Insoluble Iron Oxides Research Articles

Bio-current as an indicator for biogenic Fe(II) generation driven by dissimilatory iron reducing bacteria

Microbial reduction of insoluble iron minerals by dissimilatory iron reducing bacteria (DIRB) is an important environment process in the iron biogeochemical cycle. We reported that the bio-current generated from oxidation of organic matter by these bacteria in the presence of iron oxides can be used as an indicator for microbial dissolution of insoluble iron oxides. Bioelectrochemical experiments were conducted to investigate the effects of the specific bacteria and the phase identity of iron oxides on bio-current generation by recording the current response as a result of a poised constant potential. Experimental results indicated that the bio-current generation can be greatly enhanced by iron oxide addition under all the conditions varying in the type of pure culture or iron oxide. The increase in the bio-current was linearly correlated with the increased concentration of biogenic Fe(II) detected either by chemical analysis or cyclic voltammetry (CV) tests. This can be understood based on the proposed mechanism that the Fe(II)/Fe(III) couple functions as the electron mediator shuttling electrons from the microbes to the electrodes.

Modularity of the Mtr respiratory pathway of Shewanella oneidensis strain MR‐1

Four distinct pathways predicted to facilitate electron flow for respiration of externally located substrates are encoded in the genome of Shewanella oneidensis strain MR-1. Although the pathways share a suite of similar proteins, the activity of only two of these pathways has been described. Respiration of extracellular substrates requires a mechanism to facilitate electron transfer from the quinone pool in the cytoplasmic membrane to terminal reductase enzymes located on the outer leaflet of the outer membrane. The four pathways share MtrA paralogues, a periplasmic electron carrier cytochrome, and terminal reductases similar to MtrC for reduction of metals, flavins and electrodes or to DmsAB for reduction of dimethyl sulphoxide (DMSO). The promiscuity of respiratory electron transfer reactions catalysed by these pathways has made studying strains lacking single proteins difficult. Here, we present a comprehensive analysis of MtrA and MtrC paralogues in S. oneidensis to define the roles of these proteins in respiration of insoluble iron oxide, soluble iron citrate, flavins and DMSO. We present evidence that some periplasmic electron carrier components and terminal reductases in these pathways can provide partial compensation in the absence of the primary component, a phenomenon described as modularity, and discuss biochemical and evolutionary implications.

Constraints on anaerobic respiration in the hyperthermophilic Archaea Pyrobaculum islandicum and Pyrobaculum aerophilum.

Pyrobaculum islandicum uses iron, thiosulfate, and elemental sulfur for anaerobic respiration, while Pyrobaculum aerophilum uses iron and nitrate; however, the constraints on these processes and their physiological mechanisms for iron and sulfur reduction are not well understood. Growth rates on sulfur compounds are highest at pH 5 to 6 and highly reduced (<-420-mV) conditions, while growth rates on nitrate and iron are highest at pH 7 to 9 and more-oxidized (>-210-mV) conditions. Growth on iron expands the known pH range of growth for both organisms. P. islandicum differs from P. aerophilum in that it requires direct contact with insoluble iron oxide for growth, it did not produce any extracellular compounds when grown on insoluble iron, and it lacked 2,6-anthrahydroquinone disulfonate oxidase activity. Furthermore, iron reduction in P. islandicum appears to be completely independent of c-type cytochromes. Like that in P. aerophilum, NADH-dependent ferric reductase activity in P. islandicum increased significantly in iron-grown cultures relative to that in non-iron-grown cultures. Proteomic analyses showed that there were significant increases in the amounts of a putative membrane-bound thiosulfate reductase in P. islandicum cultures grown on thiosulfate relative to those in cultures grown on iron and elemental sulfur. This is the first evidence of this enzyme being used in either a hyperthermophile or an archaeon. Pyrobaculum arsenaticum and Pyrobaculum calidifontis also grew on Fe(III) citrate and insoluble iron oxide, but only P. arsenaticum could grow on insoluble iron without direct contact.

