Planktonic marine iron oxidizers drive iron mineralization under low‐oxygen conditions

Observations of modern microbes have led to several hypotheses on how microbes precipitated the extensive iron formations in the geologic record, but we have yet to resolve the exact microbial contributions. An initial hypothesis was that cyanobacteria produced oxygen which oxidized iron abiotically; however, in modern environments such as microbial mats, where Fe(II) and O₂ coexist, we commonly find microaerophilic chemolithotrophic iron‐oxidizing bacteria producing Fe(III) oxyhydroxides. This suggests that such iron oxidizers could have inhabited niches in ancient coastal oceans where Fe(II) and O₂ coexisted, and therefore contributed to banded iron formations (BIFs) and other ferruginous deposits. However, there is currently little evidence for planktonic marine iron oxidizers in modern analogs. Here, we demonstrate successful cultivation of planktonic microaerophilic iron‐oxidizing Zetaproteobacteria from the Chesapeake Bay during seasonal stratification. Iron oxidizers were associated with low oxygen concentrations and active iron redox cycling in the oxic–anoxic transition zone (<3 μm O₂, <0.2 μm H₂S). While cyanobacteria were also detected in this transition zone, oxygen concentrations were too low to support significant rates of abiotic iron oxidation. Cyanobacteria may be providing oxygen for microaerophilic iron oxidation through a symbiotic relationship; at high Fe(II) levels, cyanobacteria would gain protection against Fe(II) toxicity. A Zetaproteobacteria isolate from this site oxidized iron at rates sufficient to account for deposition of geologic iron formations. In sum, our results suggest that once oxygenic photosynthesis evolved, microaerophilic chemolithotrophic iron oxidizers were likely important drivers of iron mineralization in ancient oceans.     

Iron oxidation stimulates organic matter decomposition in humid tropical forest soils

Humid tropical forests have the fastest rates of organic matter decomposition globally, which often coincide with fluctuating oxygen (O₂) availability in surface soils. Microbial iron (Fe) reduction generates reduced iron [Fe(II)] under anaerobic conditions, which oxidizes to Fe(III) under subsequent aerobic conditions. We demonstrate that Fe (II) oxidation stimulates organic matter decomposition via two mechanisms: (i) organic matter oxidation, likely driven by reactive oxygen species; and (ii) increased dissolved organic carbon (DOC) availability, likely driven by acidification. Phenol oxidative activity increased linearly with Fe(II) concentrations (P < 0.0001, pseudo R² = 0.79) in soils sampled within and among five tropical forest sites. A similar pattern occurred in the absence of soil, suggesting an abiotic driver of this reaction. No phenol oxidative activity occurred in soils under anaerobic conditions, implying the importance of oxidants such as O₂ or hydrogen peroxide (H₂O₂) in addition to Fe(II). Reactions between Fe(II) and H₂O₂ generate hydroxyl radical, a strong nonselective oxidant of organic compounds. We found increasing consumption of H₂O₂ as soil Fe(II) concentrations increased, suggesting that reactive oxygen species produced by Fe(II) oxidation explained variation in phenol oxidative activity among samples. Amending soils with Fe(II) at field concentrations stimulated short‐term C mineralization by up to 270%, likely via a second mechanism. Oxidation of Fe(II) drove a decrease in pH and a monotonic increase in DOC; a decline of two pH units doubled DOC, likely stimulating microbial respiration. We obtained similar results by manipulating soil acidity independently of Fe(II), implying that Fe(II) oxidation affected C substrate availability via pH fluctuations, in addition to producing reactive oxygen species. Iron oxidation coupled to organic matter decomposition contributes to rapid rates of C cycling across humid tropical forests in spite of periodic O₂ limitation, and may help explain the rapid turnover of complex C molecules in these soils.     

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.     

Phototrophic Fe(II) oxidation in an atmosphere of H2: implications for Archean banded iron formations

The effect of hydrogen on the rate of phototrophic Fe(II) oxidation by two species of purple bacteria was measured at two different bicarbonate concentrations. Hydrogen slowed Fe(II) oxidation to varying degrees depending on the bicarbonate concentration, but even the slowest rate of Fe(II) oxidation remained on the same order of magnitude as that estimated to have been necessary to deposit the Hamersley banded iron formations. Given the hydrogen and bicarbonate concentrations inferred for the Archean, our data suggest that Fe(II) phototrophy could have been a viable process at this time.     

