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1. Spatial distribution and biogeochemistry of redox active species in arctic sedimentary porewaters and seeps

   https://doi.org/10.1039/d1em00505g  

   Hudson, Jeffrey M.; Michaud, Alexander B.; Emerson, David; Chin, Yu-Ping (March 2022, Environmental Science: Processes & Impacts)

   Redox active species in Arctic lacustrine sediments play an important, regulatory role in the carbon cycle, yet there is little information on their spatial distribution, abundance, and oxidation states. Here, we use voltammetric microelectrodes to quantify the in situ concentrations of redox-active species at high vertical resolution (mm to cm) in the benthic porewaters of an oligotrophic Arctic lake (Toolik Lake, AK, USA). Mn( ii ), Fe( ii ), O 2 , and Fe( iii )-organic complexes were detected as the major redox-active species in these porewaters, indicating both Fe( ii ) oxidation and reductive dissolution of Fe( iii ) and Mn( iv ) minerals. We observed significant spatial heterogeneity in their abundance and distribution as a function of both location within the lake and depth. Microbiological analyses and solid phase Fe( iii ) measurements were performed in one of the Toolik Lake cores to determine the relationship between biogeochemical redox gradients and microbial communities. Our data reveal iron cycling involving both oxidizing (FeOB) and reducing (FeRB) bacteria. Additionally, we profiled a large microbial iron mat in a tundra seep adjacent to an Arctic stream (Oksrukuyik Creek) where we observed Fe( ii ) and soluble Fe( iii ) in a highly reducing environment. The variable distribution of redox-active substances at all the sites yields insights into the nature and distribution of the important terminal electron acceptors in both lacustrine and tundra environments capable of exerting significant influences on the carbon cycle. 

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2. Enhanced terrestrial Fe(II) mobilization identified through a novel mechanism of microbially driven cave formation in Fe(III)-rich rocks

   https://doi.org/10.1038/s41598-022-21365-3  

   Parker, Ceth W.; Senko, John M.; Auler, Augusto S.; Sasowsky, Ira D.; Schulz, Frederik; Woyke, Tanja; Barton, Hazel A. (October 2022, Scientific Reports)

   Abstract Most cave formation requires mass separation from a host rock in a process that operates outward from permeable pathways to create the cave void. Given the poor solubility of Fe(III) phases, such processes are insufficient to account for the significant iron formation caves (IFCs) seen in Brazilian banded iron formations (BIF) and associated rock. In this study we demonstrate that microbially-mediated reductive Fe(III) dissolution is solubilizing the poorly soluble Fe(III) phases to soluble Fe(II) in the anoxic zone behind cave walls. The resultant Fe(III)-depleted material (termedsub muros) is unable to maintain the structural integrity of the walls and repeated rounds of wall collapse lead to formation of the cave void in an active, measurable process. This mechanism may move significant quantities of Fe(II) into ground water and may help to explain the mechanism of BIF dissolution and REE enrichment in the generation of canga. The role of Fe(III) reducing microorganism and mass separation behind the walls (outward-in, rather than inward-out) is not only a novel mechanism of speleogenesis, but it also may identify a previously overlooked source of continental Fe that may have contributed to Archaean BIF formation. 

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3. Genomic evidence that microbial carbon degradation is dominated by iron redox metabolism in thawing permafrost

   https://doi.org/10.1038/s43705-023-00326-5  

   Romanowicz, Karl J.; Crump, Byron C.; Kling, George W. (November 2023, ISME Communications)

   Abstract Microorganisms drive many aspects of organic carbon cycling in thawing permafrost soils, but the compositional trajectory of the post-thaw microbiome and its metabolic activity remain uncertain, which limits our ability to predict permafrost–climate feedbacks in a warming world. Using quantitative metabarcoding and metagenomic sequencing, we determined relative and absolute changes in microbiome composition and functional gene abundance during thaw incubations of wet sedge tundra collected from northern Alaska, USA. Organic soils from the tundra active-layer (0–50 cm), transition-zone (50–70 cm), and permafrost (70+ cm) depths were incubated under reducing conditions at 4 °C for 30 days to mimic an extended thaw duration. Following extended thaw, we found that iron (Fe)-cycling Gammaproteobacteria, specifically the heterotrophic Fe(III)-reducing Rhodoferax sp. and chemoautotrophic Fe(II)-oxidizing Gallionella sp., increased by 3–5 orders of magnitude in absolute abundance within the transition-zone and permafrost microbiomes, accounting for 65% of community abundance. We also found that the abundance of genes for Fe(III) reduction (e.g., MtrE) and Fe(II) oxidation (e.g., Cyc1) increased concurrently with genes for benzoate degradation and pyruvate metabolism, in which pyruvate is used to generate acetate that can be oxidized, along with benzoate, to CO2 when coupled with Fe(III) reduction. Gene abundance for CH4 metabolism decreased following extended thaw, suggesting dissimilatory Fe(III) reduction suppresses acetoclastic methanogenesis under reducing conditions. Our genomic evidence indicates that microbial carbon degradation is dominated by iron redox metabolism via an increase in gene abundance associated with Fe(III) reduction and Fe(II) oxidation during initial permafrost thaw, likely increasing microbial respiration while suppressing methanogenesis in wet sedge tundra. 

