Isolation of iron-oxidizing bacteria from corroded concretes of sewage treatment plants

Thirty-six strains of iron-oxidizing bacteria were isolated from corroded concrete samples obtained at eight sewage treatment plants in Japan. All of the strains isolated grew autotrophically in ferrous sulfate (3.0%), elemental sulfur (1.0%) and FeS (1.0%) media (pH 1.5). Washed intact cells of the 36 isolates had activities to oxidize both ferrous iron and elemental sulfur. Strain SNA-5, a representative of the isolated strains, was a gram-negative, rod-shaped bacterium (0.5-0.6x0.9-1.5 microm). The mean G+C content of its DNA was 55.9 mol%. The pH and temperature optima for growth were 1.5 and 30 degrees C, and the bacterium had activity to assimilate 14CO2 into the cells when ferrous iron or elemental sulfur was used as a sole source of energy. These results suggest that SNA-5 is Thiobacillus ferrooxidans strain. The pHs and numbers of iron-oxidizing bacteria in corroded concrete samples obtained by boring to depths of 0-1, 1-3, and 3-5 cm below the concrete surface were respectively 1.4, 1.7, and 2.0, and 1.2 x 10(8), 5 x 10(7), and 5 x 10(6) cells/g concrete. The degree of corrosion in the sample obtained nearest to the surface was more severe than in the deeper samples. The findings indicated that the levels of acidification and corrosion of the concrete structure corresponded with the number of iron-oxidizing bacteria in a concrete sample. Sulfuric acid produced by the chemolithoautotrophic sulfur-oxidizing bacterium Thiobacillus thiooxidansis known to induce concrete corrosion. Since not only T. thiooxidans but also T. ferrooxidans can oxidize reduced sulfur compounds and produce sulfuric acid, the results strongly suggest that T. ferrooxidans as well as T. thiooxidans is involved in concrete corrosion.     

Cytochrome c oxidase purified from a mercury-resistant strain of Acidithiobacillus ferrooxidans volatilizes mercury

We suggested in our previous study that the plasma membrane cytochrome c oxidase of the mercury-resistant iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans, SUG 202, is involved in Fe2+-dependent mercury volatilization. To study the involvement of A. ferrooxidans cytochrome c oxidase in mercury reduction, the cytochrome c oxidase was extracted from mercury-resistant and mercury-sensitive strains and purified. The Fe2+-dependent mercury volatilization activities of the oxidases from these strains were compared. The cytochrome c oxidase from strain SUG 2-2 volatilized 39% of the total Hg2+ (7 nmol) that had been added to a 10-ml reaction mixture (pH 3.8) in the presence of 10 micromol of Fe2+ after a 7-d incubation period at 30 degrees C. In contrast, the enzyme purified from the mercury-sensitive strain AP19-3 volatilized 3.5% of the total mercury under the same conditions. The boiled SUG 2-2 oxidase did not exhibit activity to volatilize mercury. Fe2+ reduced the oxidase from SUG 2-2 and Hg2+ oxidized the reduced enzyme. The purified SUG 2-2 oxidase is composed of three protein subunits with apparent molecular weights of 56,000 Da (alpha), 24,000 Da (beta), and 19,000 Da (gamma). The amount of mercury bound to the purified SUG 2-2 oxidase was 6.2 microg/mg protein and those bound to alpha-, beta- and gamma-subunits of the cytochrome c oxidase were 3.5, 2.6 and 0.7 microg/mg protein, respectively.     