Microbial production of isotopically light iron(II) in a modern chemically precipitated sediment and implications for isotopic variations in ancient rocks

The inventories and Fe isotope composition of aqueous Fe(II) and solid‐phase Fe compounds were quantified in neutral‐pH, chemically precipitated sediments downstream of the Iron Mountain acid mine drainage site in northern California, USA. The sediments contain high concentrations of amorphous Fe(III) oxyhydroxides [Fe(III)_am] that allow dissimilatory iron reduction (DIR) to predominate over Fe–S interactions in Fe redox transformation, as indicated by the very low abundance of Cr(II)‐extractable reduced inorganic sulfur compared with dilute HCl‐extractable Fe. δ⁵⁶Fe values for bulk HCl‐ and HF‐extractable Fe were ≈ 0. These near‐zero bulk δ⁵⁶Fe values, together with the very low abundance of dissolved Fe in the overlying water column, suggest that the pyrite Fe source had near‐zero δ⁵⁶Fe values, and that complete oxidation of Fe(II) took place prior to deposition of the Fe(III) oxide‐rich sediment. Sediment core analyses and incubation experiments demonstrated the production of millimolar quantities of isotopically light (δ⁵⁶Fe ≈ ?1.5 to ?0.5‰) aqueous Fe(II) coupled to partial reduction of Fe(III)_am by DIR. Trends in the Fe isotope composition of solid‐associated Fe(II) and residual Fe(III)_am are consistent with experiments with synthetic Fe(III) oxides, and collectively suggest an equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)_am of approximately ?2‰. These Fe(III) oxide‐rich sediments provide a model for early diagenetic processes that are likely to have taken place in Archean and Paleoproterozoic marine sediments that served as precursors for banded iron formations. Our results suggest pathways whereby DIR could have led to the formation of large quantities of low‐δ⁵⁶Fe minerals during BIF genesis.     

Localization of iron-reducing activity in paddy soilby profile studies

Profiles of iron speciations (porewaterFe(II) and Fe(III), solid-phase Fe(II) andFe(III)) have been studied to localize both ironreduction and oxidation in flooded paddy soil. Sulfateand nitrate were determined to analyze interactions ofredox reactions involved in the iron cycle with thoseof the sulfur and nitrogen cycle. The development ofthe iron(II) and iron(III) profiles was observed inmicroscale over a time period of 11 weeks. After 11weeks the profiles were stable and showed lowestconcentrations of solid-phase iron(II) on the soilsurface with increasing concentrations to a soil depthof 10 mm ( 100 µmol/cm³). Profilesof iron(III) showed a maximum of iron(III) at a depthof 2 to 4 mm ( 100--200 µmol/cm³).Porewater iron(II) concentrations were three orders ofmagnitude lower than extracted iron(II) and indicatedthat most iron(II) was adsorbed to the solid-phase orimmobilized as siderite and vivianite. Diffusive lossof iron from the soil was indicated by iron recovery(0.3 µmol gdw^–1) in the flooding water after12 weeks. The organic content of the soil influencedthe concentrations of solid-phase iron(II) in deepersoil layers (> 6 mm); higher Fe(II) concentrationsin soil with limiting amounts of electron donors mayindicate lower consumption of CO₂ by methanogenicbacteria and therefore a higher sideriteprecipitation. Soil planted with rice showed similariron(II) profiles of fresh paddy soil cores. However,maximal iron(III) concentrations ( 350µmol/cm³) were present in planted soil at adepth of 1 to 2.5 mm where oxygen is provided by a matof fine roots. Sulfate and nitrate concentrations inthe porewater were highest on the soil surface (10µM NO₃ ^–, 40 µM SO₄ ^2–) anddecreased with depth. Similar profiles were detectedfor malate, acetate, lactate, and propionate, theconcentrations decreased gradually from the surface toa depth of 4 mm. Profiles of oxygen showed highestconcentrations at the surface due to photosyntheticproduction and a depletion of oxygen below 3 mm depth.Methane production rates measured from soil layersincubated separately in closed vessels were zero atthe soil surface and increased with depth. In soildepths below 4 mm where iron(III) concentrationsdecreased higher methane production rates werefound.     