Respiration and Growth of Shewanella oneidensis MR-1 Using Vanadate as the Sole Electron Acceptor

Shewanella oneidensis MR-1 is a free-living gram-negative gamma-proteobacterium that is able to use a large number of oxidizing molecules, including fumarate, nitrate, dimethyl sulfoxide, trimethylamine N-oxide, nitrite, and insoluble iron and manganese oxides, to drive anaerobic respiration. Here we show that S. oneidensis MR-1 is able to grow on vanadate as the sole electron acceptor. Oxidant pulse experiments demonstrated that proton translocation across the cytoplasmic membrane occurs during vanadate reduction. Proton translocation is abolished in the presence of protonophores and the inhibitors 2-heptyl-4-hydroxyquinoline N-oxide and antimycin A. Redox difference spectra indicated the involvement of membrane-bound menaquinone and cytochromes c, which was confirmed by transposon mutagenesis and screening for a vanadate reduction-deficient phenotype. Two mutants which are deficient in menaquinone synthesis were isolated. Another mutant with disruption in the cytochrome c maturation gene ccmA was unable to produce any cytochrome c and to grow on vanadate. This phenotype could be restored by complementation with the pEC86 plasmid expressing ccm genes from Escherichia coli. To our knowledge, this is the first report of E. coli ccm genes being functional in another organism. Analysis of an mtrB-deficient mutant confirmed the results of a previous paper indicating that OmcB may function as a vanadate reductase or may be part of a vanadate reductase complex.

Assembly of Human Frataxin Is a Mechanism for Detoxifying Redox-Active Iron

Mitochondrial function depends on a continuous supply of iron to the iron-sulfur cluster (ISC) and heme biosynthetic pathways as well as on the ability to prevent iron-catalyzed oxidative damage. The mitochondrial protein frataxin plays a key role in these processes by a novel mechanism that remains to be fully elucidated. Recombinant yeast and human frataxin are able to self-associate in large molecular assemblies that bind and store iron as a ferrihydrite mineral. Moreover, either single monomers or polymers of human frataxin have been shown to serve as donors of Fe(II) to ISC scaffold proteins, oxidatively inactivated [3Fe-4S](+) aconitase, and ferrochelatase. These results suggest that frataxin can use different molecular forms to accomplish its functions. Here, stable monomeric and assembled forms of human frataxin purified from Escherichia coli have provided a tool for testing this hypothesis at the biochemical level. We show that human frataxin can enhance the availability of Fe(II) in monomeric or assembled form. However, the monomer is unable to prevent iron-catalyzed radical reactions and the formation of insoluble ferric iron oxides. In contrast, the assembled protein has ferroxidase activity and detoxifies redox-active iron by sequestering it in a protein-protected compartment.

Involvement of cyclic AMP (cAMP) and cAMP receptor protein in anaerobic respiration of Shewanella oneidensis.

Shewanella oneidensis is a metal reducer that can use several terminal electron acceptors for anaerobic respiration, including fumarate, nitrate, dimethyl sulfoxide (DMSO), trimethylamine N-oxide (TMAO), nitrite, and insoluble iron and manganese oxides. Two S. oneidensis mutants, SR-558 and SR-559, with Tn5 insertions in crp, were isolated and analyzed. Both mutants were deficient in Fe(III) and Mn(IV) reduction. They were also deficient in anaerobic growth with, and reduction of, nitrate, fumarate, and DMSO. Although nitrite reductase activity was not affected by the crp mutation, the mutants failed to grow with nitrite as a terminal electron acceptor. This growth deficiency may be due to the observed loss of cytochromes c in the mutants. In contrast, TMAO reduction and growth were not affected by loss of cyclic AMP (cAMP) receptor protein (CRP). Fumarate and Fe(III) reductase activities were induced in rich medium by the addition of cAMP to aerobically growing wild-type S. oneidensis. These results indicate that CRP and cAMP play a role in the regulation of anaerobic respiration, in addition to their known roles in catabolite repression and carbon source utilization in other bacteria.