Purification and some properties of sulfur reductase from the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1

Thiobacillus ferrooxidans strain NASF-1 grown aerobically in an Fe2+ (3%)-medium produces hydrogen sulfide (H2S) from elemental sulfur under anaerobic conditions with argon gas at pH 7.5. Sulfur reductase, which catalyzes the reduction of elemental sulfur (S0) with NAD(P)H as an electron donor to produce hydrogen sulfide (H2S) under anaerobic conditions, was purified 69-fold after 35-65% ammonium sulfate precipitation and Q-Sepharose FF, Phenyl-Toyopearl 650 ML, and Blue Sepharose FF column chromatography, with a specific activity of 57.6 U (mg protein)(-1). The purified enzyme was quite labile under aerobic conditions, but comparatively stable in the presence of sodium hydrosulfite and under anaerobic conditions, especially under hydrogen gas conditions. The purified enzyme showed both sulfur reductase and hydrogenase activities. Both activities had an optimum pH of 9.0. Sulfur reductase has an apparent molecular weight of 120,000 Da, and is composed of three different subunits (M(r) 54,000 Da (alpha), 36,000 Da (beta), and 35,000 Da (gamma)), as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This is the first report on the purification of sulfur reductase from a mesophilic and obligate chemolithotrophic iron-oxidizing bacterium.     

Isolation and some properties of Thiobacillus ferrooxidans strains with differing levels of mercury resistance from natural environments

Fifty iron-oxidizing bacteria isolated from natural environments were screened for resistance to mercuric ions (Hg2+). Thiobacillus ferrooxidans Funis 2-1, the strain found to show the greatest resistance to Hg2+ among the fifty isolates, gave a cell yield of 7.0 x 10(7) cells/ml after 8 d cultivation in an Fe2+-medium (pH 2.5) containing 0.7 microM Hg2+. Funis 2-1 volatilized 80% of the total mercury added to the medium over 8 d of cultivation. T. ferrooxidans AP19-3, more sensitive to Hg2+ than Funis 2-1, could not grow in an Fe2+-medium (pH 2.5) containing 0.7 microM Hg2+ even over a 28 d cultivation period. When resting cells of strains Funis 2-1 and AP19-3 were incubated for 3 h in a salt solution containing 0.7 microM Hg2+ (pH 3.0), 14.3% and 7.9% of the total mercury added to the reaction mixtures respectively, were volatilized. The activity of the mercuric reductase from Funis 2-1 was only 2.8 times higher than that of the enzyme from AP19-3. Since the markedly higher mercury resistance of Funis 2-1 compared with that of AP19-3 cannot be explained only by the level of the mercuric reductase activity, the levels of mercury resistance of iron oxidase and cytochrome c oxidase were studied. The 1 microM mercuric ions inhibited the 35% of iron-oxidizing activity from AP19-3. In contrast, the same concentration of Hg2+ did not inhibit the activity of iron oxidase from Funis 2-1. In the case of the cytochrome c oxidases purified from both strains, the 0.2 microM Hg2+ inhibited approximately 40% of cytochrome c oxidizing activity from AP19-3, on the contrary, the activity of the enzyme from Funis 2-1 was activated 1.8- and 1.2-fold, respectively, in the presence of 0.08 and 0.2 microM Hg2+. Since cytochrome c oxidase is one of the most important components of the iron-oxidizing system, these results indicate that both the existence of cytochrome c oxidase resistant to Hg2+ as well as that of mercuric reductase in the cells is responsible for the more rapid growth of Funis 2-1 than that of in an Fe2+-medium containing 0.7 microM Hg2+.     

Schwertmannite formation adjacent to bacterial cells in a mine water treatment plant and in pure cultures of Ferrovum myxofaciens

Schwertmannite has previously been found in iron- and sulfate-rich mine waters at pH 2.8-4.5. In the present study, schwertmannite (Fe(8)O(8)(OH)(6)SO(4)) was shown to be the major mineral in a mine water treatment plant at pH 3, in which ferrous iron is mainly oxidized by bacteria belonging to the species Ferrovum myxofaciens. Strain EHS6, which is closely related to the type strain of Fv. myxofaciens, was isolated from the pilot plant and characterized as an acidophilic, iron-oxidizing bacterium. In contrast to the pilot plant, the mineral phase formed by a pure culture of Fv. myxofaciens EHS6 was a mixture of schwertmannite and jarosite (KFe(3)(SO(4))(2)(OH)(6)). In contrast to other reports of neutrophilic, iron-oxidizing bacteria, acidophilic microorganisms in the pilot plant and cultures of strain EHS6 did not show encrustation of the cell surface or deposition of minerals inside the cell, though a few cells appeared to be in contact with jarosite crystals. It was concluded that no direct biomineralization occurred in the pilot plant or in laboratory cultures. The lack of encrustation of bacterial cells in the pilot plant is considered advantageous since the cells are still able to get in contact with ferrous iron and the iron oxidation process in the mine water treatment plant can proceed.     

