Iron

Overview#

Evolutionary Origins#

Iron's centrality to biology is not accidental—it reflects the chemistry of the planet where life originated.

Analysis of 3.33-billion-year-old carbonaceous material from the Barberton greenstone belt reveals that iron was among the key bio-functional elements in Earth's earliest ecosystems, enriched in biogenic material alongside vanadium, nickel, and cobalt.^{[1]Metallomics in deep time and the influence of ocean chemistry on the metabolic landscapes of Earth's earliest ecosystemsHickman-Lewis K, Cavalazzi B, Sorieul S et al. · 2020Open reference 1 ↓}

The pre-Great Oxygenation Event world was an iron-rich, anoxic environment where life evolved to depend on iron's redox versatility. Metals that became biologically important only after oxygenation—zinc and molybdenum—are absent or below detection in these ancient specimens.^{[1]Metallomics in deep time and the influence of ocean chemistry on the metabolic landscapes of Earth's earliest ecosystemsHickman-Lewis K, Cavalazzi B, Sorieul S et al. · 2020Open reference 1 ↓}

The modern biological dependency on iron is therefore a 3.3-billion-year inheritance from the geochemistry of early Earth.

Biological Roles#

Iron as the Ancestral Enzyme Cofactor#

A landmark finding from the Imlay lab demonstrates that iron was the ancestral metal cofactor for mononuclear enzymes, later replaced by manganese or zinc in organisms that colonized oxidizing environments.

The enzyme ribulose-5-phosphate 3-epimerase (Rpe) illustrates this: E. coli and Bacteroides metalate Rpe with iron, while B. subtilis and Lactococcus use manganese, and S. cerevisiae uses zinc—yet the metal-coordinating residues are identical across all organisms.^{[8]Rohaun 2024 — Microbes Vary Strategically in Their Metalation of Mononuclear EnzymesSanjay Kumar Rohaun, Ramakrishnan Sethu, James A Imlay · 2024Open reference 8 ↓}

The metal pool, not the protein, determines which cofactor is incorporated. Iron-charged Rpe has the highest catalytic turnover but is instantly inactivated by 0.1 mM hydrogen peroxide via Fenton chemistry at the active site; manganese-charged Rpe is fully resistant.^{[8]Rohaun 2024 — Microbes Vary Strategically in Their Metalation of Mononuclear EnzymesSanjay Kumar Rohaun, Ramakrishnan Sethu, James A Imlay · 2024Open reference 8 ↓}

This reveals a fundamental evolutionary trade-off: organisms sacrifice catalytic efficiency for Oxidative Stress resistance when they switch from iron to manganese cofactors.

E. coli can conditionally switch Rpe from iron to manganese under peroxide stress, mediated by OxyR-induced MntH manganese import and Dps miniferritin sequestration of free iron.^{[8]Rohaun 2024 — Microbes Vary Strategically in Their Metalation of Mononuclear EnzymesSanjay Kumar Rohaun, Ramakrishnan Sethu, James A Imlay · 2024Open reference 8 ↓}

This conditional metal switching is a key survival strategy that connects iron biology to the oxidative stress response.

Role in Wound Healing#

In a murine wound healing time course, iron shows a distinctive temporal profile: elevated at days 7-14 during the proliferation and remodeling phases, linked to oxidoreductase activity, heme-binding, steroid hydroxylase, and PPAR signaling pathways.^{[13]Combined Metallomics/Transcriptomics Profiling Reveals a Major Role for Metals in Wound RepairWilkinson HN, Guinn BA, Hardman MJ · 2021Open reference 13 ↓}

Metal-linked genes constitute 16% of wound-responsive genes with nearly 2-fold overrepresentation, establishing metals as active orchestrators of tissue repair rather than passive bystanders.^{[13]Combined Metallomics/Transcriptomics Profiling Reveals a Major Role for Metals in Wound RepairWilkinson HN, Guinn BA, Hardman MJ · 2021Open reference 13 ↓}

Iron-binding gene keywords in the wound transcriptome cluster with oxidoreductase, heme binding, aromatase, and metabolic processes.^{[13]Combined Metallomics/Transcriptomics Profiling Reveals a Major Role for Metals in Wound RepairWilkinson HN, Guinn BA, Hardman MJ · 2021Open reference 13 ↓}

Dietary and Environmental Sources#

Iron and the Dose-Response Paradox#

Essential elements follow a characteristic dose-response curve: deficiency causes harm at low concentrations, optimal performance at moderate levels, and toxicity at high concentrations.^{[3]Heavy Metal Pollution in the Environment and Their Toxicological Effects on HumansBriffa J, Sinagra E, Blundell R · 2020Open reference 3 ↓}

For iron this curve is unusually narrow—the margin between deficiency (anemia, impaired immunity) and excess (oxidative damage, pathogen feeding) is clinically treacherous, which is why iron status is the most frequently disrupted metal parameter across disease states.

