A new example of a tetranuclear iron(III) cluster containing the [Fe4O2] core: preparation, X-ray crystal structure, magnetochemistry and Mössbauer study of [Fe4O2(O2CMe)6(N3)2(phen)2]

Two synthetic procedures have been employed that allow access to the new tetranuclear cluster [Fe₄O₂(O₂CMe)₆(N₃)₂(phen)₂] (1), where phen is 1,10-phenanthroline. Complex 1 · 3MeCN displays an unusual structural asymmetry (observed for the second time) in its [Fe₄O₂]⁸⁺ core that can be considered as a hybrid of the bent (butterfly) and planar dispositions of four metal ions seen previously in such compounds with transition metals. Complex 1 has been characterized by variable-temperature magnetic susceptibility studies, and by IR and variable-temperature ⁵⁷Fe Mössbauer spectroscopies. Magnetochemical data reveal a diamagnetic ground state (S=0) with antiferromagnetic body-body and body-wingtip interactions between the iron(III) ions of the butterfly core (J_bb=−11 cm⁻¹, J_wb=−70 cm⁻¹). Magnetochemical and Mössbauer studies on 1 show that its structural asymmetry has practically no influence on these properties compared with the more symmetric types.     

Effect of pH and oxalic acid on the reduction of Fe3+ by a biomimetic chelator and on Fe3+ desorption/adsorption onto wood: Implications for brown-rot decay

Fenton reaction is thought to play an important role in wood degradation by brown-rot fungi. In this context, the effect of oxalic acid and pH on iron reduction by a biomimetic fungal chelator and on the adsorption/desorption of iron to/from wood was investigated. The results presented in this work indicate that at pH 2.0 and 4.5 and in the presence of oxalic acid, the phenolate chelator 2,3-dihydroxybenzoic acid (2,3-DHBA) is capable of reducing ferric iron only when the iron is complexed with oxalate to form Fe³⁺-mono-oxalate (Fe(C₂O₄)⁺). Within the pH range tested in this work, this complex formation occurs when the oxalate:Fe³⁺ molar ratio is less than 20 (pH 2.0) or less than 10 (pH 4.5). When aqueous ferric iron was passed through a column packed with milled red spruce (Picea rubens) wood equilibrated at pH 2.0 and 4.5, it was observed that ferric iron binds to wood at pH 4.5 but not at pH 2.0, and the bound iron could then be released by application of oxalic acid at pH 4.5. The release of bound iron was dependent on the amount of oxalic acid applied in the column. When the amount of oxalate was at least 20-fold greater than the amount of iron bound to the wood, all bound iron was released. When Fe–oxalate complexes were applied to the milled wood column equilibrated in the pH range of 2–4.5, iron from Fe–oxalate complexes was bound to the wood only when the pH was 3.6 or higher and the oxalate:Fe³⁺ molar ratio was less than 10. When 2,3-DHBA was evaluated for its ability to release iron bound to the milled wood, it was found that 2,3-DHBA possessed a greater affinity for ferric iron than the wood as 2,3-DHBA was capable of releasing the ferric iron bound to the wood in the pH range 3.6–5.5. These results further the understanding of the mechanisms employed by brown-rot fungi in wood biodegradation processes.     

Influence of Fe0 nanoparticles,magnetite Fe3O4 nanoparticles,and iron (II) sulfate (FeSO4) solutions on the content of photosynthetic pigments in Triticum vulgare

