Iron(III)-siderophore coordination chemistry: Reactivity of marine siderophores

Siderophore-Conjugated Antifungals: A Strategy to Potentially Cure Fungal Infections

Fungi pose a global threat to humankind due to the increasing emergence of multi-drug-resistant fungi. There is a rising incidence of invasive fungal infections. Due to the structural complexity of fungal cell membranes, only a few classes of antifungal agents are effective and have been approved by the U.S. FDA. Hence, researchers globally are focusing on developing novel strategies to cure fungal infections. One of the potential strategies is the "Trojan horse" approach, which uses the siderophore-mediated iron acquisition (SIA) system to scavenge iron to deliver potent antifungal agents for therapeutics and diagnostics. These siderophore conjugates chelate to iron and are taken up through siderophore-iron transporters, which are overexpressed exclusively on microbes such as bacteria or fungi, but not mammalian cells. Our comprehensive review delves into recent advancements in the design of siderophore-conjugated antifungal agents to gain fungal cell entry. Notably, our focus extends to unraveling the intricate relationship between the structure of natural siderophores or siderophore-like molecules and the resulting antifungal activity. By exploring these design strategies, we aim to contribute to the ongoing discourse on combating drug-resistant fungal infections and advancing the landscape of antifungal theranostics.

Iron mobilization from intact ferritin: effect of differential redox activity of quinone derivatives with NADH/O2 and in situ-generated ROS

Ferritins are multimeric nanocage proteins that sequester/concentrate excess of free iron and catalytically synthesize a hydrated ferric oxyhydroxide bio-mineral. Besides functioning as the primary intracellular iron storehouses, these supramolecular assemblies also oversee the controlled release of iron to meet physiologic demands. By virtue of the reducing nature of the cytosol, reductive dissolution of ferritin-iron bio-mineral by physiologic reducing agents might be a probable pathway operating in vivo. Herein, to explore this reductive iron-release pathway, a series of quinone analogs differing in size, position/nature of substituents and redox potentials were employed to relay electrons from physiologic reducing agent, NADH, to the ferritin core. Quinones are well known natural electron/proton mediators capable of facilitating both 1/2 electron transfer processes and have been implicated in iron/nutrient acquisition in plants and energy transduction. Our findings on the structure-reactivity of quinone mediators highlight that iron release from ferritin is dictated by electron-relay capability (dependent on E1/2 values) of quinones, their molecular structure (i.e., the presence of iron-chelation sites and the propensity for H-bonding) and the type/amount of reactive oxygen species (ROS) they generate in situ. Juglone/Plumbagin released maximum iron due to their intermediate E1/2 values, presence of iron chelation sites, the ability to inhibit in situ generation of H2O2 and form intramolecular H-bonding (possibly promotes semiquinone formation). This study may strengthen our understanding of the ferritin-iron-release process and their significance in bioenergetics/O2-based cellular metabolism/toxicity while providing insights on microbial/plant iron acquisition and the dynamic host-pathogen interactions.

Role of microalgae-bacterial consortium in wastewater treatment: A review

In the global effort to reduce CO2 emissions, the concurrent enhancement of pollutant degradation and reductions in fossil fuel consumption are pivotal aspects of microalgae-mediated wastewater treatment. Clarifying the degradation mechanisms of bacteria and microalgae during pollutant treatment, as well as regulatory biolipid production, could enhance process sustainability. The synergistic and inhibitory relationships between microalgae and bacteria are introduced in this paper. The different stimulators that can regulate microalgal biolipid accumulation are also reviewed. Wastewater treatment technologies that utilize microalgae and bacteria in laboratories and open ponds are described to outline their application in treating heavy metal-containing wastewater, animal husbandry wastewater, pharmaceutical wastewater, and textile dye wastewater. Finally, the major requirements to scale up the cascade utilization of biomass and energy recovery are summarized to improve the development of biological wastewater treatment.

A Siderophore Mimicking Gelation Component for Capturing and Self-Separation of Fe(III) from an Aqueous Solution of Mixture of Metal Ions

The carbohydrazide-based gelation component N2,N4,N6-(1,3,5-triazine-2,4,6-triyl)tris(benzene-1,3,5-tricarbohydrazide) (CBTC) was synthesized and characterized using various spectroscopic tools. CBTC and trimesic acid (TMA) get self-assembled to form metallogel with Fe3+, specifically through various noncovalent interactions in a DMSO and H2O mixture. The self-assembly shows remarkable specificity toward Fe(III) among different transition metal salts. It is pertinent to point out that the binding specificity for Fe3+ can also be found in nature in the form of siderophores, as they are mainly involved in scavenging iron selectively from the surroundings. DFT studies have been used to investigate the possible interaction between the different components of the iron metallogel. To determine the selectivity of CBTC for iron, CBTC, along with trimesic acid, is used to interact with other metal ions, including Fe(III) ions, in a single system. The gelation components CBTC and TMA selectively bind with iron(III), which leads to the formation of metallogel and gets separated as a discrete layer, leaving the other metal ions in the solution. Therefore, CBTC and TMA together show iron-scavenging properties. This selective scavenging property is explored through FE-SEM, XPS, PXRD, IR, and ICP-AES analysis. The FE-SEM analysis shows a flower-petal-like morphology for the Fe(III) metallogel. The resemblance in the CBTC-TMA-Fe metallogel and metallogel obtained from the mixture of different metal salts is established through FE-SEM images and XPS analysis. The release of iron from the metallogel is achieved with the help of ascorbic acid, which converts Fe3+ to Fe2+. In biological systems, iron also gets released similarly from siderophores. This is the first report where the synthesized gelation component CBTC molecule is capable of scavenging out iron in the form of metallogel and self-separating from the aqueous mixture in the presence of various other metal ions.

