Mechanism Of Iron Reduction Research Articles

Study on Magnetization Roasting Kinetics of High-Iron and Low-Silicon Red Mud.

High-iron and low-silicon red mud is not only an alkaline solid waste from Bayer process alumina production, but it is also a very important secondary iron resource. Magnetization roasting is considered as an effective and typical method for the iron recovery and removal of impurities in red mud. In this work, based on the characteristics of large specific surface area and high porosity of red mud, the kinetics of magnetization roasting and phase transformation of red mud were studied. Thermodynamic analysis results show that the reduction of iron oxide in red mud is more easily promoted by CO as reducing agent at low roasting temperature. The reduction reaction is prone to overreduction, and fayalite and ferrospinel can be formed in the reaction system. The phase transformation and iron reduction mechanism during the roasting process were evaluated. Most of hematite and goethite in the red mud decomposed in the process of magnetization roasting, released CO2, and transformed into strongly magnetic magnetite. The reaction process has some characteristics controlled by homogeneous reaction. The process of magnetization roasting reduction with CO was controlled by the hybrid control dynamics model, and the apparent activation energy was 38.31 kJ·mol-1.

Hydrogen on softening-melting and slag forming behavior under the operation of blast furnace with iron coke charging

The effect on the slag formation behavior in the softening melting zone of a hydrogen-rich blast furnace is investigated by charging iron coke to the iron ore and controlling different volume fractions of hydrogen. The reduction mechanism of iron coke to iron ore under hydrogen-rich conditions is expounded that iron coke and hydrogen increase the concentration of CO around iron ore and promote its further reduction. The participation of iron coke and hydrogen promotes that FeO is reduced to Fe in iron ore, which prompts the production of high melting point substances in slag. The melting point of slag iron increases the melting start temperature. Hydrogen and iron coke promote the carburization of dripping iron so that molten iron drips quickly, which narrows the melting drop interval. From the perspective of the maximum pressure difference and the comprehensive permeability S value, the blast furnace permeability will be improved.

Iron (oxyhydr)oxides shift the methanogenic community in deep sea methanic sediment - insights from long-term high-pressure incubations

Intermittent increases of dissolved ferrous iron concentrations have been observed in deep marine methanic sediments which is different from the traditional diagenetic electron acceptor cascade, where iron reduction precedes methanogenesis. Here we aimed to gain insight into the mechanism of iron reduction and the associated microbial processes in deep sea methanic sediment by setting up long-term high-pressure incubation experiments supplemented with ferrihydrite and methane. Continuous iron reduction was observed during the entire incubation period. Intriguingly, ferrihydrite addition shifted the archaeal community from the dominance of hydrogenotrophic methanogens (Methanogenium) to methylotrophic methanogens (Methanococcoides). The enriched samples were then amended with 13C-labeled methane and different iron (oxyhydr)oxides in batch slurries to test the mechanism of iron reduction. Intensive iron reduction was observed, the highest rates with ferrihydrite, followed by hematite and then magnetite, however, no anaerobic oxidation of methane (AOM) was observed in any treatment. Further tests on the enriched slurry showed that the addition of molybdate decreased iron reduction, suggesting a link between iron reduction with sulfur cycling. This was accompanied by the enrichment of microbes capable of dissimilatory sulfate reduction and sulfur/thiosulfate oxidation, which indicates the presence of a cryptic sulfur cycle in the incubation system with the addition of iron (oxyhydr)oxides. Our work suggests that under low sulfate conditions, the presence of iron (oxyhydr)oxides would trigger a cascade of microbial reactions, and iron reduction could link with the microbial sulfur cycle, changing the kinetics of the methanogenesis process in methanic sediment.

Ferric Iron Reduction in Extreme Acidophiles.