Carotenoids,but Not Vitamin A,Improve Iron Uptake and Ferritin Synthesis by Caco‐2 Cells from Ferrous Fumarate and NaFe‐EDTA

Due to the high prevalence of iron and vitamin A deficiencies and to the controversy about the role of vitamin A and carotenoids in iron absorption, the objectives of this study were to evaluate the following: (1) the effect of a molar excess of vitamin A as well as the role of tannic acid on iron uptake by Caco‐2 cells; (2) iron uptake and ferritin synthesis in presence of carotenoids without pro‐vitamin A activity: lycopene, lutein, and zeaxantin; and (3) iron uptake and ferritin synthesis from ferrous fumarate and NaFe‐EDTA. Cells were incubated 1 h at 37 °C in PBS pH 5.5, containing ⁵⁹Fe and different iron compounds. Vitamin A, ferrous fumarate, β‐carotene, lycopene, lutein, zeaxantin, and tannic acid were added to evaluate uptake. Ferritin synthesis was measured 24 h after uptake experiments. Vitamin A had no effect on iron uptake by Caco‐2 cells, and was significantly lower from NaFe‐EDTA than from ferrous fumarate (15.2 ± 2.5 compared with 52.5 ± 8.3 pmol Fe/mg cell protein, respectively). Carotenoids increase uptake up to 50% from fumarate and up to 300% from NaFe‐EDTA, since absorption from this compound is low when administered alone. We conclude the following: (1) There was no effect of vitamin A on iron uptake and ferritin synthesis by Caco‐2cells. (2) Carotenoids significantly increased iron uptake from ferrous fumarate and NaFe‐EDTA, and were capable of partially overcoming the inhibition produced by tannic acid. (3) Iron uptake by Caco‐2 cell from NaFe‐EDTA was significantly lower compared to other iron compounds, although carotenoids increased and tannic acid inhibited iron uptake comparably to ferrous fumarate.     

Microbial fuel cell operation with continuous biological ferrous iron oxidation of the catholyte

The oxygen reduction rate at the cathode is a limiting factor in microbial fuel cell (MFC) performance. In our previous study, we showed the performance of an MFC with ferric iron (Fe3+) reduction at the cathode. Instead of oxygen, ferric iron was reduced to ferrous iron (Fe2+) at the cathode with a bipolar membrane between the anode and cathode compartment. This resulted in a higher cathode potential than is usually obtained with oxygen on metal-based chemical catalysts in MFCs. In this study, we investigated the operation of the same MFC with ferric iron reduction at the cathode and simultaneous biological ferrous iron oxidation of the catholyte. We show that the immobilized microorganism Acidithiobacillus ferrooxidans is capable of oxidizing ferrous iron to ferric iron at a rate high enough to ensure an MFC power output of 1.2 W/m2 and a current of 4.4 A/m2. This power output was 38% higher than in our previous study at a similar current density without ferrous iron oxidation. The bipolar membrane is shown to split water into 65-76% of the needed protons and hydroxides. The other part of the protons was supplied as H2SO4 to the cathode compartment. The remaining charge was transported by K+ and HSO4-/SO4(2-) from the one compartment to the other. This resulted in increased salt concentrations in the cathode. The increased salt concentrations reduced the ohmic losses and enabled the improved MFC power output. Iron could be reversibly removed from the bipolar membrane by exchange with protons.     

