Iron plaque enhances phosphorus uptake by rice (Oryza sativa) growing under varying phosphorus and iron concentrations

Both solution culture and pot experiments were performed to investigate (a) the effects of external Fe (II) concentrations and forms on the formation of iron plaque on the roots of rice (Oryza sativa) and subsequent P adsorption on iron plaque and shoot P concentrations and (b) the effects of soil moisture regimes on the formation of iron plaque and P adsorption on root surfaces and P accumulation in shoots. The results showed that iron plaque was significantly increased with increasing Fe²⁺ concentrations in the solution culture. The amounts of P adsorbed on the iron plaque were increased significantly with external Fe²⁺ concentrations. Although shoot P concentration was not significantly affected by Fe²⁺ treatment after incubation for 2 days, it was significantly increased in the Fe‐treated plants compared with Fe‐deprived ones after incubation for 4 days. Soil culture experiment showed that the formation of iron plaque on root surfaces was promoted by exogenous iron, with greater amount of iron plaque being formed by addition of ferric hydroxide than of ferric oxide. Phosphorus adsorption on iron plaque also increased with the addition of iron oxides, and increasing soil P increased the amounts of P associated with the iron plaque and shoot P concentration. The amounts of iron plaque were almost sixfold higher under flooding condition than under field capacity condition. Plants pretreated under flooding condition generally had higher shoot P concentrations when they were transplanted to solutions with varying P levels, and this was most pronounced in the treatment with highest solution P concentration. The results suggest that iron plaque acts as a nutrient reservoir for phosphorus in the rhizosphere and helps enhance P acquisition by rice.     

A novel model for cyanobacteria bloom formation: the critical role of anoxia and ferrous iron

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Effect of iron ions (Fe<Superscript>2+</Superscript>, Fe<Superscript>3+</Superscript>) on the formation and structure of aerobic granular sludge

Aerobic granulation is a promising technology for wastewater treatment, but problems regarding its formation and stability need to be solved. Divalent metal ions, especially Ca²⁺, Mg²⁺ and Mn²⁺, have been demonstrated to play an important role in the process of aerobic granulation. Here, we studied whether iron ions can affect aerobic granulation. Granular sludge formed without iron ion addition (<0.02 mg Fe²⁺ L^?1) was fluffy and had a finger-type structure and filamentous out-growth. The addition of iron ions to concentrations of 1 and 10 mg Fe²⁺ L^?1 repressed the finger-type structure and filamentous out-growth. The results show that chemical precipitation in the granules with iron ion addition was higher than that in the granules without ferrous addition. The amount of precipitates was higher inside the granules than outside. This study demonstrates that iron ions (Fe²⁺/Fe³⁺) increase the size and stability of aerobic granular sludge but do not affect the granulation time, which is the time that the first granular sludge is observed. The study shows that aerobic granular sludge technology can be confidently applied to actual wastewater containing a high concentration of iron compounds.     

Iron requirement and iron uptake from various iron compounds by different plant species

The Fe requirements of four monocotyledonous plant species (Avena sativa L., Triticum aestivum L., Oryza sativa L., Zea mays L.) and of three dicotyledonous species (Lycopersicum esculentum Mill., Cucumis sativus L., Glycine maxima (L.) Merr.) in hydroponic cultures were ascertained. Fe was given as NaFe-EDDHA chelate (Fe ethylenediamine di (O-hydroxyphenylacetate). I found that the monocotyledonous species required a substantially higher Fe concentration in the nutrient solution in order to attain optimum growth than did the dicotyledonous species. Analyses showed that the process of iron uptake was less efficient with the monocotyledonous species. When the results obtained by using chelated Fe were compared with those using ionic Fe, it was shown that the inefficient species were equally inefficient in utilizing Fe³⁺ ions. However, the differences between the efficient and the inefficient species disappeared when Fe²⁺ was used. This confirms the work of others who postulated that Fe³⁺ is reduced before uptake of chelated iron by the root. In addition, it was shown that reduction also takes place when Fe is used in ionic form. The efficiency of Fe uptake seems to depend on the efficiency of the root system of the particular plant species in reducing Fe³⁺. The removal of Fe from the chelate complex after reduction to Fe²⁺ seems to present no difficulties to the various plant species.     