Biochemical processes are a key element of natural cycles occurring in the environment and enabling life on earth. With regard to microbially catalyzed iron transformation, research predominantly has focused on iron oxidation in acidophiles, whereas iron reduction played a minor role. Microbial conversion of ferric to ferrous iron has however become more relevant in recent years. While there are several reviews on neutrophilic iron reducers, this article summarizes the research on extreme acidophilic iron reducers. After the first reports of dissimilatory iron reduction by acidophilic, chemolithoautotrophic Acidithiobacillus strains and heterotrophic Acidiphilium species, many other prokaryotes were shown to reduce iron as part of their metabolism. Still, little is known about the exact mechanisms of iron reduction in extreme acidophiles. Initially, hypotheses and postulations for the occurring mechanisms relied on observations of growth behavior or predictions based on the genome. By comparing genomes of well-studied neutrophilic with acidophilic iron reducers (e.g., Ferroglobus placidus and Sulfolobus spp.), it became clear that the electron transport for iron reduction proceeds differently in acidophiles. Moreover, transcriptomic investigations indicated an enzymatically-mediated process in Acidithiobacillus ferrooxidans using respiratory chain components of the iron oxidation in reverse. Depending on the strain of At. ferrooxidans, further mechanisms were postulated, e.g., indirect iron reduction by hydrogen sulfide, which may form by disproportionation of elemental sulfur. Alternative scenarios include Hip, a high potential iron-sulfur protein, and further cytochromes. Apart from the anaerobic iron reduction mechanisms, sulfur-oxidizing acidithiobacilli have been shown to mediate iron reduction at low pH (< 1.3) under aerobic conditions. This presumably non-enzymatic process may be attributed to intermediates formed during sulfur/tetrathionate and/or hydrogen oxidation and has already been successfully applied for the reductive bioleaching of laterites. The aim of this review is to provide an up-to-date overview on ferric iron reduction by acidophiles. The importance of this process in anaerobic habitats will be demonstrated as well as its potential for application.

Role of a perhydroxyl radical in the chelator-mediated Fenton reaction

Differences in the ring substitution of dyhydroxybenzenes, modifies the iron reduction mechanism and are important to understand variations in the oxygen radical production profile at low H2O2.

Transcriptomics analysis of the metabolic mechanisms of iron reduction induced by sulfate reduction mediated by sulfate-reducing bacteria.

Sulfate-reducing bacteria (SRB) play an important role in sulfur, iron and carbon cycling. The majority of studies have illustrated the role of SRB in biogeochemical cycling in pure cultures. In this study, we established three SRB enrichment cultures (designated HL, NB and WC) from different paddy soils and conducted a transcriptomic analysis of their metabolic characteristics under sulfate and sulfate-free conditions. In the HL cultures, there was no sulfate consumption but ferrihydrite was reduced. This indicated that bacteria in the HL samples can reduce ferrihydrite and preferentially utilize ferrihydrite as the electron acceptor in the absence of both ferrihydrite and sulfate. Sulfate consumption was equal in the NB and the WC cultures, although more ferrihydrite was reduced in the NB cultures. Transcriptomics analysis showed that (i) upregulation of O-acetylserine sulfhydrylase gene expression indicating sulfate assimilation in the WC samples; (ii) the energy conservation trithionate pathway is commonly employed by SRB and (iii) sulfate not only enhanced iron reduction by its conversion to sulfide but also promoted enzymatic electron transfer via c-type cytochromes.

Iron acquisition in Pseudomonas aeruginosa by the siderophore pyoverdine: an intricate interacting network including periplasmic and membrane proteins

Pyoverdine (PVDI) has been reported to act both as a siderophore for scavenging iron (a key nutrient) and a signaling molecule for the expression of virulence factors. This compound is itself part of a core set of virulence factors produced by Pseudomonas aeruginosa during infections. Once secreted into the bacterial environment and having scavenged ferric iron, PVDI-Fe3+ is taken back into the P. aeruginosa periplasm via the outer membrane transporters FpvAI and FpvB. Iron release from PVDI in the bacterial periplasm involves numerous proteins encoded by the fpvGHJKCDEF genes and a mechanism of iron reduction. Here, we investigated the global interacting network between these various proteins using systematic bacterial two-hybrid screening. We deciphered a network of five interacting proteins composed of two inner-membrane proteins, FpvG (iron reductase) and FpvH (unknown function), and three periplasmic proteins, FpvJ (unknown function), FpvF (periplasmic PVDI-binding protein), and FpvC (iron periplasmic-binding protein). This interacting network strongly suggests the existence of a large protein machinery composed of these five proteins, all playing a role in iron acquisition by PVDI. Furthermore, we discovered an interaction between the periplasmic siderophore binding protein FpvF and the PvdRT-OpmQ efflux pump, also suggesting a role for FpvF in apo-PVDI recycling and secretion after iron delivery. These results highlight a multi-protein complex that drives iron release from PVDI in the periplasm of P. aeruginosa.