Seeds and seedlings of soft wheat (Triticum vulgare Vill.) were used to study seed germination, leaf elongation, and the content of photosynthetic pigments (chlorophylls a, b and carotenoids) as affected by five concentrations of iron-containing nanoparticles (NP): spherical Fe⁰ NP with the diameter of 80 ± 5 nm and the magnetite Fe₃O₄ NP measuring 50–80 nm in width and 4–10 nm in height. The effects of FeSO₄ solutions were also tested for comparison. The parameters examined varied as a function of the exogenous agent applied, the agent concentration, and the exposure duration. The highest sensitivity of seedlings was observed in the presence of increasing concentrations of iron (II) sulfate in the nutrient medium. This was evident from the decrease in seed germination percentage, inhibition of leaf growth, and the diminished content of photosynthetic pigments. The apparent toxicity of iron nanoforms varied depending on the parameter examined. (1) The strongest inhibition of germination was exerted by Fe⁰ NP (toxicity assessed from germination percentage was 3.3% higher with Fe⁰ NP than with magnetite NP); (2) the inhibition of leaf elongation on the 4th day after germination was most evident in the presence of Fe⁰ NP (a 12% stronger inhibition in the presence of Fe⁰ NP than in the presence of magnetite NP), whereas on the 7th day the inhibition was most pronounced with magnetite NP (a 9% stronger inhibition in the presence of Fe₃SO₄ NP than in the presence of Fe⁰ NP); (3) the lowest total content of photosynthetic pigments on the 4th day of seedling growth was noted in the presence of magnetite NP (8% lower in the presence of Fe₃SO₄ NP than in the presence of Fe⁰ NP), whereas on the 7th day the lowest pigment pool was observed in the presence Fe⁰ NP (a 3% reduction compared to that in the presence of magnetite NP). The highest content of photosynthetic pigments was recorded in the presence of 0.125 and 0.001 g/L of Fe⁰ NP, 0.5 g/L and 1 μg/L of Fe₃O₄ NP, and 1 mg/L FeSO₄.     

Competitive Inhibition of Ferrous Iron Oxidation by Thiobacillus ferrooxidans by Increasing Concentrations of Cells

The oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) with dioxygen (O₂) by various strains of Thiobacillus ferrooxidans was studied by measuring the rate of O₂ consumption at various Fe²⁺ concentrations and cell concentrations. The apparent K_m values for Fe²⁺ remained constant at different cell concentrations of laboratory strains ATCC 13661 and ATCC 19859 but increased with increasing cell concentrations of mine isolates SM-4 and SM-5. The latter results are explained by the competitive inhibition of the Fe²⁺-binding site of a cell by other cells in the reaction mixture. Possible mechanisms involving cell surface properties are discussed.     

Investigation of iron pools in cucumber roots by Mössbauer spectroscopy: direct evidence for the Strategy I iron uptake mechanism

Distinct chemical species of iron were investigated by Mössbauer spectroscopy during iron uptake into cucumber roots grown in unbuffered nutrient solution with or without ⁵⁷Fe-citrate. Mössbauer spectra of iron deficient roots supplied with 10–500 μM ⁵⁷Fe-citrate for 30–180 min and 24 h and iron-sufficient ones, were recorded. The roots were analysed for Fe concentration and Fe reductase activity. The Mössbauer parameters in the case of iron-sufficient roots revealed high-spin iron(III) components suggesting the presence of Fe^III-carboxylate complexes, hydrous ferric oxides and sulfate–hydroxide containing species. No Fe^II was detected in these roots. However, iron-deficient roots supplied with 0.5 mM ⁵⁷Fe^III-citrate for 30 min contained significant amount of Fe^II in a hexaaqua complex form. This is a direct evidence for the Strategy I iron uptake mechanism. Correlation was found between the decrease in Fe reductase activity and the ratio of Fe^II–Fe^III components as the time of iron supply was increased. The data may refer to a higher iron reduction rate as compared to its uptake/reoxidation in the cytoplasm in accordance with the increased reduction rate in iron deficient Strategy I plants.     

Phosphorus Sequestration in Lake Sediment with Iron Mine Tailings

Internal phosphorus loading can lead to eutrophication in lakes when anoxic sediments release bioavailable phosphorus into the water column. In laboratory experiments, iron mine tailings helped to sequester phosphorus in sediment from a eutrophic lake. Phosphorus release from the sediments after extraction with distilled water or 0.02 N H ₂ SO ₄ was significantly reduced when mine tailings were added (1:1 w/w), even when the system was anaerobic (～ 1 mg O ₂ /L). The degree of sequestration was enhanced when glucose (1% w/w) was added to stimulate the growth of microorganisms, suggesting that the process was microbially mediated. We suggest that oxidized iron in the mine tailings served as an electron sink for microbial respiration via dissimilatory Fe³⁺ reduction. The reduced iron released into solution sequestered phosphorus, either as it re-oxidized and formed hydrous ferric oxide complexes containing phosphorus (HFO-P), or through precipitation. Since mine tailings are inexpensive, they may prove useful for preventing phosphorus from entering surface waters, as well as reducing internal phosphorus loading.     