Determination of threshold value of iron in soils and plants for the response of rice and lentil to iron application in calcareous soil

Summary In a pot experiment with 26 calcareous soils, the critical limit of Fe in soils and plants was evaluated. DTPA-extractable Fe was found significanty correlated with Bray's per cent yield in rice. The Fe²⁺ (iron) in rice and lentil was also found significantly correlated with DTPA-extractable Fe as well as Bray's per cent yield showing thereby the superiority of Fe²⁺ (iron) in leaves over DTPA-extractable soil Fe to differentiate Fe responsive soils from non-responsive ones. The total Fe content in plant tissues does not seem correlated with the occurrence of Fe deficiency. The threshold values of DTPA-extractable soil Fe and Fe²⁺ (iron) in rice and lentil leaves were 6.95, 44 and 74.5 ppm, respectively below which appreciable responses to Fe application were observed. The optimum Fe level for these soils was found to be 10 ppm in which the dry matter yield response in all the 19 rice soils and 16 lentil soils ranged from 14.28 to 56.16 (Av. 25.75%) and 13.31 to 53.97 (Av. 22.47%), respectively.     

Molecular mechanisms of non‐transferrin‐bound and transferring‐bound iron uptake in primary hippocampal neurons

The molecular mechanisms of iron trafficking in neurons have not been elucidated. In this study, we characterized the expression and localization of ferrous iron transporters Zip8, Zip14 and divalent metal transporter 1 (DMT1), and ferrireductases Steap2 and stromal cell‐derived receptor 2 in primary rat hippocampal neurons. Steap2 and Zip8 partially co‐localize, indicating these two proteins may function in Fe³⁺ reduction prior to Fe²⁺ permeation. Zip8, DMT1, and Steap2 co‐localize with the transferrin receptor/transferrin complex, suggesting they may be involved in transferrin receptor/transferrin‐mediated iron assimilation. In brain interstitial fluid, transferring‐bound iron (TBI) and non‐transferrin‐bound iron (NTBI) exist as potential iron sources. Primary hippocampal neurons exhibit significant iron uptake from TBI (Transferrin‐⁵⁹Fe³⁺) and NTBI, whether presented as ⁵⁹Fe²⁺‐citrate or ⁵⁹Fe³⁺‐citrate; reductase‐independent ⁵⁹Fe²⁺ uptake was the most efficient uptake pathway of the three. Kinetic analysis of Zn²⁺ inhibition of Fe²⁺ uptake indicated that DMT1 plays only a minor role in the uptake of NTBI. In contrast, localization and knockdown data indicate that Zip8 makes a major contribution. Data suggest also that cell accumulation of ⁵⁹Fe from TBI relies at least in part on an endocytosis‐independent pathway. These data suggest that Zip8 and Steap2 play a major role in iron accumulation from NTBI and TBI by hippocampal neurons.

     

Magnetite as a precursor for green rust through the hydrogenotrophic activity of the iron‐reducing bacteria Shewanella putrefaciens

Magnetite (Fe^IIFe^III₂O₄) is often considered as a stable end product of the bioreduction of Fe^III minerals (e.g., ferrihydrite, lepidocrocite, hematite) or of the biological oxidation of Fe^II compounds (e.g., siderite), with green rust (GR) as a mixed Fe^II‐Fe^III hydroxide intermediate. Until now, the biotic transformation of magnetite to GR has not been evidenced. In this study, we investigated the capability of an iron‐reducing bacterium, Shewanella putrefaciens, to reduce magnetite at circumneutral pH in the presence of dihydrogen as sole inorganic electron donor. During incubation, GR and/or siderite (Fe^IICO₃) formation occurred as secondary iron minerals, resulting from the precipitation of Fe^II species produced via the bacterial reduction of Fe^III species present in magnetite. Taking into account the exact nature of the secondary iron minerals and the electron donor source is necessary to understand the exergonic character of the biotic transformation of magnetite to GR, which had been considered to date as thermodynamically unfavorable at circumneutral pH. This finding reinforces the hypothesis that GR would be the cornerstone of the microbial transformations of iron‐bearing minerals in the anoxic biogeochemical cycle of iron and opens up new possibilities for the interpretation of the evolution of Earth's history and for the understanding of biocorrosion processes in the field of applied science.     