Chelating Effect of Siderophore Desferrioxamine-B on Uranyl Biomineralization Mediated by Shewanella putrefaciens

In contaminated water and soil, little is known about the role and mechanism of the biometabolic molecule siderophore desferrioxamine-B (DFO) in the biogeochemical cycle of uranium due to complicated coordination and reaction networks. Here, a joint experimental and quantum chemical investigation is carried out to probe the biomineralization of uranyl (UO22+, referred to as U(VI) hereafter) induced by Shewanella putrefaciens (abbreviated as S. putrefaciens) in the presence of DFO and Fe3+ ion. The results show that the production of mineralized solids {hydrogen-uranium mica [H2(UO2)2(PO4)2·8H2O]} via S. putrefaciens binding with UO22+ is inhibited by DFO, which can both chelate preferentially UO22+ to form a U(VI)-DFO complex in solution and seize it from U(VI)-biominerals upon solvation. However, with Fe3+ ion introduced, the strong specificity of DFO binding with Fe3+ causes re-emergence of biomineralization of UO22+ {bassetite [Fe(UO2)2(PO4)2·8(H2O)]} by S. putrefaciens, owing to competitive complexation between Fe3+ and UO22+ for DFO. As DFO possesses three hydroxamic functional groups, it forms hexadentate coordination with Fe3+ and UO22+ ions via these functional groups. The stability of the Fe3+-DFO complex is much higher than that of U(VI)-DFO, resulting in some DFO-released UO22+ to be remobilized by S. putrefaciens. Our finding not only adds to the understanding of the fate of toxic U(VI)-containing substances in the environment and biogeochemical cycles in the future but also suggests the promising potential of utilizing functionalized DFO ligands for uranium processing.

Plant Growth-Promoting Soil Bacteria: Nitrogen Fixation, Phosphate Solubilization, Siderophore Production, and Other Biological Activities

This review covers the literature data on plant growth-promoting bacteria in soil, which can fix atmospheric nitrogen, solubilize phosphates, produce and secrete siderophores, and may exhibit several different behaviors simultaneously. We discuss perspectives for creating bacterial consortia and introducing them into the soil to increase crop productivity in agrosystems. The application of rhizosphere bacteria-which are capable of fixing nitrogen, solubilizing organic and inorganic phosphates, and secreting siderophores, as well as their consortia-has been demonstrated to meet the objectives of sustainable agriculture, such as increasing soil fertility and crop yields. The combining of plant growth-promoting bacteria with mineral fertilizers is a crucial trend that allows for a reduction in fertilizer use and is beneficial for crop production.

Optimization of Siderophore Production in Three Marine Bacterial Isolates along with Their Heavy-Metal Chelation and Seed Germination Potential Determination

Siderophores are low-molecular-weight and high-affinity molecules produced by bacteria under iron-limited conditions. Due to the low iron (III) (Fe+3) levels in surface waters in the marine environment, microbes produce a variety of siderophores. In the current study, halophilic bacteria Bacillus taeanensis SMI_1, Enterobacter sp., AABM_9, and Pseudomonas mendocina AMPPS_5 were isolated from marine surface water of Kalinga beach, Bay of Bengal (Visakhapatnam, Andhra Pradesh, India) and were investigated for siderophore production using the Chrome Azurol S (CAS) assay. The effect of various production parameters was also studied. The optimum production of siderophores for SMI_1 was 93.57% siderophore units (SU) (after 48 h of incubation at 30 °C, pH 8, sucrose as carbon source, sodium nitrate as nitrogen source, 0.4% succinic acid), and for AABM_9, it was 87.18 %SU (after 36 h of incubation period at 30 °C, pH 8, in the presence of sucrose, ammonium sulfate, 0.4% succinic acid). The maximum production of siderophores for AMPPS_5 was 91.17 %SU (after 36 h of incubation at 35 °C, pH 8.5, glucose, ammonium sulfate, 0.4% citric acid). The bacterial isolates SMI_1, AABM_9, and AMPPS_5 showed siderophore production at low Fe+3 concentrations of 0.10 µM, 0.01 µM, and 0.01 µM, respectively. The SMI_1 (73.09 %SU) and AMPPS_5 (68.26 %SU) isolates showed siderophore production in the presence of Zn+2 (10 µM), whereas AABM_9 (50.4 %SU) exhibited siderophore production in the presence of Cu+2 (10 µM). Additionally, these bacterial isolates showed better heavy-metal chelation ability and rapid development in seed germination experiments. Based on these results, the isolates of marine-derived bacteria effectively produced the maximum amount of siderophores, which could be employed in a variety of industrial and environmental applications.

Insights into the physiological and genomic characterization of three bacterial isolates from a highly alkaline, terrestrial serpentinizing system

The terrestrial serpentinite-hosted ecosystem known as "The Cedars" is home to a diverse microbial community persisting under highly alkaline (pH ~ 12) and reducing (Eh < -550 mV) conditions. This extreme environment presents particular difficulties for microbial life, and efforts to isolate microorganisms from The Cedars over the past decade have remained challenging. Herein, we report the initial physiological assessment and/or full genomic characterization of three isolates: Paenibacillus sp. Cedars ('Paeni-Cedars'), Alishewanella sp. BS5-314 ('Ali-BS5-314'), and Anaerobacillus sp. CMMVII ('Anaero-CMMVII'). Paeni-Cedars is a Gram-positive, rod-shaped, mesophilic facultative anaerobe that grows between pH 7-10 (minimum pH tested was 7), temperatures 20-40°C, and 0-3% NaCl concentration. The addition of 10-20 mM CaCl₂ enhanced growth, and iron reduction was observed in the following order, 2-line ferrihydrite > magnetite > serpentinite ~ chromite ~ hematite. Genome analysis identified genes for flavin-mediated iron reduction and synthesis of a bacillibactin-like, catechol-type siderophore. Ali-BS5-314 is a Gram-negative, rod-shaped, mesophilic, facultative anaerobic alkaliphile that grows between pH 10-12 and temperatures 10-40°C, with limited growth observed 1-5% NaCl. Nitrate is used as a terminal electron acceptor under anaerobic conditions, which was corroborated by genome analysis. The Ali-BS5-314 genome also includes genes for benzoate-like compound metabolism. Anaero-CMMVII remained difficult to cultivate for physiological studies; however, growth was observed between pH 9-12, with the addition of 0.01-1% yeast extract. Anaero-CMMVII is a probable oxygen-tolerant anaerobic alkaliphile with hydrogenotrophic respiration coupled with nitrate reduction, as determined by genome analysis. Based on single-copy genes, ANI, AAI and dDDH analyses, Paeni-Cedars and Ali-BS5-314 are related to other species (P. glucanolyticus and A. aestuarii, respectively), and Anaero-CMMVII represents a new species. The characterization of these three isolates demonstrate the range of ecophysiological adaptations and metabolisms present in serpentinite-hosted ecosystems, including mineral reduction, alkaliphily, and siderophore production.

