Microbial activity plays a pivotal role in the biogeochemical cycling of iron and manganese, especially at the redox gradients characteristic of stratified aquatic environments. These cycles are driven by microbial transformations between oxidized and reduced forms of the metals, allowing organisms to exploit them for metabolic energy and structural purposes.
In neutral, oxygen-rich surface waters, iron is predominantly found in its oxidized, insoluble ferric [Fe(III)] form. To access this essential nutrient, certain bacteria synthesize and secrete siderophores—low molecular weight organic compounds with a high affinity for ferric iron. These molecules bind ferric iron, rendering it soluble and available for uptake. Once internalized, ferric iron is enzymatically reduced to ferrous [Fe(II)] form, which is more bioavailable and supports various cellular functions.
At the oxic-anoxic interface, where redox conditions fluctuate, bacteria such as Leptothrix oxidize soluble ferrous iron to ferric iron, forming extracellular iron oxide sheaths. These sheaths serve a dual purpose: they prevent cell encrustation and may offer structural support. Magnetotactic bacteria, also situated in this zone, internalize iron to biomineralize magnetite (Fe₃O₄) crystals. These crystals act as intracellular compasses, aligning with Earth’s magnetic field to guide cells toward low-oxygen environments optimal for their metabolism.
Acidophilic bacteria such as Acidithiobacillus facilitate iron cycling in acidic, oxygen-rich environments. These organisms thrive at low pH and contribute to iron turnover by oxidizing Fe²⁺ (ferrous iron) to Fe³⁺ (ferric iron). This oxidation process is critical in environments like acid mine drainage systems.
Leptothrix species also contribute to manganese cycling by oxidizing soluble manganous ions [Mn(II)] into insoluble manganese dioxide [MnO₂], which sinks into the underlying anoxic sediments. In these layers, metal-reducing bacteria such as Geobacter enzymatically reduce manganese dioxide back to Mn(II), completing the cycle. Similarly, phototrophic bacteria under anoxic conditions use Fe(II) as an electron donor in anoxygenic photosynthesis, while facultative anaerobes like Shewanella reduce ferric iron through dissimilatory pathways to harness energy.
These microbial processes collectively maintain iron and manganese homeostasis in aquatic ecosystems, mediating their availability and distribution across redox interfaces.
Microbes drive iron and manganese cycling by shifting these elements between their oxidized and reduced forms.
In neutral, oxygen-rich habitats, ferric iron predominates in an insoluble, precipitated form.
Some bacteria secrete small molecules called siderophores, which bind to and solubilize ferric iron.
Cells then take up this soluble iron and reduce it to the ferrous form for metabolic use.
At the oxic–anoxic interface, where oxygen levels are low, magnetotactic bacteria such as Magnetospirillum use iron to form magnetite crystals, aligning themselves with Earth’s magnetic field to reach anoxic layers.
In the same zone, filamentous bacteria such as Leptothrix oxidize ferrous iron, producing sheaths coated with iron oxides.
They also oxidize manganese(II) into insoluble manganese dioxide, which may sink into deeper anoxic layers, where metal-reducing bacteria like Geobacter convert it back to manganese (II).
In anoxic zones, bacteria like Shewanella perform dissimilatory iron reduction, where ferric iron is the terminal electron acceptor.
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