Effects of added organic matter on iron and manganese redox systems in sediment

Addition of five types of organic matter to Lake Washington sediments resulted in release of high concentrations of iron, organic carbon, and manganese into the interstitial water, and caused an increase in observed sediment oxygen consumption rates. The depressed electrode potentials (E_h < —150 mV) that should accompany such reduction processes did not occur, indicating that E_h was being poised by redox systems present in the sediment. Iron redox systems [Fe(OH)₃‐Fe²⁺, Fe₃(OH)₈‐Fe²⁺, and Fe(OH)₃‐Fe₃(OH)₈] were shown to be poising the E_h of control sediments throughout 13 weeks of incubation and dominating the potential of several of the organically amended sediments following the first three weeks of incubation. Depression of calculated iron system E_o values relative to that of the control sediment early in the incubation appeared to be due to the decreased pH and non‐equilibrium conditions in the organic matter‐amended sediment during the first weeks of incubation. Manganese redox systems exerted no discernable impact on the E_h of the sediment.     

Effects of ferrous iron on the performance and microbial community in aerobic granular sludge in relation to nutrient removal

Lab‐scale experiments were conducted to investigate the effects of ferrous iron on nutrient removal performance and variations in the microbial community inside aerobic granular sludge for 408 days. Two reactors were simultaneously operated, one without added ferrous iron (SBR1), and one with 10 mg Fe²⁺ L^?1 of added ferrous iron (SBR2). A total of 1 mg Fe²⁺ L^?1 of added ferrous iron was applied to SBR1 starting from the 191st day to observe the resulting variations in the nutrient removal performance and the microbial community. The results show that ammonia‐oxidizing bacteria (AOB) could not oxidize ammonia due to a lack of iron compounds, but they could survive in the aerobic granular sludge. Limited ferrous iron addition encouraged nitrification. Enhanced biological phosphorus removal (EBPR) from both reactors could not be maintained regardless of the amount of ferrous iron that was applied. EBPR was established in both reactors when the concentration of mixed liquor suspended solid (MLSS) and the percentage of Accumulibacteria increased. A total of 10 mg Fe²⁺ L^?1 of added ferrous iron had a relatively adverse effect on the growth of AOB species compared to 1 mg Fe²⁺ L^?1 of added ferrous iron, but it encouraged the growth of Nitrospira sp. and Accumulibacteria, which requires further study. It could be said that the compact and stable structure of aerobic granular sludge preserved AOB and NOB from Fe‐deficient conditions, and wash‐out during the disintegration period. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:716–725, 2017     

Evidence for a specific uptake system for iron phytosiderophores in roots of grasses

Roots of grasses in response to iron deficiency markedly increase the release of chelating substances (`phytosiderophores') which are highly effective in solubilization of sparingly soluble inorganic Fe<sup>III</sup> compounds by formation of Fe<sup>III</sup>phytosiderophores. In barley (Hordeum vulgare L.), the rate of iron uptake from Fe<sup>III</sup>phytosiderophores is 100 to 1000 times faster than the rate from synthetic Fe chelates (e.g. Fe ethylenediaminetetraacetate) or microbial Fe siderophores (e.g. ferrichrome). Reduction of Fe<sup>III</sup> is not involved in the preferential iron uptake from Fe<sup>III</sup>phytosiderophores by barley. This is indicated by experiments with varied pH, addition of bicarbonate or of a strong chelator for Fe<sup>II</sup> (e.g. batho-phenanthrolinedisulfonate). The results indicate the existence of a specific uptake system for Fe<sup>III</sup>phytosiderophores in roots of barley and all other graminaceous species. In contrast to grasses, cucumber plants (Cucumis sativus L.) take up iron from Fe<sup>III</sup>phytosiderophores at rates similar to those from synthetic Fe chelates. Furthermore, under Fe deficiency in cucumber, increased rates of uptake of Fe<sup>III</sup>phytosiderophores are based on the same mechanism as for synthetic Fe chelates, namely enhanced Fe<sup>III</sup> reduction and chelate splitting. Two strategies are evident from the experiments for the acquisition of iron by plants under iron deficiency. Strategy I (in most nongraminaceous species) is characterized by an inducible plasma membrane-bound reductase and enhancement of H<sup>+</sup> release. Strategy II (in grasses) is characterized by enhanced release of phytosiderophores and by a highly specific uptake system for Fe<sup>III</sup>phytosiderophores. Strategy II seems to have several ecological advantages over Strategy I such as solubilization of sparingly soluble inorganic Fe<sup>III</sup> compounds in the rhizosphere, and less inhibition by high pH. The principal differences in the two strategies have to be taken into account in screening methods for resistance to `lime chlorosis'.     

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).     