Discovery of New Siderophores from a Marine Streptomycetes sp. via Combined Metabolomics and Analysis of Iron-Chelating Activity

The marine-derived Streptomyces sp. FIMYZ-003 strain was found to produce novel siderophores with yields negatively correlated with the iron concentration in the medium. Mass spectrometry (MS)-based metabolomics coupled with metallophore assays identified two novel α-hydroxycarboxylate-type siderophores, fradiamines C and D (3 and 4), together with two related known siderophores, fradiamines A and B (1 and 2). Their chemical structures were elucidated by nuclear magnetic resonance (NMR) and MS experiments. The annotation of a putative fra biosynthetic gene cluster enabled us to propose the biosynthetic pathway of fradiamines A-D. Furthermore, the solution-phase iron-binding activity of fradiamines was evaluated using metabolomics, confirming them as general iron scavengers. Fradiamines A-D exhibited Fe(III) binding activity equivalent to that of deferoxamine B mesylate. Growth analysis of pathogenic microbes demonstrated that fradiamine C promoted the growth of Escherichia coli and Staphylococcus aureus, but fradiamines A, B, and D did not. The results indicate that fradiamine C may serve as a novel iron carrier applicable to antibiotic delivery strategies to treat and prevent foodborne pathogens.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Mechanistic Insights and Potential Use of Siderophores Producing Microbes in Rhizosphere for Mitigation of Stress in Plants Grown in Degraded Land

Plant growth performance under a stressful environment, notably in the agriculture field, is directly correlated with the rapid growth of the human population, which triggers the pressure on crop productivity. Plants perceived many stresses owing to degraded land, which induces low plant productivity and, therefore, becomes a foremost concern for the future to face a situation of food scarcity. Land degradation is a very notable environmental issue at the local, regional, and global levels for agriculture. Land degradation generates global problems such as drought desertification, heavy metal contamination, and soil salinity, which pose challenges to achieving many UN Sustainable Development goals. The plant itself has a varied algorithm for the mitigation of stresses arising due to degraded land; the rhizospheric system of the plant has diverse modes and efficient mechanisms to cope with stress by numerous root-associated microbes. The suitable root-associated microbes and components of root exudate interplay against stress and build adaptation against stress-mediated mechanisms. The problem of iron-deficient soil is rising owing to increasing degraded land across the globe, which hampers plant growth productivity. Therefore, in the context to tackle these issues, the present review aims to identify plant-stress status owing to iron-deficient soil and its probable eco-friendly solution. Siderophores are well-recognized iron-chelating agents produced by numerous microbes and are associated with the rhizosphere. These siderophore-producing microbes are eco-friendly and sustainable agents, which may be managing plant stresses in the degraded land. The review also focuses on the molecular mechanisms of siderophores and their chemistry, cross-talk between plant root and siderophores-producing microbes to combat plant stress, and the utilization of siderophores in plant growth on degraded land.

The Purified Siderophore from Streptomyces tricolor HM10 Accelerates Recovery from Iron-Deficiency-Induced Anemia in Rats

Iron-deficiency-induced anemia is associated with poor neurological development, including decreased learning ability, altered motor functions, and numerous pathologies. Siderophores are iron chelators with low molecular weight secreted by microorganisms. The proposed catechol-type pathway was identified based on whole-genome sequences and bioinformatics tools. The intended pathway consists of five genes involved in the biosynthesis process. Therefore, the isolated catechol-type siderophore (Sid) from Streptomyces tricolor HM10 was evaluated through an anemia-induced rat model to study its potential to accelerate recovery from anemia. Rats were subjected to an iron-deficient diet (IDD) for 42 days. Anemic rats (ARs) were then divided into six groups, and normal rats (NRs) fed a standard diet (SD) were used as a positive control group. For the recovery experiment, ARs were treated as a group I; fed an IDD (AR), group II; fed an SD (AR + SD), group III, and IV, fed an SD with an intraperitoneal injection of 1 μg Sid Kg^-1 (AR + SD + Sid1) and 5 μg Sid Kg^-1 (AR + SD + Sid5) twice per week. Group V and VI were fed an iron-enriched diet (IED) with an intraperitoneal injection of 1 μg Sid Kg^-1 (AR + IED + Sid1) and 5 μg Sid Kg^-1 (AR + IED + Sid5) twice per week, respectively. Weight gain, food intake, food efficiency ratio, organ weight, liver iron concentration (LIC) and plasma (PIC), and hematological parameters were investigated. The results showed that ~50-60 mg Sid L^-1 medium could be producible, providing ~25-30 mg L^-1 purified Sid under optimal conditions. Remarkably, the AR group fed an SD with 5 μg Sid Kg^-1 showed the highest weight gain. The highest feed efficiency was observed in the AR + SD + Sid5 group, which did not significantly differ from the SD group. Liver, kidneys, and spleen weight indicated that diet and Sid concentration were related to weight recovery in a dose-dependent manner. Liver iron concentration (LIC) in the AR + IED + Sid1 and AR + IED + Sid5 groups was considerably higher than in the AR + SD + Sid1 AR + SD + Sid5 groups or the AR + SD group compared to the AR group. All hematological parameters in the treated groups were significantly closely attenuated to SD groups after 28 days, confirming the efficiency of the anemia recovery treatments. Significant increases were obtained in the AR + SD + Sid5 and AR + IED + Sid5 groups on day 14 and day 28 compared to the values for the AR + SD + Sid1 and AR + IED + Sid1 groups. The transferrin saturation % (TSAT) and ferritin concentration (FC) were significantly increased with time progression in the treated groups associatively with PIC. In comparison, the highest significant increases were noticed in ARs fed IEDs with 5 μg Kg^-1 Sid on days 14 and 28. In conclusion, this study indicated that Sid derived from S. tricolor HM10 could be a practical and feasible iron-nutritive fortifier when treating iron-deficiency-induced anemia (IDA). Further investigation focusing on its mechanism and kinetics is needed.

