Ferric Iron Reduction in Extreme Acidophiles

Biochemical processes are a key element of natural cycles occurring in the environment and enabling life on earth. With regard to microbially catalyzed iron transformation, research predominantly has focused on iron oxidation in acidophiles, whereas iron reduction played a minor role. Microbial conversion of ferric to ferrous iron has however become more relevant in recent years. While there are several reviews on neutrophilic iron reducers, this article summarizes the research on extreme acidophilic iron reducers. After the first reports of dissimilatory iron reduction by acidophilic, chemolithoautotrophic Acidithiobacillus strains and heterotrophic Acidiphilium species, many other prokaryotes were shown to reduce iron as part of their metabolism. Still, little is known about the exact mechanisms of iron reduction in extreme acidophiles. Initially, hypotheses and postulations for the occurring mechanisms relied on observations of growth behavior or predictions based on the genome. By comparing genomes of well-studied neutrophilic with acidophilic iron reducers (e.g., Ferroglobus placidus and Sulfolobus spp.), it became clear that the electron transport for iron reduction proceeds differently in acidophiles. Moreover, transcriptomic investigations indicated an enzymatically-mediated process in Acidithiobacillus ferrooxidans using respiratory chain components of the iron oxidation in reverse. Depending on the strain of At. ferrooxidans, further mechanisms were postulated, e.g., indirect iron reduction by hydrogen sulfide, which may form by disproportionation of elemental sulfur. Alternative scenarios include Hip, a high potential iron-sulfur protein, and further cytochromes. Apart from the anaerobic iron reduction mechanisms, sulfur-oxidizing acidithiobacilli have been shown to mediate iron reduction at low pH (< 1.3) under aerobic conditions. This presumably non-enzymatic process may be attributed to intermediates formed during sulfur/tetrathionate and/or hydrogen oxidation and has already been successfully applied for the reductive bioleaching of laterites. The aim of this review is to provide an up-to-date overview on ferric iron reduction by acidophiles. The importance of this process in anaerobic habitats will be demonstrated as well as its potential for application.

Resonant X-ray photo-oxidation of light-harvesting iron (II/III) N-heterocyclic carbene complexes

Two photoactive iron N-heterocyclic carbene complexes $${[\hbox {Fe}^{{{\rm{II}}}}(\hbox {btz})_2(\hbox {bpy})]^{2+}}$$ [ Fe II ( btz ) 2 ( bpy ) ] 2 + and $${[\hbox {Fe}^{{\rm{III}}}(\hbox {btz})_3]^{3+}}$$ [ Fe III ( btz ) 3 ] 3 + , where btz is 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene) and bpy is 2,2’-bipyridine, have been investigated by Resonant Photoelectron Spectroscopy (RPES). Tuning the incident X-ray photon energy to match core-valence excitations provides a site specific probe of the electronic structure properties and ligand-field interactions, as well as information about the resonantly photo-oxidised final states. Comparing measurements of the Fe centre and the surrounding ligands demonstrate strong mixing of the Fe $${\hbox {t}_{{\rm{2g}}}}$$ t 2 g levels with occupied ligand $$\pi$$ π orbitals but weak mixing with the corresponding unoccupied ligand orbitals. This highlights the importance of $$\pi$$ π -accepting and -donating considerations in ligand design strategies for photofunctional iron carbene complexes. Spin-propensity is also observed as a final-state effect in the RPES measurements of the open-shell $$\hbox {Fe}^{{\rm{III}}}$$ Fe III complex. Vibronic coupling is evident in both complexes, where the energy dispersion hints at a vibrationally hot final state. The results demonstrate the significant impact of the iron oxidation state on the frontier electronic structure and highlights the differences between the emerging class of $$\hbox {Fe}^{{\rm{III}}}$$ Fe III photosensitizers from those of more traditional $$\hbox {Fe}^{{\rm{II}}}$$ Fe II complexes.

Quantum chemical insight into the effects of the local electron environment on T2*-based MRI

T2* relaxation is an intrinsic magnetic resonance imaging (MRI) parameter that is sensitive to local magnetic field inhomogeneities created by the deposition of endogenous paramagnetic material (e.g. iron). Recent studies suggest that T2* mapping is sensitive to iron oxidation state. In this study, we evaluate the spin state-dependence of T2* relaxation using T2* mapping. We experimentally tested this physical principle using a series of phantom experiments showing that T2* relaxation times are directly proportional to the spin magnetic moment of different transition metals along with their associated magnetic susceptibility. We previously showed that T2* relaxation time can detect the oxidation of Fe2+. In this paper, we demonstrate that T2* relaxation times are significantly longer for the diamagnetic, d10 metal Ga3+, compared to the paramagnetic, d5 metal Fe3+. We also show in a cell culture model that cells supplemented with Ga3+ (S = 0) have a significantly longer relaxation time compared to cells supplemented with Fe3+ (S = 5/2). These data support the hypothesis that dipole–dipole interactions between protons and electrons are driven by the strength of the electron spin magnetic moment in the surrounding environment giving rise to T2* relaxation.