Oxygen Dependency of Neutrophilic Fe(II) Oxidation by Leptothrix Differs from Abiotic Reaction

Neutrophilic Fe(II) oxidizing microorganisms are found in many natural environments. It has been hypothesized that, at low oxygen concentrations, microbial iron oxidation is favored over abiotic oxidation. Here, we compare the kinetics of abiotic Fe(II) oxidation to oxidation in the presence of the bacterium Leptothrix cholodnii Appels isolated from a wetland sediment. Rates of Fe(II) oxidation were determined in batch experiments at 20°C, pH 7 and oxygen concentrations between 3 and 120 μmol/l. The reaction progress in experiments with and without cells exhibited two distinct phases. During the initial phase, the oxygen dependency of microbial Fe(II) oxidation followed a Michaelis-Menten rate expression (K_M = 24.5 ± 10 μmol O₂/l, v_max = 1.8 ± 0.2 μmol Fe(II)/(l min) for 10⁸ cells/ml). In contrast, abiotic rates increased linearly with increasing oxygen concentrations. At similar oxygen concentrations, initial Fe(II) oxidation rates were faster in the experiments with bacteria. During the second phase, the accumulated iron oxides catalyzed further oxidative iron precipitation in both abiotic and microbial reaction systems. That is, abiotic oxidation also dominated the reaction progress in the presence of bacteria. In fact, in some experiments with bacteria, iron oxidation during the second phase proceeded slower than in the absence of bacteria, possibly due to an inhibitory effect of extracellular polymeric substances on the growth of Fe(III) oxides. Thus, our results suggest that the competitive advantage of microbial iron oxidation in low oxygen environments may be limited by the autocatalytic nature of abiotic Fe(III) oxide precipitation, unless the accumulation of Fe(III) oxides is prevented, for example, through a close coupling of Fe(II) oxidation and Fe(III) reduction.     

Iron binding and oxidation kinetics in frataxin CyaY of Escherichia coli

Friedreich's ataxia is associated with a deficiency in frataxin, a conserved mitochondrial protein of unknown function. Here, we investigate the iron binding and oxidation chemistry of Escherichia coli frataxin (CyaY), a homologue of human frataxin, with the aim of better understanding the functional properties of this protein. Anaerobic isothermal titration calorimetry (ITC) demonstrates that at least two ferrous ions bind specifically but relatively weakly per CyaY monomer (K(d) approximately 4 microM). Such weak binding is consistent with the hypothesis that the protein functions as an iron chaperone. The bound Fe(II) is oxidized slowly by O(2). However, oxidation occurs rapidly and completely with H(2)O(2) through a non-enzymatic process with a stoichiometry of two Fe(II)/H(2)O(2), indicating complete reduction of H(2)O(2) to H(2)O. In accord with this stoichiometry, electron paramagnetic resonance (EPR) spin trapping experiments indicate that iron catalyzed production of hydroxyl radical from Fenton chemistry is greatly attenuated in the presence of CyaY. The Fe(III) produced from oxidation of Fe(II) by H(2)O(2) binds to the protein with a stoichiometry of six Fe(III)/CyaY monomer as independently measured by kinetic, UV-visible, fluorescence, iron analysis and pH-stat titrations. However, as many as 25-26 Fe(III)/monomer can bind to the protein, exhibiting UV absorption properties similar to those of hydrolyzed polynuclear Fe(III) species. Analytical ultracentrifugation measurements indicate that a tetramer is formed when Fe(II) is added anaerobically to the protein; multiple protein aggregates are formed upon oxidation of the bound Fe(II). The observed iron oxidation and binding properties of frataxin CyaY may afford the mitochondria protection against iron-induced oxidative damage.     