Melanin production and use as a soluble electron shuttle for Fe(III) oxide reduction and as a terminal electron acceptor by Shewanella algae BrY.

Dissimilatory metal-reducing bacteria (DMRB) utilize numerous compounds as terminal electron acceptors, including insoluble iron oxides. The mechanism(s) of insoluble-mineral reduction by DMRB is not well understood. Here we report that extracellular melanin is produced by Shewanella algae BrY. The extracted melanin served as the sole terminal electron acceptor. Upon reduction the reduced, soluble melanin reduced insoluble hydrous ferric oxide in the absence of bacteria, thus demonstrating that melanin produced by S. algae BrY is a soluble Fe(III)-reducing compound. In the presence of bacteria, melanin acted as an electron conduit to Fe(III) minerals and increased Fe(III) mineral reduction rates. Growth of S. algae BrY occurred in anaerobic minimal medium supplemented with melanin extracted from previously grown aerobic cultures of S. algae BrY. Melanin produced by S. algae BrY imparts increased versatility to this organism as a soluble Fe(III) reductant, an electron conduit for iron mineral reduction, and a sole terminal electron acceptor that supports growth.

Metabolic diversity in aromatic compound utilization by anaerobic microbes.

A vast array of structurally diverse aromatic compounds is continually released into the environment due to the decomposition of green plants and as a consequence of human industrial activities. Increasing numbers of bacteria that utilize aromatic compounds in the absence of oxygen have been brought into pure culture in recent years. These include most major metabolic types of anaerobic heterotrophs and acetogenic bacteria. Diverse microbes utilize aromatic compounds for diverse purposes. Chlorinated aromatic compounds can serve as electron acceptors in dehalorespiration. Humic substances serve as electron shuttles to enable the use of inorganic electron acceptors, such as insoluble iron oxides, that are not always easily reduced by microbes. Substituents that are attached to aromatic rings may serve as carbon or energy sources for microbes. Examples include acyl side chains and methyl groups. Finally, aromatic compounds can be completely degraded to serve as carbon and energy sources. Routes by which various types of aromatic compounds, including toluene, ethylbenzene, phenol, benzoate, and dihydroxylated compounds, are degraded have been elucidated in recent years. Biochemical strategies employed by microbes to destabilize the aromatic ring in preparation for degradation have become apparent from this work.

Role for Outer Membrane Cytochromes OmcA and OmcB of Shewanella putrefaciens MR-1 in Reduction of Manganese Dioxide

Shewanella putrefaciens MR-1 can use a wide variety of terminal electron acceptors for anaerobic respiration, including certain insoluble manganese and iron oxides. To examine whether the outer membrane (OM) cytochromes of MR-1 play a role in Mn(IV) and Fe(III) reduction, mutants lacking the OM cytochrome OmcA or OmcB were isolated by gene replacement. Southern blotting and PCR confirmed replacement of the omcA and omcB genes, respectively, and reverse transcription-PCR analysis demonstrated loss of the respective mRNAs, whereas mRNAs for upstream and downstream genes were retained. The omcA mutant (OMCA1) resembled MR-1 in its growth on trimethylamine N-oxide (TMAO), dimethyl sulfoxide, nitrate, fumarate, thiosulfate, and tetrathionate and its reduction of nitrate, nitrite, ferric citrate, FeOOH, and anthraquinone-2,6-disulfonic acid. Similarly, the omcB mutant (OMCB1) grew on fumarate, nitrate, TMAO, and thiosulfate and reduced ferric citrate and FeOOH. However, OMCA1 and OMCB1 were 45 and 75% slower than MR-1, respectively, at reducing MnO(2). OMCA1 lacked only OmcA. While OMCB1 lacked OmcB, other OM cytochromes were also missing or markedly depressed. The total cytochrome content of the OM of OMCB1 was less than 15% of that of MR-1. Western blots demonstrated that OMCB1 still synthesized OmcA, but most of it was localized in the cytoplasmic membrane and soluble fractions rather than in the OM. OMCB1 had therefore lost the ability to properly localize multiple OM cytochromes to the OM. Together, the results suggest that the OM cytochromes of MR-1 participate in the reduction of Mn(IV) but are not required for the reduction of Fe(III) or other electron acceptors.