Iron-Deficiency in Atopic Diseases: Innate Immune Priming by Allergens and Siderophores

Although iron is one of the most abundant elements on earth, about a third of the world's population are affected by iron deficiency. Main drivers of iron deficiency are beside the chronic lack of dietary iron, a hampered uptake machinery as a result of immune activation. Macrophages are the principal cells distributing iron in the human body with their iron restriction skewing these cells to a more pro-inflammatory state. Consequently, iron deficiency has a pronounced impact on immune cells, favoring Th2-cell survival, immunoglobulin class switching and primes mast cells for degranulation. Iron deficiency during pregnancy increases the risk of atopic diseases in children, while both children and adults with allergy are more likely to have anemia. In contrast, an improved iron status seems to protect against allergy development. Here, the most important interconnections between iron metabolism and allergies, the effect of iron deprivation on distinct immune cell types, as well as the pathophysiology in atopic diseases are summarized. Although the main focus will be humans, we also compare them with innate defense and iron sequestration strategies of microbes, given, particularly, attention to catechol-siderophores. Similarly, the defense and nutritional strategies in plants with their inducible systemic acquired resistance by salicylic acid, which further leads to synthesis of flavonoids as well as pathogenesis-related proteins, will be elaborated as both are very important for understanding the etiology of allergic diseases. Many allergens, such as lipocalins and the pathogenesis-related proteins, are able to bind iron and either deprive or supply iron to immune cells. Thus, a locally induced iron deficiency will result in immune activation and allergic sensitization. However, the same proteins such as the whey protein beta-lactoglobulin can also transport this precious micronutrient to the host immune cells (holoBLG) and hinder their activation, promoting tolerance and protecting against allergy. Since 2019, several clinical trials have also been conducted in allergic subjects using holoBLG as a food for special medical purposes, leading to a reduction in the allergic symptom burden. Supplementation with nutrient-carrying lipocalin proteins can circumvent the mucosal block and nourish selectively immune cells, therefore representing a new dietary and causative approach to compensate for functional iron deficiency in allergy sufferers.

Siderophores: an alternative bioremediation strategy?

Siderophores are small molecular weight iron scavengers that are mainly produced by bacteria, fungi, and plants. Recently, they have attracted increasing attention because of their potential role in environmental bioremediation. Although siderophores are generally considered to exhibit high specificity for iron, they have also been reported to bind to various metal and metalloid ions. This unique ability allows siderophores to solubilise and mobilise heavy metals and metalloids from soil, thereby facilitating their bioremediation. In addition, because of their redox nature, they can mediate the production of reactive oxygen species (ROS), and thus promote the biodegradation of organic contaminants. The aim of this review is to summarise the existing knowledge on the developed strategies of siderophore-assisted bioremediation of metals, metalloids, and organic contaminants. Additionally, this review also includes the biosynthesis and classification of microbial and plant siderophores.

Synergistic use of siderophores and weak organic ligands during zinc transport in the rhizosphere controlled by pH and ion strength gradients

Citrate (Cit) and Deferoxamine B (DFOB) are two important organic ligands coexisting in soils with distinct different affinities for metal ions. It has been theorized that siderophores and weak organic ligands play a synergistic role during the transport of micronutrients in the rhizosphere, but the geochemical controls of this process remain unknown. Here we test the hypothesis that gradients in pH and ion strength regulate and enable the cooperation. To this end, first we use potentiometric titrations to identify the dominant Zn(II)–Cit and Zn(II)–DFOB complexes and to determine their ionic strength dependent stability constants between 0 and 1 mol dm⁻³. We parametrise the Extended Debye-Hückel (EDH) equation and determine accurate intrinsic association constants (logβ⁰) for the formation of the complexes present. The speciation model developed confirms the presence of [Zn(Cit)]⁻, [Zn(HCit)], [Zn₂(Cit)₂(OH)₂]⁴⁻, and [Zn(Cit)₂]⁴⁻, with [Zn(Cit)]⁻ and [Zn₂(Cit)₂(OH)₂]⁴⁻ the dominant species in the pH range relevant to rhizosphere. We propose the existence of a new [Zn(Cit)(OH)₃]⁴⁻ complex above pH 10. We also verify the existence of two hexadentate Zn(II)–DFOB species, i.e., [Zn(DFOB)]⁻ and [Zn(HDFOB)], and of one tetradentate species [Zn(H₂DFOB)]⁺. Second, we identify the pH and ionic strength dependent ligand exchange points (LEP) of Zn with citrate and DFOB and the stability windows for Zn(II)–Cit and Zn(II)–DFOB complexes in NaCl and rice soil solutions. We find that the LEPs fall within the pH and ionic strength gradients expected in rhizospheres and that the stability windows for Zn(II)–citrate and Zn(II)–DFOB, i.e., low and high affinity ligands, can be distinctly set off. This suggests that pH and ion strength gradients allow for Zn(II) complexes with citrate and DFOB to dominate in different parts of the rhizosphere and this explains why mixtures of low and high affinity ligands increase leaching of micronutrients in soils. Speciation models of soil solutions using newly determined association constants demonstrate that the presence of dissolved organic matter and inorganic ligands (i.e., bicarbonate, phosphate, sulphate, or chlorides) do neither affect the position of the LEP nor the width of the stability windows significantly. In conclusion, we demonstrate that cooperative and synergistic ligand interaction between low and high affinity ligands is a valid mechanism for controlling zinc transport in the rhizosphere and possibly in other environmental reservoirs such as in the phycosphere. Multiple production of weak and strong ligands is therefore a valid strategy of plants and other soil organisms to improve access to micronutrients.

Biosynthesis Pathways, Transport Mechanisms and Biotechnological Applications of Fungal Siderophores

Iron (Fe) is the fourth most abundant element on earth and represents an essential nutrient for life. As a fundamental mineral element for cell growth and development, iron is available for uptake as ferric ions, which are usually oxidized into complex oxyhydroxide polymers, insoluble under aerobic conditions. In these conditions, the bioavailability of iron is dramatically reduced. As a result, microorganisms face problems of iron acquisition, especially under low concentrations of this element. However, some microbes have evolved mechanisms for obtaining ferric irons from the extracellular medium or environment by forming small molecules often regarded as siderophores. Siderophores are high affinity iron-binding molecules produced by a repertoire of proteins found in the cytoplasm of cyanobacteria, bacteria, fungi, and plants. Common groups of siderophores include hydroxamates, catecholates, carboxylates, and hydroximates. The hydroxamate siderophores are commonly synthesized by fungi. L-ornithine is a biosynthetic precursor of siderophores, which is synthesized from multimodular large enzyme complexes through non-ribosomal peptide synthetases (NRPSs), while siderophore-Fe chelators cell wall mannoproteins (FIT1, FIT2, and FIT3) help the retention of siderophores. S. cerevisiae, for example, can express these proteins in two genetically separate systems (reductive and nonreductive) in the plasma membrane. These proteins can convert Fe (III) into Fe (II) by a ferrous-specific metalloreductase enzyme complex and flavin reductases (FREs). However, regulation of the siderophore through Fur Box protein on the DNA promoter region and its activation or repression depend primarily on the Fe availability in the external medium. Siderophores are essential due to their wide range of applications in biotechnology, medicine, bioremediation of heavy metal polluted environments, biocontrol of plant pathogens, and plant growth enhancement.

