order at all concentrations tested. Initial reduction rates of all electron acceptors were similar for cells grown with H(2) and sulfate, while cultures grown using lactate and sulfate had similar rates of metal loss but lower sulfate reduction activities. The similarities in metal, but not sulfate, reduction with H(2) and lactate suggest divergent pathways. Respiration assays and reduced minus oxidized spectra were carried out to determine c-type cytochrome involvement in electron acceptor reduction. c-type cytochrome oxidation was immediate with Fe(III) and U(VI) in the presence of H(2), lactate, or pyruvate. Sulfidogenesis occurred with all three electron donors and effectively oxidized the c-type cytochrome in lactate- or pyruvate-reduced, but not H(2)-reduced cells. Correspondingly, electron acceptor competition assays with lactate or pyruvate as electron donors showed that Fe(III) inhibited U(VI) reduction, and U(VI) inhibited sulfate loss. However, sulfate reduction was slowed but not halted when H(2) was the electron donor in the presence of Fe(III) or U(VI). U(VI) loss was still impeded by Fe(III) when H(2) was used. Hence, we propose a modified pathway for the reduction of sulfate, Fe(III), and U(VI) which helps explain why these bacteria cannot grow using these metals. We further propose that cytochrome c(3) is an electron carrier involved in lactate and pyruvate oxidation and is the reductase for alternate electron acceptors with higher redox potentials than sulfate.

Ontogenic differences in human liver 4-hydroxynonenal detoxification are associated with in vitro injury to fetal hematopoietic stem cells

4-hydroxynonenal (4HNE) is a highly mutagenic and cytotoxic α,β-unsaturated aldehyde that can be produced in utero during transplacental exposure to prooxidant compounds. Cellular protection against 4HNE injury is provided by alcohol dehydrogenases (ADH), aldehyde reductases (ALRD), aldehyde dehydrogenases (ALDH), and glutathione S-transferases (GST). In the present study, we examined the comparative detoxification of 4HNE by aldehyde-metabolizing enzymes in a panel of adult and second-trimester prenatal liver tissues and report the toxicological ramifications of ontogenic 4HNE detoxification in vitro. The initial rates of 4HNE oxidation and reduction were two- to fivefold lower in prenatal liver subcellular fractions as compared to adult liver, and the rates of GST conjugation of 4HNE were not detectable in either prenatal or adult cytosolic fractions. GSH-affinity purification of hepatic cytosol yielded detectable and roughly equivalent rates of GST–4HNE conjugation for the two age groups. Consistent with the inefficient oxidative and reductive metabolism of 4HNE in prenatal liver, cytosolic fractions prepared from prenatal liver exhibited a decreased ability to protect against 4HNE-protein adduct formation relative to adults. Prenatal liver hematopoietic stem cells (HSC), which constitute a significant percentage of prenatal liver cell populations, exhibited ALDH activities toward 4HNE, but little reductive or conjugative capacity toward 4HNE through ALRD, ADH, and GST. Cultured HSC exposed to 5 μM 4HNE exhibited a loss in viability and readily formed one or more high molecular weight 4HNE-protein adduct(s). Collectively, our results indicate that second trimester prenatal liver has a lower ability to detoxify 4HNE relative to adults, and that the inefficient detoxification of 4HNE underlies an increased susceptibility to 4HNE injury in sensitive prenatal hepatic cell targets.

A novel antioxidant, octyl caffeate, suppression of LPS/IFN-γ-induced inducible nitric oxide synthase gene expression in rat aortic smooth muscle cells

In the present study, we investigated the effects and mechanisms of a novel potent antioxidant, octyl caffeate, on the induction of iNOS expression by lipopolysaccharide (LPS) and interferon-γ (IFN-γ) in cultured primary rat aortic smooth muscle cells (RASMCs) in vitro and LPS-induced hypotension in vivo. Octyl caffeate (0.1–1.0 μM) exerted a concentration-dependent inhibition of iron-catalyzed lipid peroxidation in rat brain homogenates. Furthermore, octyl caffeate (20, 50, and 100 μM) concentration-dependently diminished the initial rate of superoxide-induced NBT reduction and the enzymatic activity of xanthine oxidase. It also concentration-dependently (1–50 μM) inhibited the NO production, iNOS protein and messenger RNA expressions upon stimulation by LPS (100 μg/mL)/IFN-γ (100 U/mL) in RASMCs. In addition, we found that octyl caffeate did not significantly affect IκBα degradation stimulated by LPS/IFN-γ in RASMCs. On the other hand, octyl caffeate (10 and 50 μM) significantly suppressed activation of c-Jun-N-terminal kinase and extracellular signal-regulated kinase. Moreover, octyl caffeate (10 mg/kg, i.v.) significantly inhibited the fall in mean arterial pressure stimulated by LPS (7.5 mg/kg) in rats. In conclusion, we demonstrate that a novel potent antioxidant, octyl caffeate, significantly ameliorates circulatory failure of endotoxemia in vivo by a mechanism involving suppression of iNOS expression through inactivation of mitogen-activated protein kinases in RASMCs.

