Ferrous Wheel Research Articles

Sulfur cycling likely obscures dynamic biologically-driven iron redox cycling in contemporary methane seep environments.

Deep-sea methane seeps are amongst the most biologically productive environments on Earth and are often characterised by stable, low oxygen concentrations and microbial communities that couple the anaerobic oxidation of methane to sulfate reduction or iron reduction in the underlying sediment. At these sites, ferrous iron (Fe2+) can be produced by organoclastic iron reduction, methanotrophic-coupled iron reduction, or through the abiotic reduction by sulfide produced by the abundant sulfate-reducing bacteria at these sites. The prevalence of Fe2+in the anoxic sediments, as well as the availability of oxygen in the overlying water, suggests that seeps could also harbour communities of iron-oxidising microbes. However, it is unclear to what extent Fe2+ remains bioavailable and in solution given that the abiotic reaction between sulfide and ferrous iron is often assumed to scavenge all ferrous iron as insoluble iron sulfides and pyrite. Accordingly, we searched the sea floor at methane seeps along the Cascadia Margin for microaerobic, neutrophilic iron-oxidising bacteria, operating under the reasoning that if iron-oxidising bacteria could be isolated from these environments, it could indicate that porewater Fe2+ can persist is long enough for biology to outcompete pyritisation. We found that the presence of sulfate in our enrichment media muted any obvious microbially-driven iron oxidation with most iron being precipitated as iron sulfides. Transfer of enrichment cultures to sulfate-depleted media led to dynamic iron redox cycling relative to abiotic controls and sulfate-containing cultures, and demonstrated the capacity for biogenic iron (oxyhydr)oxides from a methane seep-derived community. 16S rRNA analyses revealed that removing sulfate drastically reduced the diversity of enrichment cultures and caused a general shift from a Gammaproteobacteria-domainated ecosystem to one dominated by Rhodobacteraceae (Alphaproteobacteria). Our data suggest that, in most cases, sulfur cycling may restrict the biological "ferrous wheel" in contemporary environments through a combination of the sulfur-adapted sediment-dwelling ecosystems and the abiotic reactions they influence.

Ferrous Wheel Hypothesis: Abiotic nitrate incorporation into dissolved organic matter

We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO3−) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO3− concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the 15N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO3− to nitrite (NO2−) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO3− analysis. The total recovery of 15N added as NO3− fluctuated between 63 and 101%, and the remaining 15N was measured as gaseous N2O. The 15N-labelled NO3− added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO3− (e− acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e− donors). Small amounts of N2O and CO2 provided further evidence of NO3− reduction and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO3− to DON was proven. The present results explain the high retention of NO3− in DOM from volcanic soils in ecosystems with high precipitation.

The microbial ferrous wheel: iron cycling in terrestrial, freshwater, and marine environments

The microbial ferrous wheel: iron cycling in terrestrial, freshwater, and marine environments

Iron isotope fractionation during biogeochemical cycle: Information from suspended particulate matter (SPM) in Aha Lake and its tributaries, Guizhou, China

Iron isotope compositions of suspended particulate matters (SPM) collected from the Aha Lake, an artificial lake in the karst area of Yun-Gui Plateau, and its tributaries in summer and winter were investigated for our understanding of the behavior of Fe isotopes during iron biogeochemical cycling in lake. δ 56Fe values of SPM display statistically negative shift relative to IRMM-014. Samples from the lake display a range from −1.36‰ to −0.10‰ in summer and from −0.30‰ to −0.07‰ in winter, while river samples vary from −0.88‰ to 0.07‰ in summer and from −0.35‰ to −0.03‰ in winter. The average iron isotope composition of aerosol samples is + 0.10‰, which is very similar to that of igneous rocks (0.09‰). The SPM in most rivers and water column showed seasonal variation in δ 56Fe value: the δ 56Fe values of SPM in summer were lower than in winter. The seasonal variation in δ 56Fe value of the riverine SPM should be ascribed to the change in source of particulate Fe and geochemical process in the watershed: More particulate Fe was leached from soil and produced by weathering of pyrite widely distributed in coal-containing strata. It is suggested that both allochthonous inputs and the redox iron cycling control the variations of δ 56Fe values for SPM in lake. During summer stratification, an Fe cycle named “ferrous wheel” is established near the redox boundary where the upwardly diffusing Fe(II) is oxidized and the reactive Fe oxides formed will continuously sink back into the reduction zone to complete the cycle. The δ 56Fe values for SPM reach the minima, −0.88‰ for DB station and −1.36‰ for LJK station, just near the redox boundary as a result of the Fe cycling, where a rough 45% to 76% of Fe in these particles was produced by the repetitive cycle. Due to random transportation and diffusion, δ 56Fe values of the particles near the redox zone distributed into approximately a Gaussian shape. The good negative correlation existed between δ 56Fe values and Fe/Al ratios for DB station, suggesting that they together can be used as good indicators of the redox-driven Fe transformations.

