Reductive dissolution of As-bearing iron oxides: Mediating mechanism of fulvic acid and dissimilated iron reducing bacteria.

Fulvic acid (FA) and iron oxides often play regulating roles in the geochemical behavior and ecological risk of arsenic (As) in terrestrial ecosystems. FA can act as electron shuttles to facilitate the reductive dissolution of As-bearing iron (hydr)oxides. However, the influence of FA from different sources on the sequential conversion of Fe/As in As-bearing iron oxides under biotic and abiotic conditions remains unclear. In this work, we exposed prepared As-bearing iron oxides to FAs derived from lignite (FAL) and plant peat (FAP) under anaerobic conditions, tracked the fate of Fe and As in the aqueous phase, and investigated the reduction transformation of Fe(III)/As(V) with or without the presence of Shewanella oneidensis MR-1. The results showed that the reduction efficiency of Fe(III)/As(V) was increased by MR-1, through its metabolic activity and using FAs as electron shuttles. The reduction of Fe(III)/As(V) was closely associated with goethite being more conducive to Fe/As reduction compared to hematite. It is determined that functional groups such as hydroxy, carboxy, aromatic, aldehyde, ketone and aliphatic groups are the primary electron donors. Their reductive capacities rank in the following sequence: hydroxy> carboxy, aromatic, aldehyde, ketone> aliphatic group. Notably, our findings suggest that in the biotic reduction, Fe significantly reduction precedes As reduction, thereby influencing the latter's reduction process across all incubation systems. This work provides empirical support for understanding iron's role in modulating the geochemical cycling of As and is of significant importance for assessing the release risk of arsenic in natural environments.

Lack of physiological evidence for cytochrome filaments functioning as conduits for extracellular electron transfer.

Extracellular cytochrome filaments are proposed to serve as conduits for long-range extracellular electron transfer. The primary functional physiological evidence has been the reported inhibition of Geobacter sulfurreducens Fe(III) oxide reduction when the gene for the filament-forming cytochrome OmcS is deleted. Here we report that the OmcS-deficient strain from that original report reduces Fe(III) oxide as well as the wild-type, as does a triple mutant in which the genes for the other known filament-forming cytochromes were also deleted. The triple cytochrome mutant displayed filaments with the same 3 nm diameter morphology and conductance as those produced by Escherichia coli heterologously expressing the G. sulfurreducens PilA pilin gene. Fe(III) oxide reduction was inhibited when the pilin gene in cytochrome-deficient mutants was modified to yield poorly conductive 3 nm diameter filaments. The results are consistent with the concept that 3 nm diameter electrically conductive pili (e-pili) are required for G. sulfurreducens long-range extracellular electron transfer. In contrast, rigorous physiological functional evidence is lacking for cytochrome filaments serving as conduits for long-range electron transport. IMPORTANCE: Unraveling microbial extracellular electron transfer mechanisms has profound implications for environmental processes and advancing biological applications. This study on Geobacter sulfurreducens challenges prevailing beliefs on cytochrome filaments as crucial components thought to facilitate long-range electron transport. The discovery of an OmcS-deficient strain's unexpected effectiveness in Fe(III) oxide reduction prompted a reevaluation of the key conduits for extracellular electron transfer. By exploring the impact of genetic modifications on G. sulfurreducens' performance, this research sheds light on the importance of 3-nm diameter electrically conductive pili in Fe(III) oxide reduction. Reassessing these mechanisms is essential for uncovering the true drivers of extracellular electron transfer in microbial systems, offering insights that could revolutionize applications across diverse fields.

The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal.

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2-) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

Mechanism of Peroxynitrite Interaction with Ferric M. tuberculosis Nitrobindin: A Computational Study.