Iron cycling in the anoxic cryo-ecosystem of Antarctic Lake Vida

Iron redox cycling in metal-rich, hypersaline, anoxic brines plays a central role in the biogeochemical evolution of life on Earth, and similar brines with the potential to harbor life are thought to exist elsewhere in the solar system. To investigate iron biogeochemical cycling in a terrestrial analog we determined the iron redox chemistry and isotopic signatures in the cryoencapsulated liquid brines found in frozen Lake Vida, East Antarctica. We used both in situ voltammetry and the spectrophotometric ferrozine method to determine iron speciation in Lake Vida brine (LVBr). Our results show that iron speciation in the anoxic LVBr was, unexpectedly, not free Fe(II). Iron isotope analysis revealed highly depleted values of ?2.5‰ for the ferric iron of LVBr that are similar to iron isotopic signatures of Fe(II) produced by dissimilatory iron reduction. The presence of Fe(III) in LVBr therefore indicates dynamic iron redox cycling beyond iron reduction. Furthermore, extremely low δ¹⁸O–SO₄ ^2? values (?9.7‰) support microbial iron-sulfur cycling reactions. In combination with evidence for chemodenitrification resulting in iron oxidation, we conclude that coupled abiotic and biotic redox reactions are driving the iron cycle in Lake Vida brine. Our findings challenge the current state of knowledge of anoxic brine chemistry and may serve as an analogue for icy brines found in the outer reaches of the solar system.     

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt,South Africa): Evidence for early microbial iron reduction

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7–2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28‐ to 3.26‐Gyr‐old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ⁵⁶Fe values of ?1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass‐independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ³³S values > +3‰, while the other groups display more variable and closer to 0‰ Δ³³S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ³³S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass‐independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro‐organisms closely related to the last common ancestor had the ability to reduce Fe(III).     

A Zn isotope perspective on the rise of continents

Zinc isotope abundances are fairly constant in igneous rocks and shales and are left unfractionated by hydrothermal processes at pH < 5.5. For that reason, Zn isotopes in sediments can be used to trace the changing chemistry of the hydrosphere. Here, we report Zn isotope compositions in Fe oxides from banded iron formations (BIFs) and iron formations of different ages. Zinc from early Archean samples is isotopically indistinguishable from the igneous average (δ⁶⁶Zn ~0.3‰). At 2.9–2.7 Ga, δ⁶⁶Zn becomes isotopically light (δ⁶⁶Zn < 0‰) and then bounces back to values >1‰ during the ~2.35 Ga Great Oxygenation Event. By 1.8 Ga, BIF δ⁶⁶Zn has settled to the modern value of FeMn nodules and encrustations (~0.9‰). The Zn cycle is largely controlled by two different mechanisms: Zn makes strong complexes with phosphates, and phosphates in turn are strongly adsorbed by Fe hydroxides. We therefore review the evidence that the surface geochemical cycles of Zn and P are closely related. The Zn isotope record echoes Sr isotope evidence, suggesting that erosion starts with the very large continental masses appearing at ~2.7 Ga. The lack of Zn fractionation in pre‐2.9 Ga BIFs is argued to reflect the paucity of permanent subaerial continental exposure and consequently the insignificant phosphate input to the oceans and the small output of biochemical sediments. We link the early decline of δ⁶⁶Zn between 3.0 and 2.7 Ga with the low solubility of phosphate in alkaline groundwater. The development of photosynthetic activity at the surface of the newly exposed continents increased the oxygen level in the atmosphere, which in turn triggered acid drainage and stepped up P dissolution and liberation of heavy Zn into the runoff. Zinc isotopes provide a new perspective on the rise of continents, the volume of carbonates on continents, changing weathering conditions, and compositions of the ocean through time.     

The role of microaerophilic Fe‐oxidizing micro‐organisms in producing banded iron formations

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe‐oxidizing micro‐organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O₂ co‐exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O₂ concentrations, measured as low as 2 μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O₂ concentrations, many FeOM appear to prefer and thrive at low O₂ concentrations (~3–25 μm ). These are similar to the estimated dissolved O₂ concentrations in the few hundred million years prior to the ‘Great Oxidation Event’ (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O₂ and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well‐known BIFs associated with the GOE, as well as afterward when global O₂ levels increased.     