Crystal structure of the Siderophore-interacting protein SIP from Aeromonas hydrophila

Siderophores acquire iron from hosts under iron-limiting conditions and play an essential role in the survival of microorganisms. Siderophore-interacting proteins (SIPs) from microbes release iron from the siderophore complex by reducing ferric iron to ferrous iron, but the molecular mechanism of iron reduction remains unclear. To better understand the molecular mechanism of SIPs, we herein report the crystal structure of Aeromonas hydrophila SIP (AhSIP) in complex with flavin adenine dinucleotide (FAD) as a cofactor. AhSIP consists of an N-terminal FAD binding domain and a C-terminal NADH binding domain, which are connected by a linker region. AhSIP showed unique structural differences in the orientation of the cofactor binding lobes when compared with SIP homologs. This study identified a cluster of three basic residues (Lys48, His259 and Arg262) in AhSIP distributed around a potential substrate binding pocket. In addition, AhSIP, containing the NADH binding motif E(L)VL-X3-GE, belongs to the group I subfamily. Our results show the diverse cofactor and substrate binding sites of the SIP family.

Photo-induced redox coupling of dissolved organic matter and iron in biochars and soil system: Enhanced mobility of arsenic

Dissolved organic matter (DOM) elucidated from biochars enhances the dissolution of iron oxides and reduction of iron. However, given that reduction mechanism of iron (Fe(III)) in the practical biochar applications for soil amendment and environmental remediation have not been fully elucidated, this study laid great emphasis on the photo-induced Fe(II) liberated from DOM-Fe(III) complexes. Thus, pyrolysis of biomass was carried out at 300 °C to maximize DOM release from biochars. Moreover, three different biomass samples (rice straw (R), granular sludge (G) from an anaerobic digester, and spent coffee grounds (C)) were chosen as carbon substrates for biochars preparation. To demonstrate the transformation of Fe(III), 1 and 5 wt% biochar was applied to the clean (S1) and arsenic-contaminated (S2) soil with/without the light. The results indicate that the light condition produces more Fe(II). The amount of Fe(II) accounts for 25.3, 28.6, and 30.7% of total iron under the light with 5 wt% GB, RB, and CB in S1, and 10.6, 13.1, and 13.8% in S2. This study demonstrates that Fe(II) is generated more under ultraviolet irradiation than visible light and dark condition. In addition, a control experiment without biochar showed that DOM plays an important role in the reduction of Fe(III). The mobility of arsenic increased under the light condition since the intermediates of DOM photo-degradation accelerates the dissolution of iron oxides and arsenic competes with DOM for the adsorption. Therefore, there was no significant correlation between the elution of arsenic and the formation of Fe(II) during the reductive dissolution of iron oxide under the light condition.

Molecular dissection of a putative iron reductase from Desulfotomaculum reducens MI-1

Desulfotomaculum reducens MI-1 is a Firmicute strain capable of reducing a variety of heavy metal ions and has a great potential in heavy metal bioremediation. We recently identified Dred_2421 as a potential iron reductase through proteomic study of D. reducens. The current study examines its iron-reduction mechanism. Dred_2421, like its close homolog from Escherichia coli (2, 4-dienoyl-CoA reductase), has an FMN-binding N-terminal domain (NTD), an FAD-binding C-terminal domain (CTD), and a 4Fe–4S cluster between the two domains. To understand the mechanism of the iron-reduction activity and the role of each domain, we generated a series of variants for each domain and investigated their iron-reduction activity. Our results suggest that CTD is the main contributor of the iron-reduction activity, and that NTD and the 4Fe–4S cluster are not directly involved in such activity. This study provides a mechanistic understanding of the iron–reductase activity of Dred_2421 and may also help to elucidate other physiological activities this enzyme may have.

