Microbes and Other Elemental Cycles

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.

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.

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.

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.

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.

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.

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.