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2–5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe, <0.15 µm) mainly (>85%) consisted of Fe(II) and gradually increased from 0.4 to 15 μM at the sediment surface to ~100–170 µM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) (>85% sFe, <0.02 µm). Sub-surface dFe(II) maxima were enriched in light isotopes (δ⁵⁶Fe ?2.0 to ?1.5‰), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in light isotopes (down to ?3.0‰). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water oxygen in late spring can lead to a substantial release of Fe(II). Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after >7 h. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23–31 µmol m^?2 day^?1) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusion-reaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.     

Abiotic oxidation of Fe(II) by reactive nitrogen species in cultures of the nitrate‐reducing Fe(II) oxidizer Acidovorax sp. BoFeN1 – questioning the existence of enzymatic Fe(II) oxidation

Nitrate‐reducing, Fe(II)‐oxidizing bacteria were suggested to couple with enzymatic Fe(II) oxidation to nitrate reduction. Denitrification proceeds via intermediates (, NO) that can oxidize Fe(II) abiotically at neutral and particularly at acidic pH. Here, we present a revised Fe(II) quantification protocol preventing artifacts during acidic Fe extraction and evaluate the contribution of abiotic vs. enzymatic Fe(II) oxidation in cultures of the nitrate‐reducing, Fe(II) oxidizer Acidovorax sp. BoFeN1. Sulfamic acid used instead of HCl reacts with nitrite and prevents abiotic Fe(II) oxidation during Fe extraction. Abiotic experiments without sulfamic acid showed that acidification of oxic Fe(II) nitrite samples leads to 5.6‐fold more Fe(II) oxidation than in anoxic samples because the formed NO becomes rapidly reoxidized by O₂, therefore leading to abiotic oxidation and underestimation of Fe(II). With our revised protocol using sulfamic acid, we quantified oxidation of approximately 7 mm of Fe(II) by BoFeN1 within 4 days. Without addition of sulfamic acid, the same oxidation was detected within only 2 days. Additionally, abiotic incubation of Fe(II) with nitrite in the presence of goethite as surface catalyst led to similar abiotic Fe(II) oxidation rates as observed in growing BoFeN1 cultures. BoFeN1 growth was observed on acetate with N₂O as electron acceptor. When adding Fe(II), no Fe(II) oxidation was observed, suggesting that the absence of reactive N intermediates (, NO) precludes Fe(II) oxidation. The addition of ferrihydrite [Fe(OH)₃] to acetate/nitrate BoFeN1 cultures led to growth stimulation equivalent to previously described effects on growth by adding Fe(II). This suggests that elevated iron concentrations might provide a nutritional effect rather than energy‐yielding Fe(II) oxidation. Our findings therefore suggest that although enzymatic Fe(II) oxidation by denitrifiers cannot be fully ruled out, its contribution to the observed Fe(II) oxidation in microbial cultures is probably lower than previously suggested and has to be questioned in general until the enzymatic machinery‐mediating Fe(II) oxidation is identified.     

The oxidation,fate and effects of iron during on-site bioremediation of groundwater contaminated by a mixture of polychlorophenols

Kinetics of simultaneous iron and polychlorophenol (CP) oxidation by groundwater enriched cultures were studied in laboratory and during actual remediation in orderto reveal the fate and effects of iron on aerobic on-site bioremediation of boreal groundwater. 2,4,6-tri- (TCP), 2,3,4,6-tetra- (TeCP) and pentachlorophenol (PCP)were degraded in fluidized-bed bioreactor (FBR) by over 99%, over 99%, and over96%, respectively. The oxygen consumption rate for CP-biodegradation was 1.31mol DO L^-1 min^-1 and 0.29 mol DO L^-1 min^-1 for iron oxidation, i.e. approximately 12% of the oxygen was consumed by iron oxidation during normal FBR operation. Mineralization of CPs was confirmed by DOC removal and chloride release of 158% and 78%, respectively. Excess DOC removal was due to partial degradation of the natural organic matter (NOM) (1.1 mg L^-1 or 24% DOC removal) in the groundwater. Removal of NOM consumed 0.91 mol DO L^-1min^-1. Iron oxidation in the FBR was over 94% of which chemical Fe(II) oxidation accounted for up to 10%. Fe(III) partially accumulated (58 to 69%) in the system. The TCP- and CP-biodegradation consumed DO at two times higher rates than the Fe(II)-oxidation in both, laboratory and full-scale, respectively. The batch assays atvarious TCP and Fe(II) ratios and DO concentrations showed simultaneous oxygenconsumption by TCP and Fe-oxidizers and that increased Fe concentrations do notoutcompete the bioremediation of CP's for available oxygen. Abbreviations: DAPI: 4',6'-Diamidino-phenylindole; DO:Dissolved oxygen; DOC: Dissolved organic carbon; FBR: Fluidized-bed bioreactor;HRT: Hydraulic retention time; NOM: Natural organic matter; PCP: Pentachlorophenol; SEM: Scanning-electron microscopy, TeCP: 2,3,4,6-Tetrachlorophenol; TCP: 2,4,6-Trichlorophenol     