The role of Fe(III) bioreduction by methanogens in the preservation of organic matter in smectite

Organic matter in sediments is largely associated with clay minerals and can be preserved over geological time. However, microbial activity can possibly influence this association and release organic matter from clay minerals via reductive or oxidative dissolution of clay minerals. In this study, the relationship between bioreduction of structural Fe(III) in smectite and organic matter release from smectite structure was investigated. A model organic compound, 12-aminolauric acid (ALA) was intercalated into the interlayer region of an iron-rich smectite (nontronite, NAu-2). Two methanogens: mesophilic Methanosarcina mazei and thermophilic Methanothermobacter thermautotrophicus were selected to reduce structural Fe(III) in ALA-intercalated nontronite. As a comparison, sodium dithionite was used to chemically reduce structural Fe(III) in the same mineral. The results showed that the intercalation of ALA into the nontronite interlayer decreased both the rate and the extent of Fe(III) bioreduction. Furthermore, methanogenesis was more inhibited by the presence of intercalated ALA in the nontronite structure relative to pure nontronite. After the bioreduction, the intercalated ALA was partially released, and the extent of release was positively correlated with the extent of Fe(III) reduction. A low reduction extent of bioreduction (<30%) resulted in little ALA release, whereas a nearly complete chemical reduction by sodium dithionite released all intercalated ALA. SEM observations and aqueous chemistry data suggested that reductive dissolution was a main mechanism for the observed ALA release. Because naturally prevalent biological reduction is a slow process with a low reduction extent relative to chemical reduction, the results of this study demonstrated that organic matter preserved within smectite structure should not be released by the mechanism of iron reduction.

Peculiarities of the iron reduction mechanism in Fe-Al-K system

Temperature-programmed reduction was used in combination with measurements of magnetization to determine the peculiarities of iron reduction in the Fe-K-Al system. It was found that reduction by hydrogen proceeds with the formation of metallic iron through the stage of magnetite formation (Fe3O4); the effective activation energies are 63 and 39 kJ/mol for the I and II stages, respectively. It was shown that substituting carbon oxide for hydrogen leads to iron reduction proceeding only to the stage of magnetite formation (EFe3O4 = 94 kJ/mol). The magnetite interacts with CO to produce carbide (presumably Hegge carbide Fe2C). Iron reduction in the synthesis gas occurs with the preferential participation of hydrogen or carbon dioxide, depending on the rate of temperature rise.

Changes of metabolic responses to direct and induced Fe deficiency of two Pisum sativum cultivars

The aim of this work was to investigate the effect of Fe deficiency (whether direct or bicarbonate-induced) on the glucose metabolism, by determining the activities of several enzymes (PK, PFK, G3PDH and G6PDH), together with other enzymes involved in the organic acids metabolism (PEPC, MDH, CS and ICDH) in two cultivars of Pisum sativum (Kelvedon and Lincoln, respectively tolerant and sensitive to Fe deficiency). Moreover, we studied the effects of Fe deficiency on iron reduction mechanism and proton extrusion (FCR and H +-ATPase activity, respectively), in addition to the concentrations in roots and exudates of some organic compounds (malate, citrate and phenols). Three treatments were used: (+Fe), complete medium (CM) containing 30 μM Fe; (−Fe), direct deficiency, CM without iron; (+Bic.), indirect deficiency, CM containing 30 μM Fe + lime. Our results showed that all glycolytic enzymes activities increased in the extracts of Fe-deficient roots of both cultivars when compared to the control. The above increases in the activity were particularly evident for Fe-deficient roots of Kelvedon. FCR and H +-ATPase activity was stimulated by Fe deficiency in both cultivars. PEPC activity increased by 254% in root extracts of Kelvedon particularly when grown in the absence of Fe. MDH, CS and ICDH activities showed a marked increase in the −Fe treated roots of both cultivars. The bicarbonate treatment resulted in a significant induction of root MDH and CS activities only in Kelvedon cultivar. In leaves, CS activity increased significantly in Fe deficient plants of both cultivars. However, Fe deficiency (whether direct or indirect) had no effect on the leaf activity of MDH and ICDH. Citrate concentration increased in both cultivars, particularly in Kelvedon (187% and 50% of the control, respectively in the −Fe and +bicarbonate treatments). Higher concentrations of phenols were observed in Fe-deficient roots of both cultivars than in the control plants. The increase in phenol concentration in roots was greater in Kelvedon than in Lincoln. We demonstrate that the metabolic responses of the two cultivars are differently affected by Fe deficiency both direct and induced by bicarbonate supply. The effective modulation of glycolytic and organic acid metabolisms may account for the better tolerance of Kelvedon when compared to Lincoln.