Microaerophilic Fe‐oxidizing micro‐organisms in Middle Jurassic ferruginous stromatolites and the paleoenvironmental context of their formation (Southern Carpathians,Romania)

Ferruginous stromatolites occur associated with Middle Jurassic condensed deposits in several Tethyan and peri‐Tethyan areas. The studied ferruginous stromatolites occurring in the Middle Jurassic condensed deposits of Southern Carpathians (Romania) preserve morphological, geochemical, and mineralogical data that suggest microbial iron oxidation. Based on their macrofabrics and accretion patterns, we classified stromatolites: (1) Ferruginous microstromatolites associated with hardground surfaces and forming the cortex of the macro‐oncoids and (2) Domical ferruginous stromatolites developed within the Ammonitico Rosso‐type succession disposed above the ferruginous microstromatolites (type 1). Petrographic and scanning electron microscope (SEM) examinations reveal that different types of filamentous micro‐organisms were the significant framework builders of the ferruginous stromatolitic laminae. The studied stromatolites yield a large range of δ⁵⁶Fe values, from ?0.75‰ to +0.66‰ with predominantly positive values indicating the prevalence of partial ferrous iron oxidation. The lowest negative δ⁵⁶Fe values (up to ?0.75‰) are present only in domical ferruginous stromatolites samples and point to initial iron mobilization where the Fe(II) was produced by dissimilatory Fe(III) reduction of ferric oxides by Fe(III)‐reducing bacteria. Rare‐earth elements and yttrium (REE + Y) are used to decipher the nature of the seawater during the formation of the ferruginous stromatolites. Cerium anomalies display moderate to small negative values for the ferruginous microstromatolites, indicating weakly oxygenated conditions compatible with slowly reducing environments, in contrast to the domical ferruginous stromatolites that show moderate positive Ce anomalies suggesting that they formed in deeper, anoxic–suboxic waters. The positive Eu anomalies from the studied samples suggest a diffuse hydrothermal input on the seawater during the Middle Jurassic on the sites of ferruginous stromatolite accretion. This study presents the first interpretation of REE + Y in the Middle Jurassic ferruginous stromatolites of Southern Carpathians, Romania.     

The ferroxidase center is essential for ferritin iron loading in the presence of phosphate and minimizes side reactions that form Fe(III)-phosphate colloids

Ferritin iron loading was studied in the presence of physiological serum phosphate concentrations (1 mM), elevated serum concentrations (2–5 mM), and intracellular phosphate concentrations (10 mM). Experiments compared iron loading into homopolymers of H and L ferritin with horse spleen ferritin. Prior to studying the reactions with ferritin, a series of control reactions were performed to study the solution chemistry of Fe²⁺ and phosphate. In the absence of ferritin, phosphate catalyzed Fe²⁺ oxidation and formed soluble polymeric Fe(III)-phosphate complexes. The Fe(III)-phosphate complexes were characterized by electron microscopy and atomic force microscopy, which revealed spherical nanoparticles with diameters of 10–20 nm. The soluble Fe(III)-phosphate complexes also formed as competing reactions during iron loading into ferritin. Elemental analysis on ferritin samples separated from the Fe(III)-phosphate complexes showed that as the phosphate concentration increased, the iron loading into horse ferritin decreased. The composition of the mineral that does form inside horse ferritin has a higher iron/phosphate ratio (~1:1) than ferritin purified from tissue (~10:1). Phosphate significantly inhibited iron loading into L ferritin, due to the lack of the ferroxidase center in this homopolymer. Spectrophotometric assays of iron loading into H ferritin showed identical iron loading curves in the presence of phosphate, indicating that the ferroxidase center of H ferritin efficiently competes with phosphate for the binding and oxidation of Fe²⁺. Additional studies demonstrated that H ferritin ferroxidase activity could be used to oxidize Fe²⁺ and facilitate the transfer of the Fe³⁺ into apo transferrin in the presence of phosphate.     