Magnetite formation from ferrihydrite by hyperthermophilic archaea from Endeavour Segment,Juan de Fuca Ridge hydrothermal vent chimneys

Hyperthermophilic iron reducers are common in hydrothermal chimneys found along the Endeavour Segment in the northeastern Pacific Ocean based on culture‐dependent estimates. However, information on the availability of Fe(III) (oxyhydr) oxides within these chimneys, the types of Fe(III) (oxyhydr) oxides utilized by the organisms, rates and environmental constraints of hyperthermophilic iron reduction, and mineral end products is needed to determine their biogeochemical significance and are addressed in this study. Thin‐section petrography on the interior of a hydrothermal chimney from the Dante edifice at Endeavour showed a thin coat of Fe(III) (oxyhydr) oxide associated with amorphous silica on the exposed outer surfaces of pyrrhotite, sphalerite, and chalcopyrite in pore spaces, along with anhydrite precipitation in the pores that is indicative of seawater ingress. The iron sulfide minerals were likely oxidized to Fe(III) (oxyhydr) oxide with increasing pH and E_h due to cooling and seawater exposure, providing reactants for bioreduction. Culture‐dependent estimates of hyperthermophilic iron reducer abundances in this sample were 1740 and 10 cells per gram (dry weight) of material from the outer surface and the marcasite‐sphalerite‐rich interior, respectively. Two hyperthermophilic iron reducers, Hyperthermus sp. Ro04 and Pyrodictium sp. Su06, were isolated from other active hydrothermal chimneys on the Endeavour Segment. Strain Ro04 is a neutrophilic (pH_opt 7–8) heterotroph, while strain Su06 is a mildly acidophilic (pH_opt 5), hydrogenotrophic autotroph, both with optimal growth temperatures of 90–92 °C. Mössbauer spectroscopy of the iron oxides before and after growth demonstrated that both organisms form nanophase (<12 nm) magnetite [Fe₃O₄] from laboratory‐synthesized ferrihydrite [Fe₁₀O₁₄(OH)₂] with no detectable mineral intermediates. They produced up to 40 mm Fe²⁺ in a growth‐dependent manner, while all abiotic and biotic controls produced <3 mm Fe²⁺. Hyperthermophilic iron reducers may have a growth advantage over other hyperthermophiles in hydrothermal systems that are mildly acidic where mineral weathering at increased temperatures occurs.     

Dynamics of Iron Uptake and Fe3O4 Biomineralization during Aerobic and Microaerobic Growth of Magnetospirillum gryphiswaldense

Iron uptake and magnetite (Fe₃O₄) crystal formation could be studied in the microaerophilic magnetic bacterium Magnetospirillum gryphiswaldense by using a radioactive tracer method for iron transport and a differential light-scattering technique for magnetism. Magnetite formation occurred only in a narrow range of low oxygen concentration, i.e., 2 to 7 μM O₂ at 30°C. Magnetic cells stored up to 2% iron as magnetite crystals in intracytoplasmic vesicles. This extraordinary uptake of iron was coupled tightly to the biomineralization of up to 60 magnetite crystals with diameters of 42 to 45 nm.     

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H₂O₂ when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe^IV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H₂O₂. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H₂O₂ in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O–O bond cleavage of an iron(III) hydroperoxide porphyrin (or H₂O₂–iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O–O bond cleavage.     

Coordination chemistry of acrylamide 6: Formation and structural characterization of [Fe(O-OC(NH2)CHCH2)6][Fe2OCl6]

The new acrylamide iron(II)/iron(III) complex [Fe(O-OC(NH₂)CHCH₂)₆][Fe₂OCl₆] (1) was obtained by the reaction of a mixture of anhydrous FeCl₂ and anhydrous FeCl₃ with acrylamide (molar ratio 1:2:6) in 98% pure commercial nitromethane under nitrogen atmosphere. According to an X-ray structural analysis, the acrylamide ligands in the cation are coordinated via the amide-oxygen atoms. The formation of the (μ-oxo)bis[trichloroferrate(III)]²⁻ anion presumably resulted from partial hydrolysis of FeCl₃ or [FeCl₄]⁻ by small amounts of water in the nitromethane and/or by the nitromethane itself.     