Abiotic reaction of nitrite with dissolved organic carbon? Testing the Ferrous Wheel Hypothesis

The Ferrous Wheel Hypothesis (Davidson et al. 2003) postulates the abiotic formation of dissolved organic N (DON) in forest floors, by the fast reaction of NO2− with dissolved organic C (DOC). We investigated the abiotic reaction of NO2− with dissolved organic matter extracted from six different forest floors under oxic conditions. Solutions differed in DOC concentrations (15–60 mg L−1), NO2− concentrations (0, 2, 20 mg NO2−-N L−1) and DOC/DON ratio (13.4–25.4). Concentrations of added NO2− never decreased within 60 min, therefore, no DON formation from added NO2− took place in any of the samples. Our results suggest that the reaction of NO2− with natural DOC in forest floors is rather unlikely.

Abiotic nitrate incorporation, anaerobic microsites, and the ferrous wheel

Nitrate has long been thought to be chemically unreactive in soil. This view was challenged by the report of an apparently abiotic process whereby nitrate (NO3 −) is incorporated into organic compounds (Dail et al. 2001). In Colman et al. (2007), we examined how common this process might be by testing for it in 45 soils collected from across a range of ecosystem types. We found no evidence of this process occurring in any of the soils, but found evidence of an analytical artifact that creates the appearance of incorporation. We suggested that prior evidence of this process might be due in part or in total to this analytical artifact. Davidson et al. (2008), however, challenged our results and conclusions, suggesting that we failed to observe the abiotic incorporation because we eliminated the anaerobic microsites they argue are necessary for the process. We address the criticisms, and show that they actually raise questions about the robustness of the only study to have reported abiotic NO3 − incorporation in sterile soils. We argue that this area of research needs new artifact-free experiments if the controversy is going to be resolved.

Iron interference in the quantification of nitrate in soil extracts and its effect on hypothesized abiotic immobilization of nitrate

Human alteration of the nitrogen cycle has stimulated research on nitrogen cycling in many aquatic and terrestrial ecosystems, where analyses of nitrate (NO3 −) by standard laboratory methods are common. A recent study by Colman et al. (Biogeochemistry 84:161–169, 2007) identified a potential analytical interference of soluble iron (Fe) with NO3 − quantification by standard flow-injection analysis of soil extracts, and suggested that this interference may have led Dail et al. (Biogeochemistry 54:131–146, 2001) to make an erroneous assessment of abiotic nitrate immobilization in prior 15N pool dilution studies of Harvard Forest soils. In this paper, we reproduce the Fe interference problem systematically and show that it is likely related to dissolved, complexed-Fe interfering with the colorimetric analysis of NO2 −. We also show how standard additions of NO3 − and NO2 − to soil extracts at native dissolved Fe concentrations reveal when the Fe interference problem occurs, and permit the assessment of its significance for past, present, and future analyses. We demonstrate low soluble Fe concentrations and good recovery of standard additions of NO3 − and NO2 − in extracts of sterilized Harvard Forest soils. Hence, we maintain that rapid NO3 − immobilization occurred in sterilized samples of the Harvard Forest O horizon in the study by Dail et al. (2001). Furthermore, additional evidence is accumulating in the literature for rapid disappearance of NO3 − added to soils, suggesting that our observations were not the result of an isolated analytical artifact. The conditions for NO3 − reduction are likely to be highly dependent on microsite properties, both in situ and in the laboratory. The so-called “ferrous wheel hypothesis” (Davidson et al., Glob Chang Biol 9:228–236, 2003) remains an unproven, viable explanation for published observations.

Spinning the “Ferrous Wheel”: The importance of the microbial community in an iron budget during the FeCycle experiment

Several studies have shown the importance of the microbial community in specific aspects of the biogeochemical iron (Fe) cycle such as uptake or regeneration. During FeCycle, a 10‐day study of Fe biogeochemistry within an unperturbed mesoscale in situ SF6 labeled patch of HNLC waters, we investigated the role of both microzooplankton (herbivores and bacterivores) and viruses in regenerating Fe in the upper ocean. In summer 2003 we measured grazer‐mediated Fe regeneration rates. The proportion of bacterial Fe released via grazing was severalfold greater than that mobilized from phytoplankton during herbivory. However, as the algal Fe pool (mainly Synechococcus) was severalfold larger than the bacterial pool, the absolute Fe regeneration rates were similar for both herbivores (17 pmol Fe L−1 d−1) and bacterivores (20 pmol Fe L−1 d−1). In all grazing experiments we observed that 90% (bacterivory) and 25% (herbivory) of the labeled Fe resided in the dissolved fraction after 24 hours. This trend has previously been reported in similar laboratory culture studies, which invoked the formation of dissolved, and/or colloidal metal ligands, associated with digestion, to make the released Fe less bioavailable. This explanation may not be valid for our study as another FeCycle experiment (Maldonado et al., 2005) demonstrated that resident phytoplankton could obtain Fe bound to a wide range of strong‐binding ligands. In situ estimates of virally mediated Fe regeneration during FeCycle ranged from 0.4 to 28 pmol L−1 d−1. It is not known why such a wide range of virally mediated regeneration rates was observed. Such variability prevented a direct comparison on the relative roles of grazers and viruses in Fe recycling. The rates of grazer‐mediated regeneration accounted for 30% to >100% of the bacterial and phytoplankton Fe demand measured during FeCycle, indicating the key role of the microbial food web in Fe recycling.