Obligatory reduction of ferric chelates in iron uptake by soybeans

The contrasting Fe²⁺ and Fe³⁺ chelating properties of the synthetic chelators ethylenediaminedi (o-hydroxyphenylacetate) (EDDHA) and 4,7-di(4-phenylsulfonate)-1, 10-phenanthroline (bathophenanthrolinedisulfonate) (BPDS) were used to determine the valence form of Fe absorbed by soybean roots supplied with Fe³⁺-chelates. EDDHA binds Fe³⁺ strongly, but Fe²⁺ weakly; BPDS binds Fe²⁺ strongly but Fe³⁺ weakly. Addition of an excess of BPDS to nutrient solutions containing Fe³⁺-chelates inhibited soybean Fe uptake-translocation by 99+%; [Fe(II) (BPDS)₃]⁴⁻ accumulated in the nutrient solution. The addition of EDDHA caused little or no inhibition. These results were observed with topped and intact soybeans. Thus, separation and absorption of Fe from Fe³⁺-chelates appear to require reduction of Fe³⁺-chelate to Fe²⁺-chelate at the root, with Fe²⁺ being the principal form of Fe absorbed by soybean.     

Kinetics and mechanism of iron release from the bacterial ferric binding protein<Emphasis Type="Italic"> n</Emphasis>Fbp: exogenous anion influence and comparison with mammalian transferrin

Ferric binding protein, Fbp, serves an essential biological function in shuttling naked (hydrated) Fe³⁺ across the periplasmic space of many Gram-negative bacteria. In this process, iron must be released at the cytoplasmic membrane to a permease. How iron is released from Fbp has yet to be resolved. Consequently, understanding the dynamics of iron release from Fbp is of both biological and chemical interest. Fbp requires an exogenous anion, e.g. phosphate when isolated from cell lysates, for tight iron sequestration. To address the role of exogenous anion identity and lability on Fe_aq ³⁺ dissociation from Fbp, the kinetics of PO₄ ^3– exchange in Fe³⁺ nFbp(PO₄) (nFbp=recombinant Fbp from Neisseria meningitidis) were investigated by dynamic ³¹P NMR and the kinetics of Fe³⁺ dissociation from Fe³⁺ nFbp(X) (X=PO₄ ^3–, citrate anion) were investigated by stopped-flow pH-jump measurements. We justify the use of non-physiological low-pH conditions because a high [H⁺] will drive the Fe_aq ³⁺ dissociation reaction to completion without using competing chelators, whose presence may complicate or influence the dissociation mechanism. For perspective, these studies of nFbp (which has been referred to as a bacterial transferrin) are compared to new and previously published kinetic and thermodynamic data for mammalian transferrin. Significantly, we address the lability of the Fe³⁺ coordination shell in nFbp, Fe³⁺ nFbp(X) (X=PO₄ ^3–, citrate), with respect to exogenous anion (X ^n–) exchange and dissociation, and ultimately complete dissociation of the protein to yield naked (hydrated) Fe_aq ³⁺. These findings are a first step in understanding the process of iron donation to the bacterial permease for transport across the cytoplasmic membrane.Electronic Supplementary Material Supplementary material is available in the online version of this article at . Abbreviations DTPP diethylenetriaminepenta(methylenephosphonic acid) - Fbp ferric binding protein - H₃cit citric acid - hFbp Fbp from Haemophilus influenzae - H₂ox oxalic acid - hTf human serum transferrin - 3,4-LICAMS N,N,N-tris(5-sulfo-2,3-dihydroxybenzoyl)-1,5,10-triazadecane - nFbp recombinant Fbp from Neisseria meningitidis - NTA nitrilotriacetic acid - TRENSOX tris[2-aminoethyl(8-hydroxyquinoline-5-sulfonato-7-carbonyl)]amine     

Functional characterization of the ferroxidase, permease high-affinity iron transport complex from Candida albicans

Saccharomyces cerevisiae expresses two proteins that together support high‐affinity Fe‐uptake. These are a multicopper oxidase, Fet3p, with specificity towards Fe²⁺ and a ferric iron permease, Ftr1p, which supports Fe‐accumulation. Homologues of the genes encoding these two proteins are found in all fungal genomes including those for the pathogens, Candida albicans and Cryptococcus neoformans. At least one of these loci represents a virulence factor for each pathogen suggesting that this complex would be an appropriate pharmacologic target. However, the mechanism by which this protein pair supports Fe‐uptake in any fungal pathogen has not been elucidated. Taking advantage of the robust molecular genetics available in S. cerevisiae, we identify the two of five candidate ferroxidases likely involved in high‐affinity Fe‐uptake in C. albicans, Fet31 and Fet34. Both localize to the yeast plasma membrane and both support Fe‐uptake along with an Ftr1 protein, either from C. albicans or from S. cerevisiae. We express and characterize Fet34, demonstrating that it is functionally homologous to ScFet3p. Using S. cerevisiae as host for the functional expression of the C. albicans Fe‐uptake proteins, we demonstrate that they support a mechanism of Fe‐trafficking that involves channelling of the CaFet34‐generated Fe³⁺ directly to CaFtr1 for transport into the cytoplasm.     

