10.18
Microbes and Other Elemental Cycles
Microbial activity plays a pivotal role in the biogeochemical cycling of iron and manganese, especially at the redox gradients characteristic of strat…
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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Q1: How do bacteria access iron in oxygen-rich environments?
In neutral, oxygen-rich habitats, iron exists as insoluble ferric iron. Bacteria secrete siderophores—small organic molecules with high affinity for ferric iron—that bind and solubilize it. Cells then take up this soluble iron and reduce it to ferrous form for metabolic use.
Q2: What role do magnetotactic bacteria play in iron cycling?
Magnetotactic bacteria like Magnetospirillum inhabit the oxic-anoxic interface where oxygen levels are low. They internalize iron to form magnetite crystals that act as intracellular compasses, aligning with Earth's magnetic field to guide cells toward anoxic layers optimal for their metabolism.
Q3: How do filamentous bacteria contribute to manganese cycling?
Filamentous bacteria such as Leptothrix oxidize soluble manganous ions into insoluble manganese dioxide at the oxic-anoxic interface. This manganese dioxide sinks into underlying anoxic sediments, where metal-reducing bacteria like Geobacter enzymatically reduce it back to manganous form, completing the cycle.
Q4: What is the function of iron oxide sheaths produced by Leptothrix?
Leptothrix bacteria oxidize ferrous iron and produce extracellular iron oxide sheaths. These sheaths serve dual purposes: they prevent cell encrustation and provide structural support, allowing bacteria to thrive at the oxic-anoxic interface where redox conditions fluctuate.
Q5: How do metal-reducing bacteria like Shewanella obtain energy in anoxic zones?
In anoxic zones, bacteria like Shewanella perform dissimilatory iron reduction, using ferric iron as the terminal electron acceptor. This metabolic process allows them to extract energy from iron reduction, supporting growth and cellular functions in oxygen-depleted environments.
Q6: Why are redox gradients important for microbial iron and manganese cycling?
Redox gradients in stratified aquatic environments create distinct zones where microbes exploit different oxidation states of iron and manganese for metabolic energy. These gradients enable diverse bacterial communities to thrive at different depths, collectively maintaining metal homeostasis and availability across ecosystems.
Q7: How do acidophilic bacteria like Acidithiobacillus affect iron cycling in acidic environments?
Acidithiobacillus species thrive at low pH in oxygen-rich environments and facilitate iron cycling by oxidizing ferrous iron to ferric iron. This oxidation process is critical in acidic habitats like acid mine drainage systems, where these organisms contribute significantly to iron turnover.