Inhibition of biological reductive dissolution of hematite by ferrous iron

Bacterial dissimilatory iron reduction is self-inhibited by the production of ferrous [Fe(II)] iron resulting in diminished iron reduction as Fe(II) accumulates. Experiments were conducted to investigate the mechanisms of Fe(II) inhibition employing the dissimilatory metal-reducing bacterium Shewanella putrefaciens strain CN32 under nongrowth conditions in a system designed to minimize precipitation of ferrous iron minerals. After an initial period (ca. 1 day) of relatively rapid iron reduction, hematite reduction rates were controlled by mass transfer of Fe(II). Experiments in which hematite was equilibrated with Mn(II) prior to inoculation indicated that the observed inhibition was not due to Fe(II) sorption. At longer times, soluble Fe(II) accumulated such that the reaction was slowed due to a decreased thermodynamic driving force. The thermodynamic evaluation also supported the prior conclusion that hydrated hematite surface sites may yield substantially more energy during bioreduction than "bulk" hematite. For well-mixed conditions, the rates of hematite reduction were directly proportional to the biologically available reaction potential.     

Availability of iron from milk-based formulas and fruit juices containing milk and cereals estimated by in vitro methods (solubility,dialysability) and uptake and transport by Caco-2 cells

Iron solubility, dialysability and transport and uptake (retention + transport) by Caco-2 cells as indicators of iron availability have been estimated in the in vitro gastrointestinal digests of infant foods (adapted, follow-up and toddler milk-based formulas and fruit juices containing milk and cereals (FMC)). Low correlation coefficients (in all cases R-squared ? 37.1%) were obtained between iron solubility or dialysability versus transport or uptake efficiency – a fact emphasizing the importance of incorporating Caco-2 cell cultures to in vitro systems in order to adapt the conditions to those found in in vivo assays. The highest uptake efficiency corresponded to FMC (25.6–26.1%) and toddler formulas (32.1–41.9%), the samples with the highest ascorbic acid contents and ascorbic acid/iron molar ratios. In addition, the toddler formulas contained caseinphosphopeptides with the cluster sequence SpSpSpEE, representing the binding site for minerals. In adapted formulas, greater iron uptake efficiency was obtained for the formulation containing ferrous lactate (22.7%) versus ferrous sulfate (4.7%).     

Effect of immunoglobulin G from cows immunized with ferric citrate receptor (FecA) on iron uptake by Escherichia coli

The effects of immunoglobulin (Ig) G from cows immunized with the ferric citrate receptor (FecA) on iron uptake by Escherichia coli were investigated. Receptor FecA was purified from E. coli UT5600/pSV66. Cows were immunized with 400 microg purified FecA three times at 21 d intervals during late lactation and the nonlactating period. Immunoglobulin G was purified by protein G affinity chromatography from colostral whey from cows immunized with FecA and from unimmunized control cows. The purified IgG from FecA immunized cows had higher IgG titers against FecA compared with control IgG. Fifteen E. coli isolated from intramammary infections and E. coli UT5600/pSV66 were grown in an iron-depleted medium containing 1 mM citrate to induce FecA. The bacterial cells were mixed with 0, 2, and 4 mg/ml purified IgG, and 55Fe was added to the assay. After 5, 10, and 15 min incubations at 37 degrees C, samples were passed through 0.45-pm pore size filters. Filters were washed with saline three times, and the radioactivity of 55Fe taken up by the bacterial cells on the filters was measured by a liquid scintillation counter. The measurements were expressed as numbers of 55Fe atoms per colony-forming unit and transformed to log10. The assay was repeated three times for each isolate in a partially balanced incomplete block design. The presence of IgG decreased 55Fe uptake by E. coli mastitis isolates and E. coli UT5600/pSV66. Anti-FecA IgG reduced 55Fe uptake by E. coli greater than IgG from unimmunized cows.     