Prenatal and Infant Exposure#

Maternal trace element profiles shape the infant gut microbiome. In a prospective cohort of 146 mother-infant pairs, prenatal metal exposure patterns measurably affected gut microbial diversity and composition at 3, 6, and 12 months of life.^{[15]Xiong 2025 — Prenatal Exposure to Trace Elements Impacts Mother-Infant Gut Microbiome, Metabolome and Resistome During the First Year of LifeShimao Xiong, Bing Xie, Naiyi Yin et al. · 2025Open reference 15 ↓}

Iron supplementation in infants increases Enterobacteriaceae and decreases Lactobacillus; in African children, iron supplementation increased Bacteroidetes.^{[6]Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota RemodelingHonghong Bao, Yi Wang, Hanlin Xiong et al. · 2024Open reference 6 ↓} The developmental window of iron-microbiome interaction is a critical period whose long-term consequences remain poorly understood.

Microbiome Interactions#

Iron Shapes the Commensal-Pathogen Balance#

Iron availability in the gut lumen is one of the primary determinants of microbial community structure. Iron deficiency reduces commensal beneficial bacteria (Lactobacillus, Bacillota) while iron excess increases harmful bacteria (Bacteroides, E. coli, Enterobacteriaceae).^{[6]Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota RemodelingHonghong Bao, Yi Wang, Hanlin Xiong et al. · 2024Open reference 6 ↓}

Iron deficiency anemia (IDA) specifically reduced Bacillota abundance while increasing Bacteroidota, Pseudomonadota, and Patescibacteria.^{[6]Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota RemodelingHonghong Bao, Yi Wang, Hanlin Xiong et al. · 2024Open reference 6 ↓}

The mechanism is competitive: siderophore-producing Enterobacteriaceae outcompete commensals under high-iron conditions because commensals generally lack the aggressive iron acquisition systems that pathogens deploy.^{[18]Microbial Metallomics and Parkinson's Disease: A Unified Metal-Driven Framework Linking Ferroptosis, Dysbiosis, and alpha-Synuclein PathologyKaren Pendergrass · 2025Open reference 18 ↓}

Iron, manganese, and nickel availability in the gut lumen determines competitive outcomes between commensals and pathogens.^{[18]Microbial Metallomics and Parkinson's Disease: A Unified Metal-Driven Framework Linking Ferroptosis, Dysbiosis, and alpha-Synuclein PathologyKaren Pendergrass · 2025Open reference 18 ↓}

Strategic Metal Cofactor Selection#

Different gut organisms have adopted fundamentally different iron strategies. Anaerobes and facultative anaerobes (E. coli, Bacteroides thetaiotaomicron) use iron as the default enzyme cofactor, exploiting its superior catalytic properties in the low-oxygen gut environment.^{[8]Rohaun 2024 — Microbes Vary Strategically in Their Metalation of Mononuclear EnzymesSanjay Kumar Rohaun, Ramakrishnan Sethu, James A Imlay · 2024Open reference 8 ↓}

Aerotolerant organisms like Lactobacillus and Lactococcus constitutively use manganese instead, sacrificing catalytic efficiency for protection against the oxidative damage that iron cofactors invite.^{[8]Rohaun 2024 — Microbes Vary Strategically in Their Metalation of Mononuclear EnzymesSanjay Kumar Rohaun, Ramakrishnan Sethu, James A Imlay · 2024Open reference 8 ↓} This metal strategy difference is one reason why iron supplementation selectively favors iron-dependent pathobionts over manganese-dependent commensals.

Metalloproteome Plasticity During Infection#

Under immune-imposed metal restriction, bacteria can deliberately switch the metal cofactors in their enzymes. Salmonella in macrophages increases manganese(II) (Mn2+) uptake via MntH and SitABC transporters while simultaneously reducing iron(II) (Fe2+) use—a purposeful metalloproteome remodeling that trades catalytic efficiency for oxidative stress resistance.^{[9]McEwan 2024 — Metalloproteome Plasticity: A Factor in Bacterial Pathogen Adaptive Responses?Alastair G. McEwan · 2024Open reference 9 ↓}

Cambialistic enzymes like S. aureus SodM can function with either iron or manganese, giving pathogens metabolic flexibility that strict iron-dependent organisms lack.^{[9]McEwan 2024 — Metalloproteome Plasticity: A Factor in Bacterial Pathogen Adaptive Responses?Alastair G. McEwan · 2024Open reference 9 ↓}

The Microbiome as Intermediary in Neurodegeneration#

A unifying framework proposes that metal dyshomeostasis initiates vulnerability, and the microbiome amplifies and operationalizes it.