Nitrobindins (Nbs) are all-ß-barrel heme proteins present along the evolutionary ladder. They display a highly solvent-exposed ferric heme group with the iron atom being coordinated by the proximal His residue and a water molecule at the distal position. Ferric nitrobindins (Nb(III)) play a role in the conversion of toxic peroxynitrite (ONOO-) to harmless nitrate, with the value of the second-order rate constant being similar to those of most heme proteins. The value of the second-order rate constant of Nbs increases as the pH decreases; this suggests that Nb(III) preferentially reacts with peroxynitrous acid (ONOOH), although ONOO- is more nucleophilic. In this work, we shed light on the molecular basis of the ONOO- and ONOOH reactivity of ferric Mycobacterium tuberculosis Nb (Mt-Nb(III)) by dissecting the ligand migration toward the active site, the water molecule release, and the ligand binding process by computer simulations. Classical molecular dynamics simulations were performed by employing a steered molecular dynamics approach and the Jarzynski equality to obtain ligand migration free energy profiles for both ONOO- and ONOOH. Our results indicate that ONOO- and ONOOH migration is almost unhindered, consistent with the exposed metal center of Mt-Nb(III). To further analyze the ligand binding process, we computed potential energy profiles for the displacement of the Fe(III)-coordinated water molecule using a hybrid QM/MM scheme at the DFT level and a nudged elastic band approach. These results indicate that ONOO- exhibits a much larger barrier for ligand displacement than ONOOH, suggesting that water displacement is assisted by protonation of the leaving group by the incoming ONOOH.

Spectroscopic, electrochemical, and kinetic trends in Fe(III)-thiolate disproportionation near physiologic pH.

In addition to its primary oxygen-atom-transfer function, cysteamine dioxygenase (ADO) exhibits a relatively understudied anaerobic disproportionation reaction (ADO-Fe(III)-SR → ADO-Fe(II) + ½ RSSR) with its native substrates. Inspired by ADO disproportionation reactivity, we employ [Fe(tacn)Cl3] (tacn = 1,4,7-triazacyclononane) as a precursor for generating Fe(III)-thiolate model complexes in buffered aqueous media. A series of Fe(III)-thiolate model complexes are generated in situ using aqueous [Fe(tacn)Cl3] and thiol-containing ligands cysteamine, penicillamine, mercaptopropionate, cysteine, cysteine methyl ester, N-acetylcysteine, and N-acetylcysteine methyl ester. We observe trends in UV-Vis and electron paramagnetic resonance (EPR) spectra, disproportionation rate constants, and cathodic peak potentials as a function of thiol ligand. These trends will be useful in rationalizing substrate-dependent Fe(III)-thiolate disproportionation reactions in metalloenzymes.

Bioproduction of cerium-bearing magnetite and application to improve carbon-black supported platinum catalysts.

BACKGROUND: Biogeochemical processing of metals including the fabrication of novel nanomaterials from metal contaminated waste streams by microbial cells is an area of intense interest in the environmental sciences. RESULTS: Here we focus on the fate of Ce during the microbial reduction of a suite of Ce-bearing ferrihydrites with between 0.2 and 4.2 mol% Ce. Cerium K-edge X-ray absorption near edge structure (XANES) analyses showed that trivalent and tetravalent cerium co-existed, with a higher proportion of tetravalent cerium observed with increasing Ce-bearing of the ferrihydrite. The subsurface metal-reducing bacterium Geobacter sulfurreducens was used to bioreduce Ce-bearing ferrihydrite, and with 0.2 mol% and 0.5 mol% Ce, an Fe(II)-bearing mineral, magnetite (Fe(II)(III)2O4), formed alongside a small amount of goethite (FeOOH). At higher Ce-doping (1.4 mol% and 4.2 mol%) Fe(III) bioreduction was inhibited and goethite dominated the final products. During microbial Fe(III) reduction Ce was not released to solution, suggesting Ce remained associated with the Fe minerals during redox cycling, even at high Ce loadings. In addition, Fe L2,3 X-ray magnetic circular dichroism (XMCD) analyses suggested that Ce partially incorporated into the Fe(III) crystallographic sites in the magnetite. The use of Ce-bearing biomagnetite prepared in this study was tested for hydrogen fuel cell catalyst applications. Platinum/carbon black electrodes were fabricated, containing 10% biomagnetite with 0.2 mol% Ce in the catalyst. The addition of bioreduced Ce-magnetite improved the electrode durability when compared to a normal Pt/CB catalyst. CONCLUSION: Different concentrations of Ce can inhibit the bioreduction of Fe(III) minerals, resulting in the formation of different bioreduction products. Bioprocessing of Fe-minerals to form Ce-containing magnetite (potentially from waste sources) offers a sustainable route to the production of fuel cell catalysts with improved performance.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate.