Biogeochemical Conditions Favoring Magnetite Formation during Anaerobic Iron Reduction

Several anaerobic bacteria isolated from the sediments of Contrary Creek, an iron-rich environment, produced magnetite when cultured in combinations but not when cultured alone in synthetic iron oxyhydroxide medium. When glucose was added as a carbon source, the pH of the medium decreased (to 5.5) and no magnetite was formed. When the same growth medium without glucose was used, the pH increased (to 8.5) and magnetite was formed. In both cases, Fe²⁺ was released into the growth medium. Geochemical equilibrium equations with E_h and pH as master variables were solved for the concentrations of iron and inorganic carbon that were observed in the system. Magnetite was predicted to be the dominant iron oxide formed at high pHs, while free Fe²⁺ or siderite were the dominant forms of iron expected at low pHs. Thus, magnetite formation occurs because of microbial alteration of the local E_h and pH conditions, along with concurrent reduction of ferric iron (direct biological reduction or abiological oxidation-reduction reactions).     

Oncoidal granular iron formation in the Mesoarchaean Pongola Supergroup,southern Africa: Textural and geochemical evidence for biological activity during iron deposition

We document the discovery of the first granular iron formation (GIF) of Archaean age and present textural and geochemical results that suggest these formed through microbial iron oxidation. The GIF occurs in the Nconga Formation of the ca. 3.0–2.8 Ga Pongola Supergroup in South Africa and Swaziland. It is interbedded with oxide and silicate facies micritic iron formation (MIF). There is a strong textural control on iron mineralization in the GIF not observed in the associated MIF. The GIF is marked by oncoids with chert cores surrounded by magnetite and calcite rims. These rims show laminated domal textures, similar in appearance to microstromatolites. The GIF is enriched in silica and depleted in Fe relative to the interbedded MIF. Very low Al and trace element contents in the GIF indicate that chemically precipitated chert was reworked above wave base into granules in an environment devoid of siliciclastic input. Microbially mediated iron precipitation resulted in the formation of irregular, domal rims around the chert granules. During storm surges, oncoids were transported and deposited in deeper water environments. Textural features, along with positive δ⁵⁶Fe values in magnetite, suggest that iron precipitation occurred through incomplete oxidation of hydrothermal Fe²⁺ by iron‐oxidizing bacteria. The initial Fe³⁺‐oxyhydroxide precipitates were then post‐depositionally transformed to magnetite. Comparison of the Fe isotope compositions of the oncoidal GIF with those reported for the interbedded deeper water iron formation (IF) illustrates that the Fe²⁺ pathways and sources for these units were distinct. It is suggested that the deeper water IF was deposited from the evolved margin of a buoyant Fe²⁺_aq‐rich hydrothermal plume distal to its source. In contrast, oncolitic magnetite rims of chert granules were sourced from ambient Fe²⁺_aq‐depleted shallow ocean water beyond the plume.     

Facile and greener hydrothermal honey-based synthesis of Fe3O 4/Au core/shell nanoparticles for drug delivery applications

In the present research, we report a greener, faster, and low-cost synthesis of gold-coated iron oxide nanoparticles (Fe₃O₄/Au-NPs) by different ratios (1:1, 2:1, and 3:1 molar ratio) of iron oxide and gold with natural honey (0.5% w/v) under hydrothermal conditions for 20 minutes. Honey was used as the reducing and stabilizing agent, respectively. The nanoparticles were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), vibrating sample magnetometer (VSM), and fourier transformed infrared spectroscopy (FT-IR). The XRD analysis indicated the presence of Fe₃O₄/Au-NPs, while the TEM images showed the formation of Fe₃O₄/Au-NPs with diameter range between 3.49 nm and 4.11 nm. The VSM study demonstrated that the magnetic properties were decreased in the Fe₃O₄/Au-NPs compared with the Fe₃O₄-NPs. The cytotoxicity threshold of Fe₃O₄/Au-NPs in the WEHI164 cells was determined by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. It was demonstrated no significant toxicity in higher concentration up to 140.0 ppm which can become the main candidates for biological and biomedical applications, such as drug delivery.     