Fe3O4 precipitation in magnetotactic bacteria

Using Mössbauer resonance spectroscopy of ⁵⁷Fe, we have determined the nature and distribution of major iron compounds in the magnetotactic bacterium Aquaspirillum magnetotacticum. In addition to magnetite (Fe₃O₄), cells contained a low-density hydrous ferric oxide, a high-density hydrous ferric oxide (ferrihydrite), and ferrous iron. Analysis at different temperatures of whole cells harvested early and late in growth, of mutant cells unable to synthesize magnetite, and of cell fractions enriched in ⁵⁷Fe indicated that Fe₃O₄ precipitation resulted from partial reduction of the high-density hydrous ferric oxide precursor.     

The mechanisms of iron isotope fractionation produced during dissimilatory Fe(III) reduction by Shewanella putrefaciens and Geobacter sulfurreducens

Microbial dissimilatory iron reduction (DIR) is widespread in anaerobic sediments and is a key producer of aqueous Fe(II) in suboxic sediments that contain reactive ferric oxides. Previous studies have shown that DIR produces some of the largest natural fractionations of stable Fe isotopes, although the mechanism of this isotopic fractionation is not yet well understood. Here we compare Fe isotope fractionations produced by similar cultures of Geobacter sulfurreducens strain PCA and Shewanella putrefaciens strain CN32 during reduction of hematite and goethite. Both species produce aqueous Fe(II) that is depleted in the heavy Fe isotopes, as expressed by a decrease in ⁵⁶Fe/⁵⁴Fe ratios or δ⁵⁶Fe values. The low δ⁵⁶Fe values for aqueous Fe(II) produced by DIR reflect isotopic exchange among three Fe inventories: aqueous Fe(II) (Fe(II)_aq), sorbed Fe(II) (Fe(II)_sorb), and a reactive Fe(III) component on the ferric oxide surface (Fe(III)_reac). The fractionation in ⁵⁶Fe/⁵⁴Fe ratios between Fe(II)_aq and Fe(III)_reac was –2.95‰, and this remained constant over the timescales of the experiments (280 d). The Fe(II)_aq – Fe(III)_reac fractionation was independent of the ferric Fe substrate (hematite or goethite) and bacterial species, indicating a common mechanism for Fe isotope fractionation during DIR. Moreover, the Fe(II)_aq – Fe(III)_reac fractionation in ⁵⁶Fe/⁵⁴Fe ratios during DIR is identical within error of the equilibrium Fe(II)_aq – ferric oxide fractionation in abiological systems at room temperatures. This suggests that the role of bacteria in producing Fe isotope fractionations during DIR lies in catalyzing coupled atom and electron exchange between Fe(II)_aq and Fe(III)_reac so that equilibrium Fe isotope partitioning occurs. Although Fe isotope fractionation between Fe(II)_aq and Fe(III)_reac remained constant, the absolute δ⁵⁶Fe values for Fe(II)_aq varied as a function of the relative proportions of Fe(II)_aq, Fe(II)_sorb, and Fe(III)_reac during reduction. The temporal variations in these proportions were unique to hematite or goethite but independent of bacterial species. In the case of hematite reduction, the small measured Fe(II)_aq – Fe(II)_sorb fractionation of −0.30‰ in ⁵⁶Fe/⁵⁴Fe ratios, combined with the small proportion of Fe(II)_sorb, produced insignificant (<0.05‰) isotopic effects due to sorption of Fe(II). Sorption of Fe(II) produced small, but significant effects during reduction of goethite, reflecting the higher proportion of Fe(II)_sorb and larger measured Fe(II)_aq – Fe(II)_sorb fractionation of –0.87‰ in ⁵⁶Fe/⁵⁴Fe ratios for goethite. The isotopic effects of sorption on the δ⁵⁶Fe values for Fe(II)_aq were largest during the initial stages of reduction when Fe(II)_sorb was the major ferrous Fe species during goethite reduction, on the order of 0.3 to 0.4‰. With continued reduction, however, the isotopic effects of sorption decreased to <0.2‰. These results provide insight into the mechanisms that produce Fe isotope fractionation during DIR, and form the basis for interpretation of Fe isotope variations in modern and ancient natural systems where DIR may have driven Fe cycling.     

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.     

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.