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 µmol L-1 combined with total Fe(II) increased to 188.79 µmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Nitrate-dependent anaerobic methane oxidation coupled to Fe(III) reduction as a source of ammonium and nitrous oxide.

'Candidatus Methanoperedens nitroreducens' is an archaeal methanotroph with global importance that links carbon and nitrogen cycles and great potential for sustainable operation of wastewater treatment. It has been reported to mediate the anaerobic oxidation of methane through a reverse methanogenesis pathway while reducing nitrate to nitrite. Here, we demonstrate that 'Ca. M. nitroreducens' reduces ferric iron forming ammonium (23.1 %) and nitrous oxide (N2O, 46.5 %) from nitrate. These results are supported with the upregulation of genes coding for proteins responsible for dissimilatory nitrate reduction to ammonium (nrfA), N2O formation (norV, cyt P460), and multiple multiheme c-type cytochromes for ferric iron reduction. Concomitantly, an increase in the N2O-reducing SJA-28 lineage and a decrease in the nitrite-reducing 'Candidatus Methylomirabilis oxyfera' are consistent with the changes in 'Ca. M. nitroreducens' end products. These findings demonstrate the highly flexible physiology of 'Ca. M. nitroreducens' in anaerobic ecosystems with diverse electron acceptor conditions, and further reveals its roles in linking methane oxidation to global biogeochemical cycles. 'Ca. M. nitroreducens' could significantly affect the bioavailability of nitrogen sources as well as the emission of greenhouse gas in natural ecosystems and wastewater treatment plants.

Geogenic Phosphorus Enrichment in Groundwater due to Anaerobic Methane Oxidation-Coupled Fe(III) Oxide Reduction.

Accumulation of geogenic phosphorus (P) in groundwater is an emerging environmental concern, which is closely linked to coupled processes involving FeOOH and organic matter under methanogenic conditions. However, it remains unclear how P enrichment is associated with methane cycling, particularly the anaerobic methane oxidation (AMO). This study conducted a comprehensive investigation of carbon isotopes in dissolved inorganic carbon (DIC), CO2, and CH4, alongside Fe isotopes, microbial communities, and functions in quaternary aquifers of the central Yangtze River plain. The study found that P concentrations tended to increase with Fe(II) concentrations, δ56Fe, and δ13C-DIC, suggesting P accumulation due to the reductive dissolution of FeOOH under methanogenic conditions. The positive correlations of pmoA gene abundance versus δ13C-CH4 and Fe concentrations versus δ13C-CH4, and the prevalent presence of Candidatus_Methanoperedens, jointly demonstrated the potential significance of Fe(III)-mediated AMO process (Fe-AMO) alongside traditional methanogenesis. The increase of P concentration with δ13C-CH4 value, pmoA gene abundance, and Fe concentration suggested that the Fe-AMO process facilitated P enrichment in groundwater. Redundancy analysis confirmed this assertion, identifying P concentration as the primary determinant and the cooperative influence of Fe-AMO microorganisms such as Candidatus_Methanoperedens and Geobacter on P enrichment. Our work provided new insights into P dynamics in subsurface environments.

The dual roles of dissimilatory iron reduction in the carbon cycle: The "iron mesh" effect can increase inorganic carbon sequestration.

Dissimilatory iron reduction (DIR) can drive the release of organic carbon (OC) as carbon dioxide (CO2 ) by mediating electron transfer between organic compounds and microbes. However, DIR is also crucial for carbon sequestration, which can affect inorganic-carbon redistribution via iron abiotic-phase transformation. The formation conditions of modern carbonate-bearing iron minerals (ICFe ) and their potential as a CO2 sink are still unclear. A natural environment with modern ICFe , such as karst lake sediment, could be a good analog to explore the regulation of microbial iron reduction and sequential mineral formation. We find that high porosity is conducive to electron transport and dissimilatory iron-reducing bacteria activity, which can increase the iron reduction rate. The iron-rich environment with high calcium and OC can form a large sediment pore structure to support rapid DIR, which is conducive to the formation and growth of ICFe . Our results further demonstrate that the minimum DIR threshold suitable for ICFe formation is 6.65 µmol g-1 dw day-1 . DIR is the dominant pathway (average 66.93%) of organic anaerobic mineralization, and the abiotic-phase transformation of Fe2+ reduces CO2 emissions by ~41.79%. Our findings indicate that as part of the carbon cycle, DIR not only drives mineralization reactions but also traps carbon, increasing the stability of carbon sinks. Considering the wide geographic distribution of DIR and ICFe , our findings suggest that the "iron mesh" effect may become an increasingly important vector of carbon sequestration.

Biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz facilitated Cr(VI) reduction by shaping soil functional microbial communities.

Cr(VI) contamination is widely recognized as one of the major environmental hazards. To address the problem of remediation of soil Cr(VI) contamination and utilization of waste peanut shells, this study comprehensively investigated the effects of peanut shell-derived biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz on Cr(VI) reduction and microbial community succession in soil. This study confirmed that root exudate-loaded peanut shell biochar reduced soil pH while simultaneously increasing DOC, sulfide, and Fe(II) concentrations, thereby facilitating the reduction of Cr(VI), achieving a reduction efficiency of 81.8%. Based on XPS and SEM elemental mapping analyses, Cr(VI) reduction occurred concurrently with the Fe and S redox cycles. Furthermore, the microbial diversity, abundance of the functional genera (Geobacter, Arthrobacter, and Desulfococcus) and the metabolic functions associated with Cr(VI) reduction were enhanced by root exudate-loaded biochar. Root exudate-loaded biochar can promote both direct Cr(VI) reduction mediated by the Cr(VI)-reducing bacteria Arthrobacter, and indirect Cr(VI) reduction through Cr/S/Fe co-transformation mediated by the sulfate-reducing bacteria Desulfococcus and Fe(III)-reducing bacteria Geobacter. This study demonstrates the effectiveness of peanut shell biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz to promote soil Cr(VI) reduction, reveals the mechanism how root exudate-loaded biochar shapes functional microbial communities to facilitate Cr(VI) reduction, and proposes a viable strategy for Cr(VI) remediation and utilization of peanut shell.

Enhancing the bioreduction and interaction of arsenic and iron by thiosulfate in groundwater.

Thiosulfate influences the bioreduction and migration transformation of arsenic (As) and iron (Fe) in groundwater environments. The aim of this study was to investigate the impact of microbially-mediated sulfur cycling on the bioreduction and interaction of As and Fe. Microcosm experiments were conducted, including bioreduction of thiosulfate, As(V), and Fe(III) by Citrobacter sp. JH012-1, as well as the influence of thiosulfate input at different initial arsenate concentrations on the bioreduction of As(V) and Fe(III). The results demonstrate that Citrobacter sp. JH012-1 exhibited strong reduction capabilities for thiosulfate, As(V), and Fe(III). Improving thiosulfate level promoted the bioreduction of Fe(III) and As(V). When 0, 0.1, 0.5, and 1 mM thiosulfate were added, Fe(III) was completely reduced within 9 days, 3 days, 1 day, and 0.5 days, simultaneously, 72.8%, 82.2%, 85.5%, and 90.0% of As(V) were reduced, respectively. The products of As(III) binding with sulfide are controlled by the ratio of As-S. When the initial arsenate concentration was 0.025 mM, the addition of thiosulfate resulted in the accumulation of soluble thioarsenite. However, when the initial arsenate level increased to 1 mM, precipitates of orpiment or realgar were formed. In the presence of both arsenic and iron, As(V) significantly inhibits the bioreduction of Fe(III). Under the concentrations of 0, 0.025, and 1 mM As(V), the reduction rates of Fe(III) were 100%, 91%, and 83%, respectively. In this scenario, the sulfide produced by thiosulfate reduction tends to bind with Fe(II) rather than As(III). Therefore, the competition of arsenic-iron and thiosulfate concentration should be considered to study the impact of thiosulfate on arsenic and iron migration and transformation in groundwater.

Biodegradable ferroelectric molecular crystal with large piezoelectric response.