Toxicity of the iron siderophore mycobactin J in mouse macrophages: Evidence for a hypoxia response

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an obligate intracellular pathogen that lives within the phagosome of macrophages. Here we demonstrate that the siderophore mycobactin J, produced by the closely related intracellular pathogen Mycobacterium paratuberculosis, is toxic to murine macrophage cells. Its median lethal dose, 10 μM, is lower than that of the iron chelators desferrioxamine B and TrenCAM, an enterobactin analog. To determine the source of this toxicity, we conducted microarray, ELISA, and metabolite profiling experiments. The primary response is hypoxia-like, which implies iron starvation as the underlying cause of the toxicity. This observation is consistent with our recent finding that mycobactin J is a stronger iron chelator than had been inferred from previous studies. Mycobactin J is known to partition into cell membranes and hydrophobic organelles indicating that enhanced membrane penetration is also a likely factor. Thus, mycobactin J is shown to be toxic, eliciting a hypoxia-like response under physiological conditions.

Spectroscopic and computational investigations of organometallic complexation of group 12 transition metals by methanobactins from Methylocystis sp. SB2

Methanotrophic bacteria catalyze the aerobic oxidation of methane to methanol using Cu-containing enzymes, thereby exerting a modulating influence on the global methane cycle. To facilitate the acquisition of Cu ions, some methanotrophic bacteria secrete small modified peptides known as "methanobactins," which strongly bind Cu and function as an extracellular Cu recruitment relay, analogous to siderophores and Fe. In addition to Cu, methanobactins form complexes with other late transition metals, including the Group 12 transition metals Zn, Cd, and Hg, although the interplay among solution-phase configurations, metal interactions, and the spectroscopic signatures of methanobactin-metal complexes remains ambiguous. In this study, the complexation of Zn, Cd, and Hg by methanobactin from Methylocystis sp. strain SB2 was studied using a combination of absorbance, fluorescence, extended x-ray absorption fine structure (EXAFS) spectroscopy, and time-dependent density functional theory (TD-DFT) calculations. We report changes in sample absorbance and fluorescence spectral dynamics, which occur on a wide range of experimental timescales and characterize a clear stoichiometric complexation dependence. Mercury L3-edge EXAFS and TD-DFT calculations suggest a linear model for HgS coordination, and TD-DFT suggests a tetrahedral model for Zn2+ and Cd2+. We observed an enhancement in the fluorescence of methanobactin upon interaction with transition metals and propose a mechanism of complexation-hindered isomerization drawing inspiration from the wild-type Green Fluorescent Protein active site. Collectively, our results represent the first combined computational and experimental spectroscopy study of methanobactins and shed new light on molecular interactions and dynamics that characterize complexes of methanobactins with Group 12 transition metals.

Effect of salinity on the zinc(II) binding efficiency of siderophore functional groups and implications for salinity tolerance mechanisms in barley

Bacteria, fungi and grasses use siderophores to access micronutrients. Hence, the metal binding efficiency of siderophores is directly related to ecosystem productivity. Salinization of natural solutions, linked to climate change induced sea level rise and changing precipitation patterns, is a serious ecological threat. In this study, we investigate the impact of salinization on the zinc(II) binding efficiency of the major siderophore functional groups, namely the catecholate (for bacterial siderophores), α-hydroxycarboxylate (for plant siderophores; phytosiderophores) and hydroxamate (for fungal siderophores) bidentate motifs. Our analysis suggests that the order of increasing susceptibility of siderophore classes to salinity in terms of their zinc(II) chelating ability is: hydroxamate < catecholate < α-hydroxycarboxylate. Based on this ordering, we predict that plant productivity is more sensitive to salinization than either bacterial or fungal productivity. Finally, we show that previously observed increases in phytosiderophore release by barley plants grown under salt stress in a medium without initial micronutrient deficiencies, are in line with the reduced zinc(II) binding efficiency of the α-hydroxycarboxylate ligand and hence important for the salinity tolerance of whole-plant zinc(II) status.

Cyanochelins, an Overlooked Class of Widely Distributed Cyanobacterial Siderophores, Discovered by Silent Gene Cluster Awakening

Cyanobacteria require iron for growth and often inhabit iron-limited habitats, yet only a few siderophores are known to be produced by them. We report that cyanobacterial genomes frequently encode polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) biosynthetic pathways for synthesis of lipopeptides featuring β-hydroxyaspartate (β-OH-Asp), a residue known to be involved in iron chelation. Iron starvation triggered the synthesis of β-OH-Asp lipopeptides in the cyanobacteria Rivularia sp. strain PCC 7116, Leptolyngbya sp. strain NIES-3755, and Rubidibacter lacunae strain KORDI 51-2. The induced compounds were confirmed to bind iron by mass spectrometry (MS) and were capable of Fe³⁺ to Fe²⁺ photoreduction, accompanied by their cleavage, when exposed to sunlight. The siderophore from Rivularia, named cyanochelin A, was structurally characterized by MS and nuclear magnetic resonance (NMR) and found to contain a hydrophobic tail bound to phenolate and oxazole moieties followed by five amino acids, including two modified aspartate residues for iron chelation. Phylogenomic analysis revealed 26 additional cyanochelin-like gene clusters across a broad range of cyanobacterial lineages. Our data suggest that cyanochelins and related compounds are widespread β-OH-Asp-featuring cyanobacterial siderophores produced by phylogenetically distant species upon iron starvation. Production of photolabile siderophores by phototrophic cyanobacteria raises questions about whether the compounds facilitate iron monopolization by the producer or, rather, provide Fe²⁺ for the whole microbial community via photoreduction. IMPORTANCE All living organisms depend on iron as an essential cofactor for indispensable enzymes. However, the sources of bioavailable iron are often limited. To face this problem, microorganisms synthesize low-molecular-weight metabolites capable of iron scavenging, i.e., the siderophores. Although cyanobacteria inhabit the majority of the Earth's ecosystems, their repertoire of known siderophores is remarkably poor. Their genomes are known to harbor a rich variety of gene clusters with unknown function. Here, we report the awakening of a widely distributed class of silent gene clusters by iron starvation to yield cyanochelins, β-hydroxy aspartate lipopeptides involved in iron acquisition. Our results expand the limited arsenal of known cyanobacterial siderophores and propose products with ecological function for a number of previously orphan gene clusters.