Purification and characterization of phytase from Klebsiella pneumoniae 9-3B

Phytase, an enzyme that catalyzes the hydrolysis of phytate, was purified from Klebsiella pneumoniae 9-3B. The isolate was preferentially selected in a medium which contains phytate as a sole carbon and phosphate source. Phytic acid was utilized for growth and consequently stimulated phytase production. Phytase production was detected throughout growth and the highest phytase production was observed at the onset of stationary phase. The purification scheme including ion exchange chromatography and gel filtration resulted in a 240 and 2077 fold purification of the enzyme with 2% and 15% recovery of the total activity for liberation of inorganic phosphate and inositol, respectively. The purified phytase was a monomeric protein with an estimated molecular weight of 45kDa based on size exclusion chromatography and SDS-PAGE analyses. The phytase has an optimum pH of 4.0 and optimum temperature of 50°C. The phytase activity was slightly stimulated by Ca(2+) and EDTA and inhibited by Zn(2+) and Fe(2+). The phytase exhibited broad substrate specificity and the K(m) value for phytate was 0.04mM. The enzyme completely hydrolyzed myo-inositol hexakisphosphate (phytate) to myo-inositol and inorganic phosphate. The properties of the enzyme prove that it is a good candidate for the hydrolysis of phytate for industrial applications.     

Control of ferrous iron oxidation within circumneutral microbial iron mats by cellular activity and autocatalysis

Ferrous iron (Fe2+) oxidation by microbial iron mat samples, dominated by helical stalks of Gallionella ferruginea or sheaths of Leptothrix ochracea, was examined. Pseudo-first-order rate constants for the microbial mat samples ranged from 0.029 +/- 0.004 to 0.249 +/- 0.042 min(-1) and correlated well with iron content (R2 = 0.929). Rate constants for Na azide-treated (1 mM) samples estimated autocatalytic oxidation by iron oxide stalks or sheaths, with values ranging from 0.016 +/- 0.008 to 0.062 +/- 0.006 min(-1). Fe2+ oxidation attributable to cellular activities was variable with respect to sampling location and sampling time, with rate constants from 0.013 +/- 0.005 to 0.187 +/- 0.037 min(-1). Rates of oxidation of the same order of magnitude for cellular processes and autocatalysis suggested that bacteria harnessing Fe2+ as an energy source compete with their own byproducts for growth, not chemical oxidation (under conditions where aqueous oxygen concentrations are less than saturating). The use of cyclic voltammetry within this study for the simultaneous measurement of Fe2+ and oxygen allowed the collection of statistically meaningful and reproducible data, two factors that have limited aerobic, circumneutral, Fe2+ -oxidation rate studies.     

Kinetics and mechanisms for reactions of Fe(II) with iron(III) oxides

Uptake of Fe(II) onto hematite (alpha-Fe2O3), corundum (alpha-Al2O3), amorphous ferric oxide (AFO), and a mixture of hematite and AFO was measured. Uptake was operationally divided into adsorption (extractable by 0.5 N HCl within 20 h) and fixation (extractable by 3.0 N HCl within 7 d). For 0.25 mM Fe(II) onto 25 mM iron(III) hematite at pH 6.8: (i) 10% of Fe(II) was adsorbed within 1 min; (ii) 20% of Fe(II) was adsorbed within 1 d; (iii) uptake slowly increased to 24% of Fe(II) during the next 24 d, almost all adsorbed; (iv) at 30 d, the uptake increased to 28% of Fe(II) with 6% of total Fe(II) fixed; and (v) uptake slowly increased to 30% of Fe(II) by 45 d with 10% of total Fe(II) fixed. Similar results were observed for 0.125 mM Fe(II) onto 25 mM iron(III) hematite, except that percent of adsorption and fixation were increased. There was adsorption but no fixation for 0.25 mM Fe(II) onto corundum [196.2 mM Al(III)] at pH 6.8, for 0.125 mM Fe(II) onto 25 mM iron(III) hematite at pH 4.5, and for 0.25 mM Zn(II) onto 25 mM iron(III) hematite at pH 6.8. A small addition of AFO to the hematite suspension increased Fe(II) fixation when 0.25 mM Fe(II) was reacted with 25 mM iron(III) hematite and 0.025 mM Fe(III) AFO at pH 6.8. Reaction of 0.125 mM Fe(II) with 2.5 mM Fe(III) AFO resulted in rapid adsorption of 30% of added Fe(II), followed by conversion of AFO to goethite and a decrease in adsorption without Fe(II) fixation. The fixation of Fe(II) by hematite at pH 6.8 is consistent with interfacial electron transfer and the formation of new mineral phases. We propose that electron transfer from adsorbed Fe(II) to structural Fe(III) in hematite results in oxidation of Fe(II) to AFO on the surface of hematite and that solid-phase contact among hematite, AFO, and structural Fe(II) produces magnetite (Fe3O4). The unique interactions of Fe(II) with iron(III) oxides would be environmentally important to understand the fate of redox-sensitive chemicals.     