Transient implantable piezoelectric materials are desirable for biosensing, drug delivery, tissue regeneration, and antimicrobial and tumor therapy. For use in the human body, they must show flexibility, biocompatibility, and biodegradability. These requirements are challenging for conventional inorganic piezoelectric oxides and piezoelectric polymers. We discovered high piezoelectricity in a molecular crystal HOCH2(CF2)3CH2OH [2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD)] with a large piezoelectric coefficient d33 of ~138 picocoulombs per newton and piezoelectric voltage constant g33 of ~2450 × 10-3 volt-meters per newton under no poling conditions, which also exhibits good biocompatibility toward biological cells and desirable biodegradation and biosafety in physiological environments. HFPD can be composite with polyvinyl alcohol to form flexible piezoelectric films with a d33 of 34.3 picocoulombs per newton. Our material demonstrates the ability for molecular crystals to have attractive piezoelectric properties and should be of interest for applications in transient implantable electromechanical devices.

Metagenomic insights into the mechanism for the rapid enrichment and high stability of Candidatus Brocadia facilitated by Fe(â ¢).

The rapid enrichment of anammox bacteria and its fragile resistance to adverse environment are the critical problems facing of anammox processes. As an abundant component in anammox bacteria, iron has been proved to promote the activity and growth of anammox bacteria in the mature anammox systems, but the functional and metabolic profiles in Fe(III) enhanced emerging anammox systems have not been evaluated. Results indicated that the relative abundance of functional genes involved in oxidative phosphorylation, nitrogen metabolism, cofactors synthesis, and extracellular polymers synthesis pathways was significantly promoted in the system added with 5 mg/L Fe(III) (R5). These enhanced pathways were crucial to energy generation, nitrogen removal, cell activity and proliferation, and microbial self-defense, thereby accelerating the enrichment of anammox bacteria Ca. Brocadia and facilitating their resistance to adverse environments. Microbial community analysis showed that the proportion of Ca. Brocadia in R5 also increased to 64.42 %. Hence, R5 could adapt rapidly to the increased nitrogen loading rate and increase the nitrogen removal rate by 108 % compared to the system without Fe(III) addition. However, the addition of 10 and 20 mg/L Fe(III) showed inhibitory effects on the growth and activity of anammox bacteria, which exhibited the lower relative abundance of Ca. Brocadia and unstable or even collapsed nitrogen removal performance. This study not only clarified the concentration range of Fe(III) that promoted and inhibited the enrichment of anammox bacteria, but also deepened our understanding of the functional and metabolic mechanisms underlying enhanced enrichment of anammox bacteria by Fe(III), providing a potential strategy to hasten the start-up of anammox from conventional activated sludge.

Molecular insights into the mutualism that induces iron deficiency tolerance in sorghum inoculated with Trichoderma harzianum.

Iron (Fe) deficiency is a common mineral stress in plants, including sorghum. Although the soil fungus Trichoderma harzianum has been shown to mitigate Fe deficiency in some circumstances, neither the range nor mechanism(s) of this process are well understood. In this study, high pH-induced Fe deficiency in sorghum cultivated in pots with natural field soil exhibited a significant decrease in biomass, photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency, and Fe-uptake in both the root and shoot. However, the establishment of T. harzianum colonization in roots of Fe-deprived sorghum showed significant improvements in morpho-physiological traits, Fe levels, and redox status. Molecular detection of the fungal ThAOX1 (L-aminoacid oxidase) gene showed the highest colonization of T. harzianum in the root tips of Fe-deficient sorghum, a location thus targeted for further analysis. Expression studies by RNA-seq and qPCR in sorghum root tips revealed a significant upregulation of several genes associated with Fe uptake (SbTOM2), auxin synthesis (SbSAURX15), nicotianamine synthase 3 (SbNAS3), and a phytosiderophore transporter (SbYS1). Also induced was the siderophore synthesis gene (ThSIT1) in T. harzianum, a result supported by biochemical evidence for elevated siderophore and IAA (indole acetic acid) levels in roots. Given the high affinity of fungal siderophore to chelate insoluble Fe3+ ions, it is likely that elevated siderophore released by T. harzianum led to Fe(III)-siderophore complexes in the rhizosphere that were then transported into roots by the induced SbYS1 (yellow-stripe 1) transporter. In addition, the observed induction of several plant peroxidase genes and ABA (abscisic acid) under Fe deficiency after inoculation with T. harzianum may have helped induce tolerance to Fe-deficiency-induced oxidative stress and adaptive responses. This is the first mechanistic explanation for T. harzianum's role in helping alleviate Fe deficiency in sorghum and suggests that biofertilizers using T. harzianum will improve Fe availability to crops in high pH environments.