Ore mineralogy and its relevance for selection of concentration methods in processing of Brazilian iron ores

All major companies now operating in the production of iron ore in Brazil currently employ or are planning to employ some sort of concentration method to upgrade their fine sized products (sinter feed fines and pellet feed fines). The criteria for the selection of the most adequate concentration method for each application include a series of parameters, the most important among them being related to ore mineralogy. This paper reviews the concentration operations currently employed in Brazil and shows how ore mineralogy plays a fundamental role in the selection of a given method. Examples involving gravity concentration (jigs and spirals), magnetic separation (Rare Earth wet drums, ferrous wheel, and Jones type magnetic separators) and flotation (especially column flotation) are discussed. Fundamentally important for the selection of a concentration method is the mineralogy of iron bearing minerals and gangue minerals presented to the feed to the concentration unit operation. For instance, the presence of liberated quartz as the major gangue mineral for a pellet feed size stream generally indicates that flotation will be the best selection for the concentration of that stream. However, even if magnetite is present only in the form of crystal relicts within hematite grains, it may lead to the selection of either magnetic separation alone or a combination of magnetic separation and flotation for concentrating such material. Presence of gibbsite and/or kaolinite as the major phase containing alumina will also affect the selection of the concentration method in all cases. Again for the case of pellet feed size streams, kaolinite does not interfere with flotation whereas gibbsite tends to contaminate flotation concentrate (pellet feed fines) as it is depressed together with iron oxides and hydroxides during the reverse cationic flotation process.

A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

AbstractForest soils, rather than woody biomass, are the dominant long‐term sink for N in forest fertilization studies and, by inference, for N from atmospheric deposition. Recent evidence of significant abiotic immobilization of inorganic‐N in forest humus layers challenges a previously widely held view that microbial processes are the dominant pathways for N immobilization in soil. Understanding the plant, microbial, and abiotic mechanisms of N immobilization in forest soils has important implications for understanding current and future carbon budgets. Abiotic immobilization of nitrate is particularly perplexing because the thermodynamics of nitrate reduction in soils are not generally favorable under oxic conditions. Here we present preliminary evidence for a testable hypothesis that explains abiotic immobilization of nitrate in forest soils. Because iron (and perhaps manganese) plays a key role as a catalyst, with Fe(II) reducing nitrate and reduced forms of carbon then regenerating Fe(II), we call this ‘the ferrous wheel hypothesis’. After nitrate is reduced to nitrite, we hypothesize that nitrite reacts with dissolved organic matter through nitration and nitrosation of aromatic ring structures, thus producing dissolved organic nitrogen (DON). In addition to ignorance about mechanisms of DON production, little is known about DON dynamics in soil and its fate within ecosystems. Evidence from leaching and watershed studies suggests that DON production and consumption may be largely uncoupled from seasonal biological processes, although biological processes ultimately produce the DOC and reducing power that affect DON formation and the entire N cycle. The ferrous wheel hypothesis includes both biological and abiological processes, but the reducing power of plant‐derived organic matter may build up over seasons and years while the abiotic reduction of nitrate and reaction of organic matter with nitrite may occur in a matter of seconds after nitrate enters the soil solution.

Scavenging of Phosphorous by the ‘Ferrous Wheel’ in the Northern Basin of Lake Lugano (Switzerland/Italy)

Scavenging of Phosphorous by the ‘Ferrous Wheel’ in the Northern Basin of Lake Lugano (Switzerland/Italy)

Hypolimnetic redox and phosphorus cycling in hypereutrophic Lake Sebasticook, Maine

The vertical distribution of reduced species and phosphate was monitored throughout a summer stagnation period in hypereutrophic Lake Sebasticook, Maine. The oxygen demand of reduced species released from hypolimnetic sediments was dominated by methane, followed by ammonium, and included minor contributions from Fe 2+, Mn 2+ and sulfide. Release of iron and manganese from the sediments was apparently controlled by dissolution of siderite and rhodochrosite, with the requisite acidity for the dissolution provided by methanogenesis. Redox recycling of these two metals occurs in two vertically displaced “wheels”, which enhance oxidant delivery to the hypolimnion beyond that provided by oxygen diffusion alone. Phosphorus is tightly coupled to the “ferrous wheel”, in the absence of wind events, and is scavenged with a stoichiometry consistent with Tessenow's (Tessenow U., Arch. Hydrobiol. Suppl. 47, 1–79, 1974) laboratory studies.