Altered host iron handling increases labile metal pools, creating selection pressure for microbes capable of surviving and exploiting metal-rich, inflammatory environments along the gut-brain axis.^{[19]Pendergrass 2025 — From Dysbiosis to Dyshomeostasis: Why Parkinson's Requires a Metallomic–Microbiome LensKaren Pendergrass · 2025Open reference 19 ↓} Dysbiosis in this model reflects functional enrichment for metal resistance and virulence—not a nonspecific imbalance.^{[19]Pendergrass 2025 — From Dysbiosis to Dyshomeostasis: Why Parkinson's Requires a Metallomic–Microbiome LensKaren Pendergrass · 2025Open reference 19 ↓}

Gut microbiota influence the host via metabolites: SCFAs (Butyrate enhances epithelial barrier), indole derivatives, bile acids, and neurotransmitters (serotonin, dopamine, GABA).^{[6]Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota RemodelingHonghong Bao, Yi Wang, Hanlin Xiong et al. · 2024Open reference 6 ↓}

Metal-Microbiome-Behavior Link#

Heavy metal load, microbiome-associated metabolites, and catecholamine ratios together account for 32% of variance in social behaviors in children.

The gut microbiome alters the metabolic outcomes of Heavy Metals, and heavy metals influence microbiome viability and metabolism—creating a bidirectional interaction where iron, zinc, and copper displacement by toxic metals disrupts microbial enzyme function.^{[20]Krajewski 2025 -- Heavy metals, noradrenaline/adrenaline ratio, and microbiome-associated hormone precursor metabolites: biomarkers for social behaviour, ADHD symptoms, and executive function in childrenKristin Krajewski · 2025Open reference 20 ↓}

Nutritional Immunity#

Iron is the centerpiece of Nutritional Immunity (Metal Sequestration)—the host strategy of sequestering metals to starve invading pathogens. But nutritional immunity is not just about withholding iron.

It is a coordinated two-front war: the host simultaneously starves pathogens of iron in the extracellular space and floods them with toxic copper and zinc inside phagosomes.

Metal Weaponization: The Other Side of Immunity#

While iron is withheld, the immune system simultaneously floods pathogens with toxic zinc and copper. Host peptidoglycan recognition proteins (PGRPs) induce 60-100x increases in intracellular zinc(II) (Zn2+) and copper (Cu)+ in target bacteria.^{[24]Kashyap et al. 2014 — Peptidoglycan Recognition Proteins Kill Bacteria by Inducing Oxidative, Thiol, and Metal StressDipika R. Kashyap, Minhui Wang, Li-Hung Liu et al. · 2014Open reference 24 ↓}

This metal weaponization is not optional—chelation of either zinc or copper completely abolished PGRP bactericidal activity, demonstrating that metal intoxication is a required component of immune killing.^{[24]Kashyap et al. 2014 — Peptidoglycan Recognition Proteins Kill Bacteria by Inducing Oxidative, Thiol, and Metal StressDipika R. Kashyap, Minhui Wang, Li-Hung Liu et al. · 2014Open reference 24 ↓}

The iron connection is direct: copper toxicity kills bacteria primarily by destroying iron-sulfur clusters in metabolic enzymes,^{[10]Sullivan 2024 — Resisting Death by Metal: Metabolism and Cu/Zn Homeostasis in BacteriaMatthew J. Sullivan, Ignacio Teran, Kelvin GK Goh et al. · 2024Open reference 10 ↓} and zinc toxicity kills by displacing manganese from enzymes that bacteria switched to precisely to avoid iron-related oxidative damage.^{[25]Chandrangsu & Helmann 2016 — Intracellular Zn Intoxication Mis-metalates PerR, Causing Heme Toxicity and Oxidative DeathPete Chandrangsu, John D. Helmann · 2016Open reference 25 ↓}

Every axis of metal-mediated immune killing ultimately converges on disrupting the iron-dependent or iron-replacement enzyme systems.

The Siderophore Ecology#

Siderophores are small molecules evolved by bacteria to scavenge iron from the host environment. They represent the pathogen's answer to nutritional immunity, and their diversity reveals the intensity of the evolutionary arms race for iron.