Effect of cadmium on iron uptake in cucumber roots: A Mössbauer-spectroscopic study

The uptake and accumulation of iron in cucumber roots exposed to cadmium were investigated with Fe sufficient and deficient cucumber plants using Mössbauer spectroscopy, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and ferric chelate reductase activity measurements. Both Fe sufficient and Fe deficient plants were applied. In the case of Fe sufficient cucumber roots grown in nutrient solution with 10 μM Cd no changes were found in the occurrence of Fe species (mostly hydrous ferric oxides and ferric-carboxylate complexes) compared to the control where no Cd was added. In the Fe deficient roots pretreated with 0, 0.1, 1, 10 and 100 μM Cd for 3 h then supplied also with 0.5 mM ⁵⁷Fe-citrate for 30 min, Fe^II was identified in a hexaaqua complex form. The relative amount of Fe^II was decreasing simultaneously with increasing Cd concentration, while the relative occurrence of Fe^III species and total Fe concentration were increasing. The results support the inhibitory effect of Cd on Fe-chelate reduction. Although the reductase activity at 10 and 100 μM Cd treatment was lower than in the iron sufficient control plants, Fe^II could be identified by Mössbauer spectroscopy whereas in the Fe sufficient control, this form was below detection limit. These data demonstrate that the influx and the reoxidation of Fe^II was decreased by Cd, consequently, they refer to the competition of Cd²⁺ and Fe²⁺ during the membrane transport and the inhibition of the reoxidation process.     

Characterisation of the dissimilatory reduction of Fe(III)-oxyhydroxide at the microbe-mineral interface: the application of STXM-XMCD

A combination of scanning transmission X‐ray microscopy and X‐ray magnetic circular dichroism was used to spatially resolve the distribution of different carbon and iron species associated with Shewanella oneidensis MR‐1 cells. S. oneidensis MR‐1 couples the reduction of Fe(III)‐oxyhydroxides to the oxidation of organic matter in order to conserve energy for growth. Several potential mechanisms may be used by S. oneidensis MR‐1 to facilitate Fe(III)‐reduction. These include direct contact between the cell and mineral surface, secretion of either exogenous electron shuttles or Fe‐chelating agents and the production of conductive ‘nanowires’. In this study, the protein/lipid signature of the bacterial cells was associated with areas of magnetite (Fe₃O₄), the product of dissimilatory Fe(III) reduction, which was oversaturated with Fe(II) (compared to stoichiometric magnetite). However, areas of the sample rich in polysaccharides, most likely associated with extracellular polymeric matrix and not in direct contact with the cell surface, were undersaturated with Fe(II), forming maghemite‐like (γ‐Fe₂O₃) phases compared to stoichiometric magnetite. The reduced form of magnetite will be much more effective in environmental remediation such as the immobilisation of toxic metals. These findings suggest a dominant role for surface contact‐mediated electron transfer in this study and also the inhomogeneity of magnetite species on the submicron scale present in microbial reactions. This study also illustrates the applicability of this new synchrotron‐based technique for high‐resolution characterisation of the microbe–mineral interface, which is pivotal in controlling the chemistry of the Earth’s critical zone.     

聚乙二醇连接荧光Cy5.5构建超小超顺磁性氧化铁成像探针的制备与表征

摘要 目的：以超小超顺磁性氧化铁颗粒为载体通过聚乙二醇连接荧光Cy5.5构建核磁/荧光分子探针并表征。方法：取Cy5.5-NHS荧光粉末溶于二甲基甲砜（Dimethyl sulfoxide,DMSO）溶液,将PEG四氧化三铁颗粒离心超滤之后用磷酸盐缓冲液（Phosphate Buffered Saline,PBS）重悬纳米颗粒改变PEG化四氧化三铁纳米颗粒溶液pH。将配置好的Cy5.5荧光加入到四氧化三铁颗粒中,恒温摇床孵育,通过离心过滤器去除较大铁离子与未结合的荧光,静置后检测水合粒径及Zeta电位,纽麦小核磁检测其驰豫率,CCK-8实验检测其细胞毒性,激光共聚焦显微镜观察探针被细胞摄取情况。结果：合成Cy5.5-PEG-FeO₄探针,透射电镜（Transmission electron microscope,TEM）显示探针粒径为16.8±2.4nm,纳米颗粒的水合径为43.4±17.6 nm,Zeta电位为-18.0 mV。驰豫率为39.5 mM^-1?s^-1,R²为0.98。细胞毒性实验结果显示对细胞有轻微毒性,且毒性与浓度呈依赖性。激光共聚焦结果显示此款探针可顺利被细胞摄取。结论：成功合成Cy5.5-PEG-FeO₄探针。     