Maintenance of Iron Meromixis by Iron Redeposition in a Rapidly Flushed Monimolimnion

Water mass ages determined by the 3H–3He method gave a water renewal time of 2.5 ± 1 yr for the monimolimnion of softwater iron meromictic Lake 120. The water renewal time of the monimolimnion is less than, or equal to, the renewal time of the whole lake. The monimolimnion of Lake 120 was, therefore, not found to be a stratum of "perennially stagnant deep water." The rates of supply to, and degradation of, organic matter in the monimolimnion are responsible, in the first place, for the low redox potential necessary to establish the high concentrations of soluble Fe2+ observed (up to 4.2 mmol∙L−1). However, it was found that the major key to maintenance of high monimolimnetic concentrations of Fe, i.e. maintenance of iron meromixis, is recycling of Fe at the chemocline by an [Formula: see text] "Ferrous Wheel." Up to 90% recycling of iron between chemocline and monimolimnion results in an iron residence time of [Formula: see text] for the whole lake (greater than 4 times the whole lake water renewal time).Key words: meromixis, iron recycling, 3H–3He water ages, water renewal times, chemical budgets, sediment funneling, redox reactions, Experimental Lakes Area (ELA).

The Mechanism of Aconitase Action: III. DETECTION AND PROPERTIES OF ENZYME-METAL-SUBSTRATE AND ENZYME-METAL-INHIBITOR BRIDGE COMPLEXES WITH MANGANESE(II) AND IRON(II)

Abstract A magnetic resonance study of the paramagnetic contribution to the longitudinal (1/T1) and the transverse (1/T2) relaxation rates of the carbon-bound protons of citrate and trans-aconitate has been made utilizing Mn(II), Fe(II), and aconitase. The width of the methylene resonances in the nuclear magnetic resonance (NMR) spectrum of citrate (an AB pattern) increases linearly with Mn(II) concentration from 0 to 200 µm due to a paramagnetic effect on 1/T2 of these protons. Aconitase enhances the effect of Mn(II) on the 1/T1 and 1/T2 relaxation rates of the methylene protons of citrate and enhances the effect of Fe(II) on both relaxation rates of the —CH2— and —CHD—protons of pro-R monodeuterocitrate. From the Solomon-Bloembergen equation for 1/T1, the Mn(II) to proton distance in the aconitase-manganese-citrate ternary complex is 4.3 ± 0.5 A, and the Fe(II) to proton distances in the aconitase-iron-monodeuterocitrate ternary complex are 3.8 ± 0.2 A (—CH2—) and 3.1 ± 0.2 A (—CHD—). The absolute and relative values of these distances are in good agreement with crystallographic distances in the binary Mn(II)-citrate chelate complex. Hyperfine coupling of the unpaired electrons of enzyme-bound Fe(II) with the protons of citrate is detected. The data are, therefore, consistent with direct coordination of the substrate, citrate, by the metal ion on aconitase. In contrast with citrate, the NMR spectrum of the inhibitor trans-aconitate shows a nonlinear dependence of the width of the coupled methylene and methine resonances on the concentration of Mn(II) below 30 µm due to chemical exchange spin decoupling (Frankel, L. S. (1969) J. Mol. Spectrosc. 29, 273). Above 30 µm Mn(II), the resonance widths increase linearly with Mn(II) due to a simple paramagnetic effect on 1/T2. A ternary aconitase-manganese-trans-aconitate bridge complex is detected since aconitase enhances the effect of Mn(II) on the 1/T2 relaxation rate of the protons of trans-aconitate. From 1/T1, Mn(II) to proton distances in this ternary complex are 4.8 ± 0.5 A (—CH2—) and 5.3 ± 0.6 A (—CH—). As shown by NMR, both citrate and trans-aconitate compete for the metal site on aconitase at concentrations consistent with their respective dissociation constants as determined kinetically. From 1/T2, the aconitase-Fe(II)-citrate complex forms and dissociates rapidly enough to participate in enzyme catalysis. Hence the metal bridge complexes detected by NMR may be equated with the enzymatically active substrate complex and the enzymatically inactive inhibitor complex observed kinetically. Three mechanisms for the role of Fe(II) in the catalysis, consistent with the data, the forward and reverse ferrous wheel mechanisms, and the Bailar twist mechanism, are discussed.