Biosynthesis of fosfomycin in pseudomonads reveals an unexpected enzymatic activity in the metallohydrolase superfamily

The epoxide-containing phosphonate natural product fosfomycin is a broad-spectrum antibiotic used in the treatment of cystitis. Fosfomycin is produced by both the plant pathogen Pseudomonas syringae and soil-dwelling streptomycetes. While the streptomycete pathway has recently been fully elucidated, the pseudomonad pathway is still mostly elusive. Through a systematic evaluation of heterologous expression of putative biosynthetic enzymes, we identified the central enzyme responsible for completing the biosynthetic pathway in pseudomonads. The missing transformation involves the oxidative decarboxylation of the intermediate 2-phosphonomethylmalate to a new intermediate, 3-oxo-4-phosphonobutanoate, by PsfC. Crystallographic studies reveal that PsfC unexpectedly belongs to a new class of diiron metalloenzymes that are part of the polymerase and histidinol phosphatase superfamily.

Photoactive siderophores: Structure, function and biology

It is well known that bacteria and fungi have evolved sophisticated systems for acquiring the abundant but biologically inaccessible trace element iron. These systems are based on high affinity Fe(III)-specific binding compounds called siderophores which function to acquire, transport, and process this essential metal ion. Many hundreds of siderophores are now known and their numbers continue to grow. Extensive studies of their isolation, structure, transport, and molecular genetics have been undertaken in the last three decades and have been comprehensively reviewed many times. In this review we focus on a unique subset of siderophores that has only been recognized in the last 20 years, namely those whose iron complexes display photoactivity. This photoactivity, which typically results in the photooxidation of the siderophore ligand with concomitant reduction of Fe(III) to Fe(II), seemingly upsets the siderophore paradigm of forming and transporting only extremely stable Fe(III) complexes into microbial cells. Here we review their structure, synthesis, photochemistry, photoproduct coordination chemistry and explore the potential biological and ecological consequences of this photoactivity.

Insights into the chemistry of the amphibactin-metal (M3+) interaction and its role in antibiotic resistance

We have studied the diversity and specificity of interactions of amphibactin produced by Vibrio genus bacterium (Vibrio sp. HC0601C5) with iron and various metal ions in + 3 oxidation state in an octahedral (O_h) environment. To survive in the iron-deficient environment of their host, pathogenic bacteria have devised various efficient iron acquisition strategies. One such strategy involves the production of low molecular weight peptides called siderophores, which have a strong affinity and specificity to chelate Fe³⁺ and can thus facilitate uptake of this metal in order to ensure iron requirements. The Fe uptake by amphibactin and the release of iron inside the cell have been studied. Comparison of the interaction of different transition metal ions (M³⁺) with amphibactin has been studied and it reveals that Co and Ga form stable complexes with this siderophore. The competition of Co and Ga with Fe impedes iron uptake by bacteria, thereby preventing infection.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O2

The high-spin (S = ⁵/₂) meridional diastereoisomer of [Fe^III(tpena)]²⁺ (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate), mer-[Fe(tpena)]²⁺, undergoes photolytic CO₂ release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L^•). The structure of this unprecedented transient iron(II)(L^•) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L^•, namely, [Fe^III₂O(Cl)₂(L⁺)₂](ClO₄)₄(MeCN)₂ (L⁺ = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe^III₂O(Cl)₂(L⁺)₂]⁴⁺ is obtained only in the absence of O₂. Under aerobic conditions, O₂ will intercept the iron(II)(L^•) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L⁺ or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS^- adduct of [Fe(tpena)]²⁺. This has allowed for kinetic observation of the reaction of [Fe^II(L^•)]²⁺ with O₂ which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L^• to a ring-opened tautomer to allow for O₂ insertion between the C and Fe atoms of the iron(II) complex.

Iron Assimilation during Emerging Infections Caused by Opportunistic Fungi with emphasis on Mucorales and the Development of Antifungal Resistance

Iron is a key transition metal required by most microorganisms and is prominently utilised in the transfer of electrons during metabolic reactions. The acquisition of iron is essential and becomes a crucial pathogenic event for opportunistic fungi. Iron is not readily available in the natural environment as it exists in its insoluble ferric form, i.e., in oxides and hydroxides. During infection, the host iron is bound to proteins such as transferrin, ferritin, and haemoglobin. As such, access to iron is one of the major hurdles that fungal pathogens must overcome in an immunocompromised host. Thus, these opportunistic fungi utilise three major iron acquisition systems to overcome this limiting factor for growth and proliferation. To date, numerous iron acquisition pathways have been fully characterised, with key components of these systems having major roles in virulence. Most recently, proteins involved in these pathways have been linked to the development of antifungal resistance. Here, we provide a detailed review of our current knowledge of iron acquisition in opportunistic fungi, and the role iron may have on the development of resistance to antifungals with emphasis on species of the fungal basal lineage order Mucorales, the causative agents of mucormycosis.

Novel South African Rare Actinomycete Kribbella speibonae Strain SK5: A Prolific Producer of Hydroxamate Siderophores Including New Dehydroxylated Congeners

In this paper, we report on the chemistry of the rare South African Actinomycete Kribbella speibonae strain SK5, a prolific producer of hydroxamate siderophores and their congeners. Two new analogues, dehydroxylated desferrioxamines, speibonoxamine 1 and desoxy-desferrioxamine D₁2, have been isolated, together with four known hydroxamates, desferrioxamine D₁3, desferrioxamine B 4, desoxy-nocardamine 5 and nocardamine 6, and a diketopiperazine (DKP) 7. The structures of 1–7 were characterized by the analysis of HRESIMS and 1D and 2D NMR data, as well as by comparison with the relevant literature. Three new dehydroxy desferrioxamine derivatives 8–10 were tentatively identified in the molecular network of K.speibonae strain SK5 extracts, and structures were proposed based on their MS/MS fragmentation patterns. A plausible spb biosynthetic pathway was proposed. To the best of our knowledge, this is the first report of the isolation of desferrioxamines from the actinobacterial genus Kribbella.

Assignment of complex species by affinity capillary electrophoresis: The case of Th(IV)-desferrioxamine B

The electrophoretic mobility change of desferrioxamine B (DFO) was monitored by UV absorption spectrophotometry upon increasing the thorium(IV) concentration in the background electrolyte at two acidities ([HClO₄ ]_Tot = 0.0316 and 0.0100 M). These data enabled to assess the speciation model and to determine the equilibrium constant of [Th(DFO)H₂ ]³⁺ at fixed ionic strength (I = 0.1 M (H,Na)ClO₄ ). Affinity capillary electrophoresis (ACE) turned out to be most helpful in identifying the complexed species by ascertaining its charge and protonation state. The assignment of the correct stoichiometry relied on the reliable estimation of the electrophoretic mobility by assuming similar hydrodynamic radii for (DFO)H₄ ⁺ and the chelate. The value of the apparent equilibrium constant (log β₁₁₂ = 38.7 ± 0.4) obtained by ACE compares favorably well with those reported in the literature for thorium and a range of other metal ions, according to a linear free-energy relationship. This method is useful for studying metal-ligand binding equilibria and provides valuable information for further modelling the behavior of tetravalent actinides under environmental conditions. Structural information about the prevalent solution species in acidic conditions was gained by DFT calculations, confirming the bishydroxamato coordination mode of Th⁴⁺ by the diprotonated ligand.