Age-associated variation in sensory perception of iron in drinking water and the potential for overexposure in the human population

Humans interact with their environment through the five senses, but little is known about population variability in the ability to assess contaminants. Sensory thresholds and biochemical indicators of metallic flavor perception in humans were evaluated for ferrous (Fe(2+)) iron in drinking water; subjects aged 19-84 years participated. Metallic flavor thresholds for individuals and subpopulations based on age were determined. Oral lipid oxidation and oral pH were measured in saliva as potential biochemical indicators. Individual thresholds were 0.007-14.14 mg/L Fe(2+) and the overall population threshold was 0.17 mg/L Fe(2+) in reagent water. Average thresholds for individuals younger and older than 50 years of age (grouped by the daily recommended nutritional guidelines for iron intake) were significantly different (p = 0.013); the population thresholds for each group were 0.045 mg/L Fe(2+) and 0.498 mg/L Fe(2+), respectively. Many subjects >50 and a few subjects <50 years were insensitive to metallic flavor. There was no correlation between age, oral lipid oxidation, and oral pH. Standardized olfactory assessment found poor sensitivity for Fe(2+) corresponded with conditions of mild, moderate, and total anosmia. The findings demonstrate an age-dependent sensitivity to iron indicating as people age they are less sensitive to metallic perception.     

Suppressive effects of bifidobacteria on lipid peroxidation in the colonic mucosa of iron-overloaded mice

The antioxidative effects of live bifidobacteria on lipid peroxidation in the colonic mucosa were investigated. Bifidobacterium bifidum strain Yakult, which has been used for production of fermented milk, most effectively inhibited lipid peroxidation catalyzed by ferrous iron in liposomes among 10 species of bifidobacteria from human intestinal flora. Oral administration of B. bifidum strain Yakult for 2 wk significantly decreased the level of lipid peroxide (thiobarbituric acid reactive substance) in the colonic mucosa of iron-overload mice (Fe 0.07% in diet). The iron concentrations in plasma and cecum contents were not affected by administration of B. bifidum strain Yakult. Bifidobacterium bifidum strain Yakult had no chelating or incorporating activity for ferrous iron in vitro. Therefore, the antioxidative effect of B. bifidum strain Yakult in the colonic mucosa was not thought to be based on the removal of ferrous iron from the reaction system of lipid peroxidation. These results suggested that B. bifidum strain Yakult protected the colonic mucosa from oxidative injury without inhibiting iron absorption.     

Isolation and characterization of acidophilic heterotrophic iron-oxidizing bacterium from enrichment culture obtained from acid mine drainage treatment plant

