Bacterial Iron Handling Shapes Soil Communities and Toxic Metal Cleanup

Iron is everywhere in soil, yet for bacteria it is often frustratingly out of reach. A new narrative review published in Biotechnology for the Environment argues that the way bacteria solubilize and reduce iron is not merely a matter of microbial housekeeping but a central force shaping soil bacterial communities and, ultimately, the fate of toxic metals such as cadmium, chromium, arsenic, and uranium. The review, authored by Oluwafemi Bamidele Daramola and colleagues and published on 22 July 2026, synthesizes two decades of peer-reviewed literature to connect iron metabolism, community dynamics, and bioremediation strategies in a single framework.

The authors assessed their own literature search using Item 3 of the Scale for the Assessment of Narrative Review Articles (SANRA), drawing on PubMed, ScienceDirect, Google Scholar, Web of Science, and publisher platforms. Their search combined controlled vocabulary and free-text terms covering iron metabolism, bacterial iron solubilization and reduction, siderophores, soil bacterial diversity, iron speciation, iron plaque, nano zerovalent iron, manganese oxide, and organic amendments. Only peer-reviewed studies from the last twenty years with empirical evidence relevant to iron solubilization and toxic metal bioreduction were retained after title, abstract, and full-text screening. As with any narrative review, the synthesis reflects the authors’ selection of studies rather than a formal meta-analysis, and the findings summarized below are drawn from the primary research the review cites.

At the heart of the review is a simple chemical problem. Iron is the fourth most abundant element in the Earth’s crust, but in aerobic, neutral-pH soils it is locked away as highly insoluble ferric oxyhydroxides or organic-ligand complexes that microbes cannot directly reduce. Bacteria solve this with two main strategies: secreting siderophores, low-molecular-weight ligands with a very high affinity for ferric iron (Fe3+), and releasing organic acids or protons that locally acidify their surroundings, boosting iron bioavailability. Species such as Pantoea agglomerans, Priestia aryabhattai, and Enterobacter cloacae have shown notable iron-solubilizing prowess. Once ferric-siderophore complexes are captured by specific outer membrane receptors, Gram-negative bacteria internalize them via periplasmic binding proteins and inner membrane ATP-binding cassette transporters, while Gram-positive bacteria couple the proton motive force of the cytoplasmic membrane to uptake through the TonB system.

Inside the cell, the reduction of Fe3+ to ferrous iron (Fe2+) is pivotal. The review emphasizes that this internalized and reduced iron is essential for building the cofactors that power metabolic and enzymatic activity, including iron-sulfur centers in nitrate reductase, nitrite reductase, and various dehydrogenation coenzymes. Iron homeostasis is guarded by the Ferric Uptake Regulator (Fur) protein: when iron is plentiful, Fe2+ binds Fur, which then represses the transcription of iron-acquisition genes; when iron is scarce, the repression lifts and transport machinery is expressed. Under anaerobic conditions, dissimilatory iron-reducing bacteria such as Geobacter metallireducens, Shewanella putrefaciens, Bacillus infernus, Geothrix fermentens, and Wolinella succinogenes use Fe3+ as a terminal electron acceptor, coupling its reduction to the oxidation of hydrogen or organic substrates. Geobacter species deploy conductive pili for direct electron transfer to mineral surfaces, while Shewanella species secrete extracellular electron shuttles and outer membrane cytochromes.

Which iron forms dominate a soil, the review argues, is a primary driver of which bacteria thrive there. In pyrite-dominated environments, the oxidation of iron and sulfur generates extreme acidity, favoring specialized acidophiles such as Sulfurimonas, Ferroplasma, and Leptospirillum that maintain neutral internal pH through proton pumps and transporters. Studies in pyrite mining areas show these iron-solubilizing species reach high relative abundance in contaminated zones even as overall microbial richness declines compared with less polluted areas. Work cited from Xia and colleagues found that ferric iron significantly alters soil bacterial diversity, with Anaeromusa, Acinetobacter, and Bacteroides dominating and preferentially initiating iron metabolism, which can delay sulfate reduction by sulfate-reducing bacteria such as Desulfobulbus, Desulfosporosinus, and Desulfovibrio. Even the physical properties of iron ore byproducts matter: high-pH, calcium-rich ferrous slags decreased overall diversity and shifted communities toward specialized taxa such as Serpentinomonas and Meiothermus equipped with cation/proton antiporters and sulfur oxidation pathways.

Toxic metals complicate this picture considerably. Because metals such as cadmium, lead, and chromium share ionic radii and charges with iron, they are frequently imported through established iron transport systems, including ferrous importers and siderophore-mediated uptake. Once inside, they mimic iron and displace it from functional sites on enzymes and proteins, a process called mis-metallation, disrupting DNA, RNA, protein, and ATP synthesis. The review reports that a high concentration of toxic metals can induce an iron starvation response even when ferrous iron is chemically available: a study of Bacillus cereus exposed to a mixture of eight metals, including aluminum, uranium, manganese, iron, cadmium, copper, cobalt, and nickel, revealed a profound starvation response that appeared only under concurrent multi-metal exposure, not single-metal stress. Antimony was found to suppress microbial iron transport systems and reduce overall diversity by 40 to 60 percent, leaving only a few survivors such as Achromobacter, Stenotrophomonas, and Arenimonas. Arsenic reduced bacterial diversity while increasing the prevalence of Geobacter and Hydrogenophaga.