Lesson from Ecotoxicity: Revisiting the Microbial Lipopeptides for the Management of Emerging Diseases for Crop Protection

Microorganisms area treasure in terms of theproduction of various bioactive compounds which are being explored in different arenas of applied sciences. In agriculture, microbes and their bioactive compounds are being utilized in growth promotion and health promotion withnutrient fortification and its acquisition. Exhaustive explorations are unraveling the vast diversity of microbialcompounds with their potential usage in solving multiferous problems incrop production. Lipopeptides are one of such microbial compounds which havestrong antimicrobial properties against different plant pathogens. These compounds are reported to be produced by bacteria, cyanobacteria, fungi, and few other microorganisms; however, genus Bacillus alone produces a majority of diverse lipopeptides. Lipopeptides are low molecular weight compounds which havemultiple industrial roles apart from being usedas biosurfactants and antimicrobials. In plant protection, lipopeptides have wide prospects owing totheirpore-forming ability in pathogens, siderophore activity, biofilm inhibition, and dislodging activity, preventing colonization bypathogens, antiviral activity, etc. Microbes with lipopeptides that haveall these actions are good biocontrol agents. Exploring these antimicrobial compounds could widen the vistasof biological pest control for existing and emerging plant pathogens. The broader diversity and strong antimicrobial behavior of lipopeptides could be a boon for dealing withcomplex pathosystems and controlling diseases of greater economic importance. Understanding which and how these compounds modulate the synthesis and production of defense-related biomolecules in the plants is a key question—the answer of whichneeds in-depth investigation. The present reviewprovides a comprehensive picture of important lipopeptides produced by plant microbiome, their isolation, characterization, mechanisms of disease control, behavior against phytopathogens to understand different aspects of antagonism, and potential prospects for future explorations as antimicrobial agents. Understanding and exploring the antimicrobial lipopeptides from bacteria and fungi could also open upan entire new arena of biopesticides for effective control of devastating plant diseases.

Photochemistry of iron in aquatic environments

Light energy is a driver for many biogeochemical element cycles in aquatic systems. The sunlight-induced photochemical reduction of ferric iron (Fe(iii) photoreduction) to ferrous iron (Fe(ii)) by either direct ligand-to-metal charge transfer or by photochemically produced radicals can be an important source of dissolved Fe_aq²⁺ in aqueous and sedimentary environments. Reactive oxygen species (ROS) are formed by a variety of light-dependent reactions. Those ROS can oxidize Fe(ii) or reduce Fe(iii), and due to their high reactivity they are key oxidants in aquatic systems where they influence many other biogeochemical cycles. In oxic waters with circumneutral pH, the produced Fe(ii) reaches nanomolar concentrations and serves as a nutrient, whereas in acidic waters, freshwater and marine sediments, which are rich in Fe(ii), the photochemically formed Fe(ii) can reach concentrations of up to 100 micromolar and be used as additional electron donor for acidophilic aerobic, microaerophilic, phototrophic and, if nitrate is present, for nitrate-reducing Fe(ii)-oxidizing bacteria. Therefore, Fe(iii) photoreduction may not only control the primary productivity in the oceans but has a tremendous impact on Fe cycling in the littoral zone of freshwater and marine environments. In this review, we summarize photochemical reactions involving Fe, discuss the role of ROS in Fe cycling, and highlight the importance of photoreductive processes in the environment.

Siderophore (from Synechococcus sp. PCC 7002)-Chelated Iron Promotes Iron Uptake in Caco-2 Cells and Ameliorates Iron Deficiency in Rats

Siderophores are iron chelators with low molecular weight secreted by microorganisms. Siderophores have the potential to become natural iron fortifiers. To explore the feasibility of the application of Synechococcus sp. PCC7002-derived siderophores as iron fortifiers, Synechococcus sp. PCC7002, as a carrier, was fermented to produce siderophores. The absorption mechanism and anemia intervention effect of siderophores-chelated iron (SCI) were studied through the polarized Caco-2 Cell monolayers and the rat model of iron-deficiency anemia, respectively. The results indicated that siderophores (from Synechococcus sp. PCC7002) had an enhancing effect on iron absorption in polarized Caco-2 cell monolayers. The main absorption site of SCI was duodenum with pH 5.5, and the absorption methods included endocytosis and DMT1, with endocytosis being dominant. The effect of sodium phytate on SCI was less than that of ferrous sulfate. Therefore, SCI could resist inhibitory iron absorption factors in polarized Caco-2 cell monolayers. SCI showed significantly higher relative bioavailability (133.58 ± 15.42%) than ferrous sulfate (100 ± 14.84%) and ferric citrate (66.34 ± 8.715%) in the rat model. Food intake, hemoglobin concentration, and hematocrit and serum iron concentration of rats improved significantly after Fe-repletion. Overall, this study indicated that siderophores derived from Synechococcus sp. PCC7002 could be an effective and feasible iron nutritive fortifier.

Is Superoxide-Mediated Fe(III) Reduction Important in Sunlit Surface Waters?

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O₂^•-) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O₂^•- does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O₂^•- and LMCT. The reduction of AFO by O₂^•- occurs following the dissolution of AFO, and hence, the contribution of O₂^•- to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O₂^•-, there is no involvement of O₂^•- in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

Reduction-dependent siderophore assimilation in a model pennate diatom

Diatoms can access inorganic iron with remarkable efficiency, but this process is contingent on carbonate ion concentration. As ocean acidification reduces carbonate concentration, inorganic iron uptake may be discouraged in favor of carbonate-independent uptake. We report details of an iron assimilation process that needs no carbonate but requires exogenous compounds produced by cooccurring organisms. We show this process to be critical for diatom growth at high siderophore concentrations, but ineffective at acquiring iron from low-affinity organic chelators or lithogenic particulates. Understanding the caveats associated with iron source preference in diatoms will help predict the impacts of climate change on microbial community structure in high-nitrate low-chlorophyll ecosystems.