Interdependencies between Biotic and Abiotic Ferrous Iron Oxidation and Influence of pH,Oxygen and Ferric Iron Deposits

The effect of pH, oxygen and ferrous iron on growth and oxidation rates of iron-oxidizing bacteria (Gallionella spp and Leptothrix spp) as well as indirect effects, the most prominent being catalytic activity of the produced ferric iron deposits, were investigated. Deposits of biotic origin exhibit slightly lower surface oxidation rates compared to abiotically produced ferric iron. It was shown that the required habitat conditions of the studied species hardly overlap, but increase the pH/oxygen range of potential Fe(II) oxidation conditions. The study highlights the combined effect of microbial iron oxidation and catalytic properties of the Mn and Fe oxidation products.     

Investigation of the role and chemical form of iron in the ovarian carcinogenesis process

BackgroundOvarian cancer is one of the most frequent types of gynaecological malignancy among women. Despite the advances in diagnostic techniques, ovarian tumours are still detected at a late stage, thus the survival rate is very low. Iron is an essential metal in the human body, yet its potential role in ovarian carcinogenesis is yet to be determined. The aim of this study was to check if iron oxidation state in tissue and cystic fluid can be treated as an indicator of the malignancy of the ovarian tumours. Another aspect of this study was to investigate the role of iron in carcinogenesis mechanism in ovarian tumour transformation.MethodsSynchrotron radiation X-ray absorption near edge structure (SR-XANES) spectroscopy was used to analyze the human ovarian tumour tissues and cystic fluids of different types and grades of malignancy. Fresh, non-fixed, frozen samples were used to analyze the state of iron oxidation in all the biological materials. The samples were obtained from patients requiring surgical intervention. The High Energy X-ray Absorption Spectroscopy (XANES) measurements were performed at the beamline P65 at Petra III Extension, DESY – Deutsches Elektronen - Synchrotron.ResultsFe XANES spectra were collected at selected points of a few different regions of the samples. For each specimen, an average of these points was probed. Having been measured, the spectra were compared with organic and inorganic reference materials. Also, the position of the absorption edge was calculated using the integration method. In all specimens, iron occurred in the oxidation states, Fe2+ and Fe3+, although the fraction of iron in the third oxidation state was substantial, especially in malignant cases. The results also show differences in the chemical form of iron in the tissue and cystic fluids of the same patient.ConclusionsThe cryo-XANES measurement carried out for ovarian cancer tissues and cystic fluids showed changes in the chemical form of iron between non-malignant and malignant tumours. For both types of sample can be observed that they contain iron on second and iron on third oxidation state. Moreover, the tendency was observed that malignant tumours of the ovary contain a larger fraction of iron in the second oxidation state compared to non-malignant ones.     