Siderophore-antibiotic conjugates (Trojan horses) exploit the pathogen's own iron hunger against it. By attaching antibiotics to siderophore structures, drugs are actively imported through the pathogen's iron uptake machinery.

This achieves MICs 100-fold lower than passive diffusion because active transport concentrates the drug precisely where it acts.^{[29]de Carvalho & Fernandes 2014 — Siderophores as Trojan Horses Against MDR Pathogensde Carvalho, C.C.C.R., Fernandes et al. · 2014Open reference 29 ↓}

Salmochelin-beta-lactam conjugates achieve 100-1000x enhanced activity against uropathogenic E. coli while sparing commensal Lactobacillus, because the IroN salmochelin receptor is encoded on a virulence-associated genomic island absent from commensals—the first demonstration of pathogen-selective antibiotic action.^{[30]Chairatana et al. 2015 — Salmochelin Conjugates for Pathogen-Selective KillingChairatana, P., et al. · 2015Open reference 30 ↓}

Competitive iron deprivation takes a different approach: natural pyoverdine siderophores from Pseudomonas species can outcompete target pathogens' own siderophores for iron. A systematic screen of 320 pyoverdine variants identified specific structural variants that potently inhibit Acinetobacter baumannii, Klebsiella pneumoniae, and Staphylococcus aureus.^{[31]Vollenweider et al. 2024 — Pyoverdines as Iron-Depriving AntimicrobialsVollenweider, V., et al. · 2024Open reference 31 ↓}

Resistance evolution against pyoverdines is slower than against conventional antibiotics, because the target—iron acquisition—is a fundamental metabolic requirement rather than a single enzyme.^{[31]Vollenweider et al. 2024 — Pyoverdines as Iron-Depriving AntimicrobialsVollenweider, V., et al. · 2024Open reference 31 ↓}

Gallium as an iron mimic represents perhaps the most elegant exploitation of iron dependency. Ga3+ has nearly identical ionic radius and coordination geometry to iron(III) (Fe3+), and bacteria cannot distinguish between them.

But gallium is redox-inactive—once incorporated into iron-dependent enzymes like ribonucleotide reductase, it permanently inactivates them, halting DNA synthesis.^{[32]Ikhazuagbe et al. 2025 — Gallium Nanoparticles as Antimicrobial AgentsIkhazuagbe, I., et al. · 2025Open reference 32 ↓}

Galbofloxacin, a rationally designed Ga3+-siderophore-fluoroquinolone conjugate, achieves an MIC of 93 nM against S. aureus—roughly 1000-fold more potent than unconjugated fluoroquinolones.^{[33]Pandey et al. 2021 — Galbofloxacin: Rationally Designed Gallium-Siderophore Antibiotic Against S. aureusPandey, A., et al. · 2021Open reference 33 ↓} Resistance to gallium is intrinsically difficult because blocking gallium entry would simultaneously block essential iron uptake.^{[32]Ikhazuagbe et al. 2025 — Gallium Nanoparticles as Antimicrobial AgentsIkhazuagbe, I., et al. · 2025Open reference 32 ↓}

Assisted Nutritional Immunity#

The concept of "assisted nutritional immunity" uses pharmacological iron chelation not to kill bacteria directly but to amplify the host's own metal-withholding defense.

Deferiprone reduced intracellular iron by 33.1% and bacterial load by 78% in macrophages infected with Piscirickettsia salmonis, with in vivo fish mortality reduced by 34.9%.^{[34]Caruffo et al. 2020 — Deferiprone as Assisted Nutritional Immunity Against PiscirickettsiaCaruffo, M., et al. · 2020Open reference 34 ↓}

The disproportionate effect—modest iron reduction causing dramatic bacterial reduction—suggests pathogens operate near the threshold of iron sufficiency, making them exquisitely sensitive to restriction.^{[34]Caruffo et al. 2020 — Deferiprone as Assisted Nutritional Immunity Against PiscirickettsiaCaruffo, M., et al. · 2020Open reference 34 ↓}

Multi-metal chelation strategies may be more effective than single-metal approaches, as pathogens can compensate for one metal restriction by upregulating alternative pathways.^{[35]Golden et al. 2024 — Metal Chelation as Antibacterial Strategy Against Pseudomonas and AcinetobacterGolden, M., et al. · 2024Open reference 35 ↓}

Disrupting metal homeostasis disarms virulence without directly killing bacteria—a fundamentally different mechanism from conventional antibiotics that reduces resistance selection pressure.^{[35]Golden et al. 2024 — Metal Chelation as Antibacterial Strategy Against Pseudomonas and AcinetobacterGolden, M., et al. · 2024Open reference 35 ↓}