Iron acquisition from hydrous Fe(III)-oxides by an aerobic Pseudomonas sp.

Iron is a metabolic requirement of living systems, yet iron is very insoluble in aerobic, neutral environments. Therefore, the amount of iron in solution under these conditions is not sufficient to meet the nutrient requirements of microorganisms. It has been assumed that microorganisms acquire iron in these environments through the use of specific iron chelating compounds called siderophores. Interestingly, there is little quantitative data in the literature to support this hypothesis. Our studies were initiated to investigate the mechanism(s) used by an aerobic microorganism to acquire iron from a relatively insoluble iron oxide. When iron was supplied in the form of hematite, a Pseudomonas sp. was able to achieve moderate growth, as compared to growth on FeEDTA. Analysis using high-performance liquid chromatography (HPLC) of the metabolic products of this species showed significant differences between growth on hematite as compared to FeEDTA. The results of these experiments will be discussed in terms of our current knowledge of microbial enhanced iron dissolution, and compared to abiotic dissolution rates.

Particulates in hydrometallurgy: Part I. Characterization of laterite acid leach residues

Two different laterite samples, designated as Ore No. 1 and Ore. No. 2, respectively, were leached with sulfuric acid in the temperature range of 25°C to 275°C, in order to evaluate the effects of ore type and leaching conditions (e.g., temperature and solid/liquid ratio) on the settling behavior of the leach residues. The initial solids, as well as the leach residues, were characterized by chemical and physical techniques such as X-ray diffraction, electrophoretic mobility, particle size, and chemical analyses. Spectrochemical and X-ray analyses determined that Ore No. 1 was a limonitic-type (iron oxide-rich ore) material, whereas Ore No. 2 was a transitional laterite ore (silicate-rich, iron oxidecontaining material). By means of X-ray diffraction analysis, it was found that the conversion of goethite to hematite was enhanced as the temperature and/or leaching time was increased. Moreover, it was determined by chemical analysis that as the leaching temperature increased, the concentrations of Ni and Co progressively increased in solution, whereas the impurities (Fe and Al) rose through a maximum and then decreased. This behavior is attributed to the reprecipitation of iron and aluminum from solution as hydrolysis products of hematite and aluminum sulfates. The particle size studies revealed a particle size growth with temperature increase, which is attributed to the deposition of reprecipitation products onto the surface of the solid residues. Investigations of the surface chemistry of the coated particlesvia the electrophoretic mobility technique revealed that the 275°C leach residues (Ore No. 2, pzc ∼2) exhibited a point of zero charge (pzc) near pH 2, which is probably due to the redeposition of the dissolved silica. In contrast, the 250°C leach residues (Ore No. 1, iron oxide-rich ore, pzc∼2) had a pzc value of about 3.8. This shift in mobility to less acidic values suggests that in this case, the insoluble iron and aluminum oxide products on the particles dominate the surface chemistry.

Reclaimed sewage water: a hydroponic growth medium for plants

Reclaimed water from a sewage plant at Calabasas, California was a satisfactory source of nutrients, except for iron, for growing crop plants in hydroponic culture under pilot plant commercial conditions. When solutions were renewed and adequate nutrients were present, commercial culture of a number of glasshouse crop plants could be grown without toxicity from salt or other materials. The crops were grown at energy savings compared with standard crop culture in that it was not necessary to use manufactured fertilizers. To avoid adding additional soluble nutrients to the sewage water, iron was supplied either as foliar spray or as insoluble iron oxide applied to plant roots before transplanting. The latter procedure was most satisfactory. Acidification of the water to pH 5 with sulfuric acid also overcame the iron deficiency, but failed to make the plants grow because the sulfuric acid increased the availability of other trace metals to the point of excess. After the water had been used in the hydroponic tanks, much of the copper, zinc, manganese, iron, nickel, lead, and some of the nitrogen, phosphorus, and potassium had been stripped from the water.