Identification of Iron Transporters Expressed in the Magnetotactic Bacterium <Emphasis Type="Italic">Magnetospirillum magnetotacticum</Emphasis>

The magnetotactic bacterium Magnetospirillum magnetotacticum MS-1 mineralizes the magnetite (Fe₃O₄) crystal and organizes a highly ordered intracellular structure, called the magnetosome. However, the iron transport system, which supports the biogenesis of magnetite, is not fully understood. In this study, we first identified the expressions of both the ferric and the ferrous iron transporter proteins in M. magnetotacticum. The cellular protein compositions of ferric and ferrous iron-rich cultures were examined using two-dimensional electrophoresis. According to the gel patterns, two outer-membrane ferric-siderophore receptor homologues were identified as proteins strongly induced in the ferrous iron-rich condition. Also, we identified for the first time that the ferrous iron transport protein, FeoB, is expressed in the M. magnetotacticum cytoplasmic membrane using immunoblotting.     

Study of the magnetic properties of the mixed oxide Fe2O3-Fe2(MoO4)3 in the temperature range 2-300 K with different compositions

Powdered Fe₂O₃-Fe₂(MoO₄)₃ with different amounts of iron and molybdate precursors was prepared by a solvothermal route, followed by a supercritical drying and oxidation at 500 °C. The possibility to arrange Fe or Mo precursors in excess into a methanol solution makes one accessible to the preparation of iron(III) molybdate samples with different composition. The structural parameters and relationship between different phases in the composition are obtained from Rietveld profile refinement. Our intention was to modify the magnetic properties of Fe₂(MoO₄)₃ by adding the crystalline phase of Fe₂O₃, which carries a Fe-O magnetic component. A possible contribution to the magnetization and the magnetic susceptibility by this magnetic component is analyzed in the temperature range 2-300 K. The observed higher magnetic susceptibilities are compared to those reported.     

Evidence for Microbial Fe(III) Reduction in Anoxic, Mining-Impacted Lake Sediments (Lake Coeur d'Alene, Idaho)

Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg⁻¹, suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter⁻¹) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe₃O₄), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg⁻¹. Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 × 10⁵ cells g (dry weight) of sediment⁻¹. Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.     

Neutrophilic, nitrate-dependent, Fe(II) oxidation by a Dechloromonas species

A species of Dechloromonas, strain UWNR4, was isolated from a nitrate-reducing, enrichment culture obtained from Wisconsin River (USA) sediments. This strain was characterized for anaerobic oxidation of both aqueous and chelated Fe(II) coupled to nitrate reduction at circumneutral pH. Dechloromonas sp. UWNR4 was incubated in anoxic batch reactors in a defined medium containing 4.5–5 mM NO₃ ^?, 6 mM Fe²⁺ and 1–1.8 mM acetate. Strain UWNR4 efficiently oxidized Fe²⁺ with 90 % oxidation of Fe²⁺ after 3 days of incubation. However, oxidation of Fe²⁺ resulted in Fe(III)-hydroxide-encrusted cells and loss of metabolic activity, suggested by inability of the cells to utilize further additions of acetate. In similar experiments with chelated iron (Fe(II)-EDTA), encrusted cells were not produced and further additions of acetate and Fe(II)-EDTA could be oxidized. Although members of the genus Dechloromonas are primarily known as perchlorate and nitrate reducers, our findings suggest that some species could be members of microbial communities influencing iron redox cycling in anoxic, freshwater sediments. Our work using Fe(II)-EDTA also demonstrates that Fe(II) oxidation was microbially catalyzed rather than a result of abiotic oxidation by biogenic NO₂ ^?.