Bacteria are not passive victims, however, and the review details how metal-resistant communities leverage iron-dependent systems to tolerate contamination. Desulfotomaculum reducens responds to Cr(VI), U(VI), and Mn(IV) by upregulating genes for iron homeostasis and c-type cytochrome biogenesis, allowing it to use these metals as alternative electron acceptors. Exposure to toxic metals activates dormant resistance genes: arsenic-contaminated soils show increased abundance of arsC, arsB, and arrB, while cadmium and chromium induce resistance genes such as queF in Lysinibacillus sp. OR-15 and chrA. Siderophores themselves have a dual role, chelating metals such as mercury, nickel, aluminum, cadmium, lead, and zinc and sequestering them extracellularly. Dissimilatory iron-reducing bacteria can concurrently reduce Mn(IV), Cr(VI), and U(VI), and iron-reducing bacteria facilitate indirect Cr(VI) reduction by producing Fe2+ as a metabolic end product, with bioreduced iron immobilizing Cr(VI) within ferric oxyhydroxides.

The review then evaluates four strategies that enhance bacterial-mediated reduction of toxic metals: iron plaque, organic amendments, manganese oxides, and nano zerovalent iron (nZVI). Iron plaque, the reddish-brown coating that forms on wetland plant roots when iron-oxidizing bacteria convert Fe2+ to insoluble Fe3+ oxides, acts as a porous, high-affinity barrier that adsorbs toxic metals and limits their translocation into plant tissues. Arsenic-resistant strains including Bacillus sp. T2, Bacillus sp. TF1-3, and Pseudomonas sp. Yangling I4 have been documented to enhance plaque thickness and sequester arsenic, while Lysinibacillus sp. OR-15 enhances plaque formation and significantly reduced cadmium and chromium concentrations. Biogenic manganese oxides, with their high surface areas and redox potentials, oxidize arsenic from the highly toxic, mobile trivalent form to the less mobile pentavalent form. Organic amendments such as biochar, straw, and glucose stimulate bacterial enzymatic activity, though the review cautions that straw and biochar amendments can, under anaerobic conditions, drive the reduction of arsenate into the more mobile and toxic arsenite.

Nano zerovalent iron emerges as a particularly versatile tool. It provides a slow-release supply of iron ions, lowers soil oxidation-reduction potential, and, through anaerobic corrosion of its Fe0 core, releases hydrogen and Fe2+ that serve as electron donors for iron- and sulfate-reducing bacteria. Bacteria interact with nZVI through outward and inward extracellular electron transfer pathways involving conductive nanowires and MtrCAB porin-cytochrome complexes, reducing toxic metals such as Cr(VI), U(VI), Hg(II), and Pb(II) sequestered within protective extracellular polymeric substance matrices. Functionalizing nZVI with biochar, chitosan, lignin-based hydrogels, or carboxymethyl cellulose prevents nanoparticle aggregation and boosts soil enzyme activity. The review is candid about nZVI’s risks: its high reactivity can disrupt membrane integrity, generate reactive oxygen species that damage lipids, proteins, and DNA, and substantially disrupt beneficial microbial communities, and predictable field-scale application requires understanding species-specific toxicity thresholds.

The authors close with a roadmap rather than a victory lap. They note that the molecular mechanisms of toxic metal toxicity within electron transport chains during extracellular electron transfer remain uncertain, that microbially driven iron redox cycling is difficult to decouple from simultaneous abiotic processes such as nitrite- or sulfide-induced iron transformation, and that long-term ecological risks remain unresolved, since community-level successions risk destabilizing secondary biogenic iron minerals and remobilizing coprecipitated metalloids into groundwater. Most research models, they observe, still rely on single-metal matrices that fail to replicate complex real-world contamination. They call for long-term field-scale monitoring frameworks, multi-omics approaches to map active metabolic pathways in real time, bioelectrochemical systems, and climate-smart remediation strategies. For now, the review’s central message stands: bacterial iron solubilization and reduction are not peripheral curiosities but critical levers for engineering sustainable remediation of metal-contaminated soils.

Subject of Research: Bacterial iron solubilization and reduction in soil and their role in bioreducing toxic metals

Article Title: Bacterial iron solubilization and reduction: impacts on bacterial community dynamics and the bioreduction of toxic metals in the soil

Article References: Daramola, O. B., Daramola, I. V., Taiwo, T. D., Akinde, B. P., Ibitoye, R. G., & Otuyelu, F. O. (2026). Bacterial iron solubilization and reduction: impacts on bacterial community dynamics and the bioreduction of toxic metals in the soil. Biotechnology for the Environment, 3(1), Article 14. https://doi.org/10.1186/s44314-026-00049-6

Image Credits: AI Generated

DOI: 10.1186/s44314-026-00049-6

Keywords: iron solubilization, iron reduction, siderophores, soil bacteria, toxic metals, bioremediation, nano zerovalent iron, iron plaque, manganese oxide, organic amendments, dissimilatory iron-reducing bacteria, Fur regulation