Microbes and Other Elemental Cycles

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.