Physiological and biochemical responses for two cultivars of Pisum sativum (“Merveille de Kelvedon” and “Lincoln”) to iron deficiency conditions

The aim of this work was to compare the tolerance and the physiological responses to Fe deficiency of two Pisum sativum cultivars (“Merveille de kelvedon” and “Lincoln”) commonly cultivated in Tunisia. We studied the effects of Fe deficiency on: (i) chlorophyll content, relative growth rate (RGR) and Fe status, (ii) rhizosphere acidification, (iii) changes under Fe-deficient conditions in the activities of two root enzymes, the first related to the proton extrusion (H +-ATPase) and the second to iron reduction mechanism (Fe(III)-chelate-reductase: FCR). Three treatments were used: C, control, complete nutrient solution (CNS) containing 30 μM Fe; DD, direct deficiency, CNS without iron; ID, indirect deficiency, CNS containing 30 μM Fe + lime. Fe deficiency led to a significant decrease of chlorophyll content in both cultivars. The below reduction was observed in Fe-deficient plants of Merveille de Kelvedon. In addition, relative growth of shoots and whole plant was not affected by Fe deficiency. H +-ATPase and FCR activities were more stimulated in Merveille de Kelvedon than in Lincoln, under DD and ID Fe deficiency. The capacity of this cultivar to maintain plant growth and to preserve adequate chlorophyll synthesis under iron-limiting conditions is related to its better Fe-use efficiency, in addition to its high acidification and root reducing capacities. This allows us to suggest that Merveille de Kelvedon is more effective in overcoming Fe deficiency than Lincoln.

Variation and inheritance of iron reductase activity in the roots of common bean (Phaseolus vulgaris L.) and association with seed iron accumulation QTL

BackgroundIron deficiency anemia is a global problem which often affects women and children of developing countries. Strategy I plants, such as common bean (Phaseolus vulgaris L.) take up iron through a process that involves an iron reduction mechanism in their roots; this reduction is required to convert ferric iron to ferrous iron. Root absorbed iron is critical for the iron nutrition of the plant, and for the delivery of iron to the shoot and ultimately the seeds. The objectives of this study were to determine the variability and inheritance for iron reductase activity in a range of genotypes and in a low × high seed iron cross (DOR364 × G19833), to identify quantitative trait loci (QTL) for this trait, and to assess possible associations with seed iron levels.ResultsThe experiments were carried out with hydroponically grown plants provided different amounts of iron varying between 0 and 20 μM Fe(III)-EDDHA. The parents, DOR364 and G19833, plus 13 other cultivated or wild beans, were found to differ in iron reductase activity. Based on these initial experiments, two growth conditions (iron limited and iron sufficient) were selected as treatments for evaluating the DOR364 × G19833 recombinant inbred lines. A single major QTL was found for iron reductase activity under iron-limited conditions (1 μM Fe) on linkage group b02 and another major QTL was found under iron sufficient conditions (15 μM Fe) on linkage group b11. Associations between the b11 QTL were found with several QTL for seed iron.ConclusionsGenes conditioning iron reductase activity in iron sufficient bean plants appear to be associated with genes contributing to seed iron accumulation. Markers for bean iron reductase (FRO) homologues were found with in silico mapping based on common bean synteny with soybean and Medicago truncatula on b06 and b07; however, neither locus aligned with the QTL for iron reductase activity. In summary, the QTL for iron reductase activity under iron limited conditions may be useful in environments where beans are grown in alkaline soils, while the QTL for iron reductase under sufficiency conditions may be useful for selecting for enhanced seed nutritional quality.

Study on mechanism of iron reduction in magnesium alloy melt

This article studies the mechanism of iron reduction in magnesium alloy melt by investigating the iron concentrations in different parts in the melt with different holding time. The iron concentrations at the top, the center and the bottom of the melt are measured with the holding time of 300, 1800 and 7200 s. The elements in the sludge settling down in the crucible are also determined by ICP. With the increasing holding time, the iron content in the upper part of the melt decreases and that in the lower part of the melt increases. And the iron content in the sludge increases rapidly. The iron concentrations in the melt and the sludge change little with longer holding time than 1800 s. Among the three iron reduction agents B2O3, MnCl2 and TiO2, B2O3 has the highest iron reduction efficiency (IRE). IRE of B2O3 is as two times and five times as IRE of MnCl2 and IRE of TiO2 respectively. Formation of FeB and settling of it into melting sludge are believed to be the primary mechanism of iron reduction by B2O3 processing. The mechanism of iron reduction in magnesium melt by MnCl2 or TiO2 processing is studied by thermodynamic analysis. Formation of substance containing MnFeAl or TiFe with high density and high melting point and settling of them into melting sludge are suggested to be the mechanism of iron reduction by MnCl2 or TiO2 processing.