Morphology of biogenic iron oxides records microbial physiology and environmental conditions: toward interpreting iron microfossils

Despite the abundance of Fe and its significance in Earth history, there are no established robust biosignatures for Fe(II)‐oxidizing micro‐organisms. This limits our ability to piece together the history of Fe biogeochemical cycling and, in particular, to determine whether Fe(II)‐oxidizers played a role in depositing ancient iron formations. A promising candidate for Fe(II)‐oxidizer biosignatures is the distinctive morphology and texture of extracellular Fe(III)‐oxyhydroxide stalks produced by mat‐forming microaerophilic Fe(II)‐oxidizing micro‐organisms. To establish the stalk morphology as a biosignature, morphologic parameters must be quantified and linked to the microaerophilic Fe(II)‐oxidizing metabolism and environmental conditions. Toward this end, we studied an extant model organism, the marine stalk‐forming Fe(II)‐oxidizing bacterium, Mariprofundus ferrooxydans PV‐1. We grew cultures in flat glass microslide chambers, with FeS substrate, creating opposing oxygen/Fe(II) concentration gradients. We used solid‐state voltammetric microelectrodes to measure chemical gradients in situ while using light microscopy to image microbial growth, motility, and mineral formation. In low‐oxygen (2.7–28 μm ) zones of redox gradients, the bacteria converge into a narrow (100 μm–1 mm) growth band. As cells oxidize Fe(II), they deposit Fe(III)‐oxyhydroxide stalks in this band; the stalks orient directionally, elongating toward higher oxygen concentrations. M. ferrooxydans stalks display a narrow range of widths and uniquely biogenic branching patterns, which result from cell division. Together with filament composition, these features (width, branching, and directional orientation) form a physical record unique to microaerophilic Fe(II)‐oxidizer physiology; therefore, stalk morphology is a biosignature, as well as an indicator of local oxygen concentration at the time of formation. Observations of filamentous Fe(III)‐oxyhydroxide microfossils from a ~170 Ma marine Fe‐Si hydrothermal deposit show that these morphological characteristics can be preserved in the microfossil record. This study demonstrates the potential of morphological biosignatures to reveal microbiology and environmental chemistry associated with geologic iron formation depositional processes.     

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring,Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron‐rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H₂. Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid‐phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens‐like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat‐shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin–cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe‐rich, circumneutral pH geothermal environment.     

Dissolved silica affects the bulk iron redox state and recrystallization of minerals generated by photoferrotrophy in a simulated Archean ocean

Chemical sedimentary deposits called Banded Iron Formations (BIFs) are one of the best surviving records of ancient marine (bio)geochemistry. Many BIF precursor sediments precipitated from ferruginous, silica-rich waters prior to the Great Oxidation Event at ~2.43 Ga. Reconstructing the mineralogy of BIF precursor phases is key to understanding the coevolution of seawater chemistry and early life. Many models of BIF deposition invoke the activity of Fe(II)-oxidizing photoautotrophic bacteria as a mechanism for precipitating mixed-valence Fe(II,III) and/or fully oxidized Fe(III) minerals in the absence of molecular oxygen. Although the identity of phases produced by ancient photoferrotrophs remains debated, laboratory experiments provide a means to explore what their mineral byproducts might have been. Few studies have thoroughly characterized precipitates produced by photoferrotrophs in settings representative of Archean oceans, including investigating how residual Fe(II)_aq can affect the mineralogy of expected solid phases. The concentration of dissolved silica (Si) is also an important variable to consider, as silicate species may influence the identity and reactivity of Fe(III)-bearing phases. To address these uncertainties, we cultured Rhodopseudomonas palustris TIE-1 as a photoferrotroph in synthetic Archean seawater with an initial [Fe(II)_aq] of 1 mM and [Si] spanning 0–1.5 mM. Ferrihydrite was the dominant precipitate across all Si concentrations, even with substantial Fe(II) remaining in solution. Consistent with other studies of microbial iron oxidation, no Fe-silicates were observed across the silica gradient, although Si coprecipitated with ferrihydrite via surface adsorption. More crystalline phases such as lepidocrocite and goethite were only detected at low [Si] and are likely products of Fe(II)-catalyzed ferrihydrite transformation. Finally, we observed a substantial fraction of Fe(II) in precipitates, with the proportion of Fe(II) increasing as a function of [Si]. These experimental results suggest that photoferrotrophy in a Fe(II)-buffered ocean may have exported Fe(II,III)-oxide/silica admixtures to BIF sediments, providing a more chemically diverse substrate than previously hypothesized.