Performance Evaluation of Waste Materials for the Treatment of Acid Mine Drainage to Remove Heavy Metals and Sulfate

Acid Mine Drainage (AMD) is the most severe environmental problem facing the mining sector in the current scenario because of low pH and high pollutants concentration. AMD contains a high amount of sulphate viz. pyrite, FeS2, and to a lesser extent pyrrhotite and heavy metal ions, contaminate both surface water and groundwater. To treat AMD, extensive research projects have been initiated by governments, the mining industry, universities, and research establishments. The environmental impact of AMD can be minimized at these basic levels; prevention should be taken to control the infiltration of groundwater to the pollution site and control the acid-generating process. There are some conventional active methods to treat AMD, such as compost reactor and packed bed iron-oxidation bioreactors; however, these methods have associated with costly material and high maintenance cost, which increases the cost of the entire treatment. In an alternative, the use of low-cost materials such as fly ash, metallurgical slag, zero-valent iron (ZVI), cement kiln dust (CKD), and organic waste such as peat humic agent (PHA), rice husk, and eggshell can be a valuable measure for economic viability to treat the metal-rich wastewater.

Free spermidine evokes superoxide radicals that manifest toxicity

Spermidine and other polyamines alleviate oxidative stress, yet excess spermidine seems toxic to Escherichia coli unless it is neutralized by SpeG, an enzyme for the spermidine N-acetyl transferase function. Besides, a specific mechanism of SpeG function conferring pathogenic fitness to Staphylococcus aureus USA300 strain is unknown. Here, we provide evidence that although spermidine mitigates oxidative stress by lowering hydroxyl radical and hydrogen peroxide levels, excess of it simultaneously triggers the production of superoxide radicals, thereby causing toxicity in the E. coli ΔspeG strain as well as naturally SpeG-deficient S. aureus RN4220 strain. However, wild-type E. coli and S. aureus USA300 with a horizontally-acquired speG gene tolerate applied exogenous spermidine stress. Furthermore, we demonstrate that while RNA-bound spermidine inhibits iron oxidation, free spermidine interacts and oxidizes the iron to evoke superoxide radicals directly. Superoxide radicals thus generated, further affects redox balance and iron homeostasis. Since iron acquisition and metabolism in the host tissues is a challenging task for S. aureus, the current findings helped us explain that the evolutionary gain of SpeG function by S. aureus USA300 strain allows it to neutralize exogenous spermidine- and spermine-mediated toxicity towards iron metabolism inside the host tissues.

Metagenomic Insights Into the Microbial Iron Cycle of Subseafloor Habitats

Microbial iron cycling influences the flux of major nutrients in the environment (e.g., through the adsorptive capacity of iron oxides) and includes biotically induced iron oxidation and reduction processes. The ecological extent of microbial iron cycling is not well understood, even with increased sequencing efforts, in part due to limitations in gene annotation pipelines and limitations in experimental studies linking phenotype to genotype. This is particularly true for the marine subseafloor, which remains undersampled, but represents the largest contiguous habitat on Earth. To address this limitation, we used FeGenie, a database and bioinformatics tool that identifies microbial iron cycling genes and enables the development of testable hypotheses on the biogeochemical cycling of iron. Herein, we survey the microbial iron cycle in diverse subseafloor habitats, including sediment-buried crustal aquifers, as well as surficial and deep sediments. We inferred the genetic potential for iron redox cycling in 32 of the 46 metagenomes included in our analysis, demonstrating the prevalence of these activities across underexplored subseafloor ecosystems. We show that while some processes (e.g., iron uptake and storage, siderophore transport potential, and iron gene regulation) are near-universal, others (e.g., iron reduction/oxidation, siderophore synthesis, and magnetosome formation) are dependent on local redox and nutrient status. Additionally, we detected niche-specific differences in strategies used for dissimilatory iron reduction, suggesting that geochemical constraints likely play an important role in dictating the dominant mechanisms for iron cycling. Overall, our survey advances the known distribution, magnitude, and potential ecological impact of microbe-mediated iron cycling and utilization in sub-benthic ecosystems.

The The effect of iron on precipitation hardening in the Cu-Ni-Mn alloys

The peculiarities in the structure and properties formation of precipitation-hardened Сu–Ni–Mn–Fe alloys within the concentration range of Ni (19.3–21.0 %), Mn (19.5–20.5 %), Fe (0.6–2.7 %), Cu – balance (in wt. %) were investigated in this work. The methods of quantitative metallography, X-ray analysis, scanning electron microscopy, energy-dispersive spectroscopy and differential thermal analysis were applied. Two solid solutions based on a-Cu differing in composition and hardness were found in the structure of the cast Сu–Ni–Mn–Fe alloys. The temperature ranges of solutions’ formation were determined as (1010±10) °С and (890±10) °С, correspondingly. NiMn phase was also formed at (405±15) °С due to precipitation hardening. In the Сu–Ni–Mn–Fe alloys annealed at 500 and 900 °С for 60–750 hours, the volume fraction and size of NiMn precipitates increased with prolonging annealing time and lowering annealing temperature. As iron content was raised up to 2.7 wt. %, the density of NiMn precipitates increased, especially during first 60 hours of annealing at 900 °С. By adding iron, oxidation resistance was improved, but melting temperature and fluidity did not yield any significant change. Hardness of the Сu–Ni–Mn–Fe alloys with higher iron contents increased by 10 НRB on average. However, when test temperature was raised up to 400 °С, tensile strength decreased (by ~1.3 times) and elongation dropped markedly (by ~10 times).