Effects of Purification Fluxes Containing Boride on AZ91 Alloy

Purification fluxes JDMR-1 containing B2O3 and JDMR-2 containing Na2B4O7 were developed for magnesium alloy. Density, viscosity and melting points of the two fluxes were measured. In the magnesium alloy iron concentration decreased from 0.024% to 0.004% and 0.001%, and average volume fraction of nonmetallic inclusions from 2.06% to 0.86% and 0.23% after JDMR-1 and JDMR-2 processing, respectively. Corrosion resistance and tensile properties of the magnesium alloy were greatly improved. Iron reduction mechanism was studied thermodynamically.

Wettability of Graphite by CaO-SiO2-Al2O3-FeO-MgO Slag

Wettability of graphite by the CaO-SiO 2 -Al 2 O 3 -FeO-MgO slag has been investigated using the sessile drop technique. The focus of this paper is on the carbon/slag interfacial phenomena, which has a significant effect on the kinetics and mechanisms of iron reduction from the molten slag by carbon in the direct iron smelting process. In this study wettability of graphite by slags of different compositions has been investigated at 1600°C. Experiments were performed in a horizontal superkanthal tube furnace. Slag droplets on a graphite substrate were placed into the hot isothermal zone of the furnace and examined using a CCD camera. The camera assembly was connected to a video cassette recorder through a video distributor amplifier. The reaction sequence during the experiment was recorded on video tape. The images from the tape were converted into computer files and the wettability of graphite by slag was determined by direct measurement of the interfacial contact angle using a sophisticated computer software. An on-line date/time generator permitted image capturing as a function of time. The results indicated significant effect of slag composition on the slag/graphite interfacial interactions. The change in the wetting in the reactive system is predominantly affected by the Gibbs free energy of the reaction. In the initial moment, when iron oxide concentration is high, wetting is good. As the slag graphite system approaches equilibrium the effect of the reactive component decreases and wettability declines. The product iron formed as a result of the reduction reaction was seen to be deposited inside the graphite within a distance of 30-50 μm from the graphite/slag interface. In addition, lumps of iron are also deposited at the slag/graphite interface.

Chemical and biological mobilization of Fe(III) in marsh sediments

Iron reduction in marine sulfitic environments may occur via a mechanism involving direct bacterial reduction with the use of hydrogen as an electron donor, direct bacterial reduction involving carbon turnover, or by indirect reduction where sulfide acts to reduce iron. In the presented experiments, the relative importance of direct and indirect mechanisms of iron reduction, and the contribution of these two mechanisms to overall carbon turnover has been evaluated in two marsh environments. Sediments collected from two Northeastern US salt marshes each having different Fe (III) histories were incubated with the addition of reactive iron (as amorphous oxyhydroxide). These sediments were either incubated alone or in conjunction with sodium molybdate. Production of both inorganic and organic pore water constituents and a calculation of net carbon production were used as measures to compare the relative importance of direct bacterial reduction and indirect bacterial reduction. Results indicate that in the environments tested, the majority of the reduced iron found results from indirect reduction mediated by hydrogen sulfide, a result of dissolution and precipitation phenomena, or is a result of direct bacterial reduction using hydrogen as an electron donor. Direct iron reduction plays a minor role in carbon turnover in these environments.

Mechanism of iron reduction by roots of Phaseolus vulgaris L.

Abstract Iron‐deficient roots of Phaseolus vulgaris L. can reduce electron acceptors such as ferricyanide, dichlorophenolindophenol and phenazine methosulfate at increased rates compared to iron‐sufficient control plants. This indicates a trans‐plasma membrane electron transfer for Fe(III) reduction. The high Fe(III) reduction capacity can be completely inhibited by Nitro‐Blue Tetrazolium salt (Nitro‐BT) with concomitant formation of the blue reduced precipitate within the epidermis cells of the youngest root parts. Root respiration is not impaired by Nitro‐BT. The intracellular Nitro‐BT reduction can largely be prevented by excess extracellular Fe(III)EDTA. Iron‐sufficient roots reduce Nitro‐BT at a much slower rate. We conclude that cytosolic NADH or NADPH is the cellular electron donor for the extracellular Fe(III) reduction.