Iron uptake by diatoms is a biochemical process with global biogeochemical implications. In large regions of the surface ocean diatoms are both responsible for the majority of primary production and frequently experiencing iron limitation of growth. The strategies used by these phytoplankton to extract iron from seawater constrain carbon flux into higher trophic levels and sequestration into sediments. In this study we use reverse genetic techniques to target putative iron-acquisition genes in the model pennate diatom Phaeodactylum tricornutum. We describe components of a reduction-dependent siderophore acquisition pathway that relies on a bacterial-derived receptor protein and provides a viable alternative to inorganic iron uptake under certain conditions. This form of iron uptake entails a close association between diatoms and siderophore-producing organisms during low-iron conditions. Homologs of these proteins are found distributed across diatom lineages, suggesting the significance of siderophore utilization by diatoms in the marine environment. Evaluation of specific proteins enables us to confirm independent iron-acquisition pathways in diatoms and characterize their preferred substrates. These findings refine our mechanistic understanding of the multiple iron-uptake systems used by diatoms and help us better predict the influence of iron speciation on taxa-specific iron bioavailability.

Hybrid Ionic Silver and Magnetite Microgels Nanocomposites for Efficient Removal of Methylene Blue

The ionic crosslinked 2-acrylamido-2-methylpropane sulfonic acid-co-acrylic acid hydrogel, AMPS/AA and its Ag and Fe₃O₄ composites were synthesized using an in situ technique. The surface charge, particle sizes, morphology, and thermal stability of the prepared AMPS/AA-Ag and AMPS/AA-Fe₃O₄ composites were evaluated using different analytical techniques and their adsorption characteristics were evaluated to remove the methylene blue cationic dye, MB, from their aqueous solutions at optimum conditions. Also, the same monomers were used to synthesize AMPS/AA microgel and its Ag and Fe₃O₄ nanocomposites, which were synthesized using the same technique. The AMPS/AA-Fe₃O₄ nanocomposite was selected as conventional iron-supported catalyst due to the presence of both Fe(II) and Fe(III) species besides its magnetic properties that allow their easy, fast, and inexpensive separation from the aqueous solution. It was then evaluated as a heterogeneous catalyst for complete MB degradation from aqueous solution by heterogeneous Fenton oxidation. It achieved a high rate of degradation, degrading 100 mg L^-1 of MB during a short time of 35 min as compared with the reported literature.

Fueling a Hot Debate on the Application of TiO2 Nanoparticles in Sunscreen

Titanium is one of the most abundant elements in the earth's crust and while there are many examples of its bioactive properties and use by living organisms, there are few studies that have probed its biochemical reactivity in physiological environments. In the cosmetic industry, TiO2 nanoparticles are widely used. They are often incorporated in sunscreens as inorganic physical sun blockers, taking advantage of their semiconducting property, which facilitates absorbing ultraviolet (UV) radiation. Sunscreens are formulated to protect human skin from the redox activity of the TiO2 nanoparticles (NPs) and are mass-marketed as safe for people and the environment. By closely examining the biological use of TiO2 and the influence of biomolecules on its stability and solubility, we reassess the reactivity of the material in the presence and absence of UV energy. We also consider the alarming impact that TiO2 NP seepage into bodies of water can cause to the environment and aquatic life, and the effect that it can have on human skin and health, in general, especially if it penetrates into the human body and the bloodstream.

Ambiguity of NRPS Structure Predictions: Four Bidentate Chelating Groups in the Siderophore Pacifibactin

Identified through a bioinformatics approach, a nonribosomal peptide synthetase gene cluster in Alcanivorax pacificus encodes the biosynthesis of the new siderophore pacifibactin. The structure of pacifibactin differs markedly from the bioinformatic prediction and contains four bidentate metal chelation sites, atypical for siderophores. Genome mining and structural characterization of pacifibactin is reported herein, as well as characterization of pacifibactin variants accessible due to a lack of adenylation domain fidelity during biosynthesis. A spectrophotometric titration of pacifibactin with Fe(III) and ¹³C NMR spectroscopy of the Ga(III)-pacifibactin complex establish 1:1 metal:pacifibactin coordination and reveal which of the bidentate binding groups are coordinated to the metal. The photoreaction of Fe(III)-pacifibactin, resulting from Fe(III) coordination of the β-hydroxyaspartic acid ligands, is reported.

Siderophores: From natural roles to potential applications

Siderophores are secondary metabolites produced by different organisms in order to scavenge iron from their surrounding environment making this essential element available to the cell. Presenting high affinity for ferric iron, siderophores are secreted out to form soluble ferric complexes that can be taken up by the organisms. Siderophores present complex chemistry that allows them to form the strongest iron-chelating complexes. Interest in this field is always up to date and new siderophores are found with new roles and applications. For example, siderophores participate to the mobilization of iron and other elements and are involved in virulence processes. Recently, a strong relation between siderophores and oxidative stress tolerance has been also highlighted. Their application in medicine has been widely studied as well as in agriculture. However, new fields are paying attention to the use of siderophores as green-iron chelators. In particular, siderophores have been proposed for the preservation of cultural heritage.

Algae induce siderophore biosynthesis in the freshwater bacterium Cupriavidus necator H16

Cupriachelin is a photoreactive lipopeptide siderophore produced by the freshwater bacterium Cupriavidus necator H16. In the presence of sunlight, the iron-loaded siderophore undergoes photolytic cleavage, thereby releasing solubilized iron into the environment. This iron is not only available to the siderophore producer, but also to the surrounding microbial community. In this study, the cupriachelin-based interaction between C. necator H16 and the freshwater diatom Navicula pelliculosa was investigated. A reporter strain of the bacterium was constructed to study differential expression levels of the cupriachelin biosynthesis gene cucJ in response to varying environmental conditions. Not only iron starvation, but also culture supernatants of N. pelliculosa were found to induce cupriachelin biosynthesis. The transcription factors involved in this differential gene expression were identified using DNA-protein pulldown assays. Besides the well-characterized ferric uptake regulator, a two-component system was found to tune the expression of cupriachelin biosynthesis genes in the presence of diatom supernatants.

Photoinduced O2-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex

The iron(III) complex [Fe(tpena)]²⁺ (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine- N'-acetate) undergoes irreversible O₂-dependent N-demethylcarboxylation to afford [Fe^II(SBPy3)(MeCN)]²⁺ (SBPy3 = N, N-bis(2-pyridylmethyl)amine- N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO₂ and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O₂ in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [Fe^III(OH)(SBPy₃)]²⁺. Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [Fe^II(SBPy3)(X)]^2+/3+, X = MeCN, [Zn(tpena)]⁺.