An acidophilic heterotrophic bacterium, designated as HIB4, having the ability to oxidize ferrous ion was newly isolated from a sample of an enrichment culture for iron-oxidizing bacteria, using the modified washed agarose/yeast extract (WAYE) medium with ferrous sulphate. The isolate HIB4 was an acidophilic, heterotrophic, mesophilic and gram-positive bacterium. Phylogenetically, it was classified under the genus Alicyclobacillus and was the closest to Alicyclobacillus disulfidooxidans SD-11 with 99.7% 16S rDNA homology. It grew and oxidized ferrous ion in the medium containing 0.02% (w/v) yeast extract. Yeast extract was an essential substrate for this bacterium because it could not grow or oxidize ferrous ion without yeast extract. However, a higher concentration of yeast extract inhibited the growth of HIB4, so that the optimum concentration of yeast extract for this bacterium to grow was 0.02% (w/v) at 0.08 mol/l of ferrous ion. On the other hand, ferrous ion oxidation occurred almost at the end of the bacterium's logarithmic growth phase and the isolate was able to grow without ferrous ion. These results denote that HIB4 did not obtain any energy from the ferrous ion oxidation and that HIB4 is an obligate heterotrophic and aerobic bacterium even though it oxidized ferrous ion. Also, HIB4 could not utilize any organic compounds, among the several organic chemicals used in this study, as a carbon source except yeast extract. These characteristics were completely different from these of A. disulfidooxidans SD-11 so that HIB4 might be a different species.     

Reduction of organically complexed ferric iron by superoxide in a simulated natural water

Superoxide (and potentially its conjugate acid hydroperoxyl) is unique among the reactive oxygen species in that its standard redox potential in circumneutral natural waters potentially allows it to reduce ferric iron to the more soluble ferrous state. Here we have observed the superoxide/ hydroperoxyl-mediated reduction of ferric complexes with a variety of synthetic organic ligands and several complexes with natural organic matter (NOM), as well as freshly precipitated amorphous ferric oxyhydroxide, in bicarbonate buffered solutions at pH 8.1. From measurements of superoxide decay in the presence of the complexes, we calculated second-order rate constants for superoxide/ hydroperoxyl-mediated reduction that vary from (9.3+/-0.2) x 10(3) M(-1) s(-1) for the complex between Fe(III) and desferrioxamine B up to (1.9+/-0.2) x 10(5) M(-1) s(-1) for Fe(III)-salicylate and (2.3+/-0.1) x 10(5) M(-1) s(-1) for one of the Fe(III)-NOM complexes. We also verified that ferrous iron was produced from superoxide/hydroperoxyl-mediated Fe(III) reduction using ferrozine to trap free Fe(II). Low yields of the ferrozine complex when compared to the measured rates of superoxide decay suggest that ferric complexes are reduced directlyto corresponding ferrous complexes, with much of the ferrous complex reoxidizing before it is able to release free ferrous iron. This is an important consideration for microorganisms, as the kinetics of trace metal uptake is typically governed by free ion activity.     

Hydrosulfide oxidation pathways in oxic solutions containing iron(III) chelates

The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree C. At higher pH, oxygen was seen to slow down the hydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2, C(HS-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/L). In anoxia, HS- oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS- conversion rates. The presence of DO2 disrupts the HS- oligomerization process by generating thiosulfate precursors from polysulfides, a pathway that impedes the HS- uptake. At lower alkaline pH where the hydroxide-free Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides. Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur, which did not form in DO2-saturated solutions.     

Relation between the redox state of iron-based nanoparticles and their cytotoxicity toward Escherichia coli

Iron-based nanoparticles have been proposed for an increasing number of biomedical or environmental applications although in vitro toxicity has been observed. The aim of this study was to understand the relationship between the redox state of iron-based nanoparticles and their cytotoxicity toward a Gram-negative bacterium, Escherichia coli. While chemically stable nanoparticles (gammaFe2O3) have no apparent cytotoxicity, nanoparticles containing ferrous and, particularly, zerovalent iron are cytotoxic. The cytotoxic effects appear to be associated principally with an oxidative stress as demonstrated using a mutant strain of E. coli completely devoid of superoxide dismutase activity. This stress can result from the generation of reactive oxygen species with the interplay of oxygen with reduced iron species (Fe(II) and/or Fe(0)) or from the disturbance of the electronic and/or ionic transport chains due to the strong affinity of the nanoparticles for the cell membrane.