Transferrin-inspired iron delivery across the cell membrane using [(L2Fe)2(µ-O)] (L = chlorquinaldol) to harness anticancer activity of ferroptosis.

Although iron is a bio-essential metal, dysregulated iron acquisition and metabolism result in production of reactive oxygen species (ROS) due to the Fenton catalytic reaction, which activates ferroptotic cell death pathways. The lipophilic Fe(III)-chelator chlorquinaldol (L; i.e., 5,7-dichloro-8-hydroxy-2-methylquinoline) strongly favors the formation of a highly stable binuclear Fe(III) complex [(L2Fe)2(µ-O)] (1) that can mimic the function of the Fe(III)-transferrin complex in terms of the strong binding to Fe(III) and facile release of Fe(II) when the metal center is reduced. It should be noted that the cellular uptake of 1 is not transferrin receptor-mediated but enhanced by the high lipophilicity of chlorquinaldol. Once 1 is transported across the cell membrane, Fe(III) can be reduced by ferric reductase or other cellular antioxidants to be released as Fe(II), which triggers the Fenton catalytic reaction, thus harnessing the anticancer activity of iron. As the result, this transferrin-inspired iron-delivery strategy significantly reduces the cytotoxicity of 1 in normal human embryonic kidney cells (HEK 293) and the hemolytic activity of 1 in human red blood cells (hRBCs), giving rise to the unique tumor-specific anticancer activity of this Fe(III) complex.

Ultrafast removal of toxic Cr(VI) by the marine bacterium Vibrio natriegens.

The fastest-growing microbe Vibrio natriegens is an excellent platform for bioproduction processes. Until now, this marine bacterium has not been examined for bioremediation applications, where the production of substantial amounts of biomass would be beneficial. V. natriegens can perform extracellular electron transfer (EET) to Fe(III) via a single porin-cytochrome circuit conserved in Vibrionaceae. Electroactive microbes capable of EET to Fe(III) usually also reduce toxic metals such as carcinogenic Cr(VI), which is converted to Cr(III), thus decreasing its toxicity and mobility. Here, the performance of V. natriegens was explored for the bioremediation of Cr(VI). At a density of 100 mg/mL, V. natriegens removed 5-20 mg/L Cr(VI) within 30 s and 100 mg/L Cr(VI) within 10 min. In comparison, the model bacterium Escherichia coli grown to a comparable cell density removed Cr(VI) 36 times slower. To eliminate Cr(VI), V. natriegens had to be metabolically active, and functional outer-membrane c-type cytochromes were required. At the end of the Cr(VI) removal process, V. natriegens had reduced all of it into Cr(III) while adsorbing more than half of the metallic ions. These results demonstrate that V. natriegens, with its fast metabolism, is a viable option for the rapid treatment of aqueous pollution with Cr.

Uropathogenic Escherichia coli wield enterobactin-derived catabolites as siderophores.

Uropathogenic Escherichia coli (UPEC) secrete multiple siderophore types to scavenge extracellular iron(III) ions during clinical urinary tract infections, despite the metabolic costs of biosynthesis. Here, we find the siderophore enterobactin (Ent) and its related products to be prominent components of the iron-responsive extracellular metabolome of a model UPEC strain. Using defined Ent biosynthesis and import mutants, we identify lower molecular weight dimeric exometabolites as products of incomplete siderophore catabolism, rather than prematurely released biosynthetic intermediates. In E. coli, iron acquisition from iron(III)-Ent complexes requires intracellular esterases that hydrolyze the siderophore. Although UPEC are equipped to consume the products of completely hydrolyzed Ent, we find that Ent and its derivatives may be incompletely hydrolyzed to yield products with retained siderophore activity. These results are consistent with catabolic inefficiency as means to obtain more than one iron ion per siderophore molecule. This is compatible with an evolved UPEC strategy to maximize the nutritional returns from metabolic investments in siderophore biosynthesis.