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4. Catalytic and post-translational maturation roles of a conserved active site serine residue in nitrile hydratases

   https://doi.org/10.1016/j.jinorgbio.2024.112763  

   Miller, Callie; Knutson, Kylie; Liu, Dali; Bennett, Brian; Holz, Richard C (January 2025, Journal of Inorganic Biochemistry)

   NA (Ed.)

   A highly conserved second-sphere active site αSer residue in nitrile hydratase (NHase), that forms a hydrogen bond with the axial metal-bound water molecule, was mutated to Ala, Asp, and Thr, in the Co-type NHase from Pseudonocardia thermophila JCM 3095 (PtNHase) and to Ala and Thr in the Fe-type NHase from Rhodococcus equi TG328–2 (ReNHase). All five mutants were successfully purified; metal analysis via ICP-AES indicated that all three Co-type PtNHase mutants were in their apo-form while the Fe-type αSer117Ala and αSer117Thr mutants contained 85 and 50 % of their active site Fe(III) ions, respectively. The kcat values obtained for the PtNHase mutant enzymes were between 0.03 ± 0.01 and 0.2 ± 0.02 s− 1 amounting to <0.8 % of the kcat value observed for WT PtNHase. The Fe-type ReNHase mutants retained some detectable activity with kcat values of 93 ± 3 and 40 ± 2 s− 1 for the αSer117Ala and αSer117Thr mutants, respectively, which is ~5 % of WT ReNHase activity towards acrylonitrile. UV–Vis spectra coupled with EPR data obtained on the ReNHase mutant enzymes showed subtle changes in the electronic environment around the active site Fe(III) ions, consistent with altering the hydrogen bonding interaction with the axial water ligand. X-ray crystal structures of the three PtNHase mutant enzymes confirmed the mutation and the lack of active site metal, while also providing insight into the active site hydrogen bonding network. Taken together, these data confirm that the conserved active site αSer residue plays an important catalytic role but is not essential for catalysis. They also confirm the necessity of the conserved second-sphere αSer residue for the metalation process and subsequent post-translational modification of the α-subunit in Co-type NHases but not Fe-type NHases, suggesting different mechanisms for the two types of NHases. 

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5. Fe iii emission in quasars: evidence for a dense turbulent medium

   https://doi.org/10.1093/mnras/staa1717  

   Temple, Matthew J; Ferland, Gary J; Rankine, Amy L; Hewett, Paul C; Badnell, N R; Ballance, Connor P; Zanna, Giulio Del; Dufresne, Roger P (April 2020, Monthly Notices of the Royal Astronomical Society)

   Abstract Recent improvements to atomic energy-level data allow, for the first time, accurate predictions to be made for the Fe iii line emission strengths in the spectra of luminous, $$L_\text{bol}\simeq 10^{46}-10^{48}{{\rm \, erg}{\rm \, s}^{-1}\,}$$, Active Galactic Nuclei. The Fe iii emitting gas must be primarily photoionized, consistent with observations of line reverberation. We use Cloudy models exploring a wide range of parameter space, together with ≃26,000 rest-frame ultraviolet spectra from the Sloan Digital Sky Survey, to constrain the physical conditions of the line emitting gas. The observed Fe iii emission is best accounted for by dense (nH ≃ 1014 cm−3) gas which is microturbulent, leading to smaller line optical depths and fluorescent excitation. Such high density gas appears to be present in the central regions of the majority of luminous quasars. Using our favoured model, we present theoretical predictions for the relative strengths of the Fe iii UV34 λλ1895,1914,1926 multiplet. This multiplet is blended with the Si iii] λ1892 and C iii] λ1909 emission lines and an accurate subtraction of UV34 is essential when using these lines to infer information about the physics of the broad line region in quasars. 

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