Biological and physicochemical characters and functions of amorphous iron oxides produced by iron-oxidizing bacteria at ambient temperature in hydrosphere

Leptothrix species, one of the Fe/Mn-oxidizing bacteria, are ubiquitous in aqueous environments, especially at sites characterized by outwelling groundwater with a nearly neutral pH, an oxygen gradient and a source of Fe(II) and/or Mn(II). The genus Leptothrix forms lined-cell chains by repeating cell division and then forms uniquely shaped microtubular sheaths surrounding the cell chains through the precipitation of copious amounts of iron or manganese oxides which were accumulated from groundwater. The sheath is an ingenious hybrid of organic and inorganic materials produced through the interaction of bacterial exopolymer fibrils with aqueous-phase inorganics. Intriguingly, Leptothrix sheaths, that we call biogenous iron oxides (BIOXs), have a variety of unexpected functions that are suitable for industrial applications such as lithium-ion battery anode, a catalyst support, a starting material of bright red pigment among others. This review focuses on the structural and chemical properties of BIOXs produced by Leptothrix and their noteworthy functions that show promise for variety of industrial applications.