Ammonium loss microbiologically mediated by Fe(III) and Mn(IV) reduction along a coastal lagoon system.

Anaerobic ammonium oxidation, associated with both iron (Feammox) and manganese (Mnammox) reduction, is a microbial nitrogen (N) removal mechanism recently identified in natural ecosystems. Nevertheless, the spatial distributions of these non-canonical Anammox (NC-Anammox) pathways and their environmental drivers in subtidal coastal sediments are still unknown. Here, we determined the potential NC-Anammox rates and abundance of dissimilatory metal-reducing bacteria (Acidomicrobiaceae A6 and Geobacteraceae) at different horizons (0-20 cm at 5 cm intervals) of subtidal coastal sediments using the 15N isotope-tracing technique and molecular analyses. Sediments were collected across three sectors (inlet, transition, and inner) in a coastal lagoon system (Bahia de San Quintin, Mexico) dominated by seagrass meadows. The positive relationship between 30N2 production rates and dissimilatory Fe and Mn reduction provided evidence for Feammox's and Mnammox's co-occurrence. N loss through NC-Anammox was detected in subtidal sediments, with potential rates of 0.07-0.62 µg N g-1 day-1. NC-Anammox process in vegetated sediments tended to be higher than those in adjacent unvegetated ones. NC-Anammox rates showed a subsurface peak (between 5 and 15 cm) in the vegetated sediments but decreased consistently with depth in the adjacent bare bottoms. Thus, the presence/absence of seagrasses and sediment characteristics, particularly the availability of organic carbon and microbiologically reducible Fe(III) and Mn(IV), affected the abundance of dissimilatory metal-reducing bacteria, which mediated NC-Anammox activity and the associated N removal. An annual loss of 32.31 ± 3.57 t N was estimated to be associated with Feammox and Mnammox within the investigated area, accounting for 2.8-4.7% of the gross total import of reactive N from the ocean into the Bahia de San Quintin. Taken as a whole, this study reveals the distribution patterns and controlling factors of the NC-Anammox pathways along a coastal lagoon system. It improves our understanding of the coupling between N and trace metal cycles in coastal environments.

Insights into nitrate-reducing Fe(II) oxidation by Diaphorobacter caeni LI3T through kinetic, nitrogen isotope fractionation, and genome analyses.

Nitrate (NO3-)-reducing Fe(II) oxidation (NRFO) is prevalent in anoxic environments. However, it is uncertain in which step(s) the biological Fe(II) oxidation is coupled with denitrification during NRFO. In this study, a heterotrophic NRFO bacterium, Diaphorobacter caeni LI3T, was isolated from paddy soil and used to investigate the transformation of Fe(II) and nitrogen as well as nitrogen isotopic fractionation (δ15N-N2O) during NRFO. Fe(II) oxidation was observed in the Cell+NO3- +Fe(II), Cell+NO2- + Fe(II), and NO2- + Fe(II) treatments, resulting in precipitation of amorphous Fe(III) minerals and lepidocrocite on the surface and in the periplasm of cells. The presence of Fe(II) slightly accelerated microbial NO3- reduction in the Cell+NO3- + Fe(II) treatment relative to the Cell+NO3- treatment, but slowed down the NO2- reduction in the Cell+NO2- + Fe(II) treatment relative to the Cell+NO2- treatment likely due to cell encrustation that blocking microbial NO2- reduction in the periplasm. The δ15N-N2O results in the Cell+NO3- + Fe(II) treatment were close to those in the Cell+NO3- and Cell+NO2- treatments, indicating that the accumulative N2O is primarily of biological origin during NRFO. The genome analysis found a complete set of denitrification and oxidative phosphorylation genes in strain LI3T, the metabolic pathways of which were closely related with cyc2 and cytc as indicated by protein-protein interactions network analysis. It is proposed that Fe(II) oxidation is catalyzed by the outer membrane protein Cyc2, with the resulting electrons being transferred to the nitrite reductase NirS via CytC in the periplasm, and the CytC can also accept electrons from the oxidative phosphorylation in the cytoplasmic membrane. Overall, our findings provide new insights into the potential pathways of biological Fe(II) oxidation coupled with nitrate reduction in heterotrophic NRFO bacteria.