The Effect of Particle Size on the Regional Deposition of Inhaled Aerosols in the Human Respiratory Tract

Abstract The effect of particle size on the regional deposition of aerosols inhaled through the mouth was determined in 93 studies on 34 subjects. The test aerosols were spherical monodisperse insoluble iron oxide particles (specific gravity 2.5) containing radioactive tags, ranging in median unit density diameter from 2.1 to 12.5-microns (σ ≅ 1.08). Particles deposited on the bronchial tree were translocated to the stomach by mucociliary clearance which was essentially complete within the first day. The proportion of the initial lung burden of radioactive particles removed during the first 24 hours provided a functional measure of tracheo-bronchial deposition. A portion of the inhaled aerosol was deposited in the head by impaction. As an impactor, the tracheobronchial tree is more efficient. For each individual subject, head and tracheobronchial deposition increased with increasing particle size. Alveolar depositions decreased with size for particles larger than 4-microns.

The effect of particle size on the regional deposition of inhaled aerosols in the human respiratory tract.

Abstract The effect of particle size on the regional deposition of aerosols inhaled through the mouth was determined in 93 studies on 34 subjects. The test aerosols were spherical monodisperse insoluble iron oxide particles (specific gravity 2.5) containing radioactive tags, ranging in median unit density diameter from 2.1 to 12.5-microns (σ ≅ 1.08). Particles deposited on the bronchial tree were translocated to the stomach by mucociliary clearance which was essentially complete within the first day. The proportion of the initial lung burden of radioactive particles removed during the first 24 hours provided a functional measure of tracheo-bronchial deposition. A portion of the inhaled aerosol was deposited in the head by impaction. As an impactor, the tracheobronchial tree is more efficient. For each individual subject, head and tracheobronchial deposition increased with increasing particle size. Alveolar depositions decreased with size for particles larger than 4-microns.

A compact electric-motor driven spinning disc aerosol generator.

Abstract The design and performance characteristics of a spinning disc aerosol generator are presented and discussed. Two generators of the type described have been in use for about two years for the production of monodisperse insoluble iron oxide particles containing various nonleaching radioactive tags. Aerosols with median diameters between 1 and 10 microns and a geometric standard deviation averaging 1.08 have been produced routinely for studies of aerosol deposition in the respiratory tract. The generator operates from a 110 volt ac line, has a low (5.5 cfm) volumetric output and convenient external adjustment features.

The iron ores of Cerro Bolivar, Venezuela

The ores were formed by leaching of silica and residual concentration of insoluble iron oxides in extensively deformed, recrystallized ferruginous cherts of the Precambrian Imataca series. Physical types include so-called crustal ores, consisting of hematite grains in a matrix of secondary goethite and characterized by a banded structure inherited from the bedding of the iron formations, and friable ores, porous aggregates of hematite grains which extend downward below a transitional zone of semifriable, platy ores to depths of as much as 800 feet. Sampling of spring waters which issue from levels at or below the lower margins of the ore bodies and other evidence indicate that silica is being removed at the rate of about nine tons per year, that iron ore is being formed at the rate of 18 tons per year, and that the amount of ore now present could have been formed (under present-day climatic conditions) in only 20 million years, or since Oligocene time.

Protein Digestion in the Human Stomach.

The primary function of the stomach being to digest protein, it would be desirable to be able to determine the actual accomplishment of the stomach in health and disease as far as protein digestion is concerned, instead of relying on acidity determinations as an indication. Determinations of the ratio of soluble to insoluble protein in gastric contents following a test meal might give useful information provided that the gastric contents remained a uniform suspension, and provided solid and liquid left the stomach at the same rate. On the other hand, if insoluble iron oxide were mixed with the flour and baked into the bread used as a test meal and if this oxide adhered to the undissolved portion of the gastric contents, the ratio of undissolved protein to iron would enable one to determine the percentage of protein of the test meal that had been put in solution by the gastric juice, and such a result would be fairly accurate whether or not there were stratification of gastric contents or selective evacuat...