Fe(III) Oxide Reactivity Toward Biological versus Chemical Reduction

Initial rates of biological (Shewanella putrefaciens strain CN32, pH 6.8) and chemical (ascorbate, pH 3.0) reduction of synthetic Fe(III) oxides with a broad range of crystallinity and specific surface area were examined to assess how variations in these properties are likely to influence the kinetics of bacterial Fe(III) oxide reduction in heterogeneous natural Fe(III) oxide assemblages. The results indicate that bacterial Fe(III) oxide reduction does not respond strongly to oxide crystal thermodynamic properties (ΔGf) which exert a significant impact on the kinetics of abiotic reductive dissolution. These findings suggest that oxide mineral heterogeneity in natural soils and sediments is likely to affect initial rates of bacterial reduction (e.g. during the early stages of anaerobic metabolism following the onset of anoxic conditions) mainly via an influence on reactive surface site density and that inferences regarding the competitiveness of bacterial Fe(III) oxide reduction as a pathway for organic matter oxidation in anoxic environments cannot be based on assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.

Effects of SiO2 and Al2O3 on the Lattice Parameter and CO Gas Reduction of CaO-containing Dense Wustite.

The effects of SiO2 and Al2O3 on the lattice parameter and the initial reduction rate of CaO-containing wustite were studied at 1 273 K. Wustite plates containing 0.5-2 mass% CaO, 0.2-2 mass% SiO2 and 1 mass% Al2O3 were prepared at the mass fractions of CaO/SiO2=0.5-5. The lattice parameter of CaO–SiO2- containing wustite decreased with decreasing CaO/SiO2 ratio at a constant CaO content. This corresponded to a decrease of the dissolved content of CaO in the wustite due to a formation of CaO-FeO-SiO2 compounds. Similarly, the dissolved content of CaO decreased in CaO-SiO2-Al2O3-containing wustite. The process of CO gas reduction within the wustite phase field was traced with the lattice parameter measurement of the partially reduced samples and was analyzed with a mixed-control model of chemical reaction at the surface and the solid state diffusion in the wustite. The chemical reaction rate constant, k´, decreased with increasing the amount of SiO2 addition at a constant CaO content. The dependence of k´ on SiO2 (and Al2O3) addition was in good accordance with the change in the dissolved content of CaO in the wustite owing to the formation of the compounds containing CaO. The inter-diffusion coefficient, D, tended to decrease slightly with increasing the amount of compound.

Anhydrite precipitation and the extent of hydrothermal recharge zones at ocean ridge crests

We develop mathematical models to investigate anhydrite precipitation in hydrothermal recharge zones at ocean ridge axes. We first consider constant flow during hydrothermal recharge to estimate the depth, the depth interval over which anhydrite precipitation would occur, and the initial rate of porosity reduction. We find that the smaller the downward mass flux, the shallower and more broadly distributed the region of precipitation. For values of downward mass flux similar to that for high‐temperature black smoker systems, the porosity would be clogged significantly in a matter of months to years depending upon the initial porosity. We then consider anhydrite precipitation in a buoyancy‐driven single‐pass model in which the reduction in permeability resulting from anhydrite precipitation exerts a feedback on the flow rate. This model gives results similar to the constant flow model for the initial depth, depth interval, and rate of porosity reduction. The effect of the permeability clogging on the flow rate, however, is twofold. First, the zone of anhydrite precipitation shallows and broadens as the flow rate decreases, thus allowing deeply deposited anhydrite to be preserved temporarily. Second, if the recharge area of the same size as the discharge area, anhydrite precipitation rapidly reduces the flow rate and the heat output of the hydrothermal system. If the initial porosity of the recharge zone is 1%, the area of the recharge zone must be ∼10–100 times that of the discharge zone to maintain near steady state conditions for time periods of decades to hundreds of years. This result implies that recharge zones may be quite large and extend off axis as well as along axis.