Siderophores Beyond Infection#

Siderophores have therapeutic applications beyond antimicrobials. Desferrioxamine (DFO, originally a bacterial siderophore from Streptomyces pilosus) inhibits P. falciparum growth by depriving the malaria parasite of iron required for intraerythrocytic development.^{[36]Passari et al. 2023 — Siderophores: Medical Applications Beyond AntimicrobialsPassari, A.K., et al. · 2023Open reference 36 ↓}

Enterobactin causes iron-depletion-driven cancer cell apoptosis, and siderophore-hapten conjugates function as vaccines against uropathogenic E. coli by exploiting obligate siderophore receptor expression to target immune responses specifically to pathogens.^{[36]Passari et al. 2023 — Siderophores: Medical Applications Beyond AntimicrobialsPassari, A.K., et al. · 2023Open reference 36 ↓}

Siderophores loaded with radiometals (68Ga, 89Zr) enable PET imaging of bacterial infections by tracking where bacteria are actively scavenging iron.^{[36]Passari et al. 2023 — Siderophores: Medical Applications Beyond AntimicrobialsPassari, A.K., et al. · 2023Open reference 36 ↓}

Conditions Associated#

Major Depressive Disorder#

Shotgun metagenomic analysis of MDD patients reveals pathway-level disruptions connecting iron metabolism to depression.

MDD-enriched pathways include LPS biosynthesis, serotonin degradation, and folate transformations, while protective taxa depleted in MDD (Faecalibacterium prausnitzii, Coprococcus) are SCFA producers whose loss compromises gut barrier integrity and anti-inflammatory signaling.^{[51]Maes 2026 — Functional Shotgun Metagenomic Insights into Gut Microbial Pathway and Enzyme Disruptions Linking Metabolism, Affect, Cognition, and Suicidal Ideation in Major Depressive DisorderMichael Maes, Abbas F. Almulla, Asara Vasupanrajit et al. · 2026Open reference 51 ↓}

The NIMETOX framework positions iron redistribution (not necessarily deficiency) as part of the neuro-immune-metabolic cascade in depression.^{[51]Maes 2026 — Functional Shotgun Metagenomic Insights into Gut Microbial Pathway and Enzyme Disruptions Linking Metabolism, Affect, Cognition, and Suicidal Ideation in Major Depressive DisorderMichael Maes, Abbas F. Almulla, Asara Vasupanrajit et al. · 2026Open reference 51 ↓}

Cross-References#

• Ferroptosis—iron-dependent lipid peroxidation as convergent cell death mechanism
• Nutritional Immunity (Metal Sequestration)—host iron sequestration as primary antimicrobial defense
• Siderophores—pathogen iron acquisition and Trojan horse antimicrobial strategies
• Mis-Metallation—iron-sulfur clusters as the convergent target of metal toxicity
• Parkinson's Disease—iron accumulation in substantia nigra; ferroptotic dopaminergic neuron death
• Alzheimer's Disease—iron accumulation in hippocampus and cortex; amyloid-beta aggregation
• Postpartum Depression—iron deficiency anemia increases PPD risk 1.89-fold
• gut microbiome—iron status shapes commensal vs pathogen balance
• Gut-Brain Axis—iron-microbiome-neurodegeneration link
• Nickel—nickel (Ni) disrupts iron (Fe) homeostasis via enzyme active site substitution and iron-S cluster damage
• Copper—copper (Cu) destroys iron-S clusters; iron/copper ratio as cardiovascular biomarker
• Manganese—iron/manganese (Mn) interchangeability in enzymes; competitive DMT1 transport
• Zinc—zinc (Zn) flooding targets manganese-dependent enzymes that replaced iron-dependent ones
• Cadmium—increased cadmium (Cd) absorption in iron-depleted individuals; proteome-wide mis-metallation
• Gallium—redox-inactive iron mimic; Trojan horse antimicrobial strategy
• Selenium—cooperates with iron in GPX4-mediated antioxidant defense
• Staphylococcus aureus—elaborate siderophore and heme acquisition systems
• Streptococcus pneumoniae—cell wall as metal reservoir; heme as sole iron source
• Escherichia coli—siderophore competition; salmochelin as virulence marker
• Lactoferrin—iron-binding glycoprotein central to mucosal immune defense
• Glutathione (GSH)—GSH required for GPX4 defense against ferroptosis; linchpin of triple-stress immune killing
• Dyshomeostasis—iron dyshomeostasis as the prototype metal imbalance across disease domains
• Metal Chelation Therapy—deferiprone, deferoxamine in neurodegeneration and iron overload