It is a cycle of iron through the atmosphere, hydrosphere, biosphere, and lithosphere where Fe is abundant in the earth's crust, it is less common in oxygenated surface waters.
The iron cycle is the conversion of ferrous iron to ferric iron and then back again.
Ferric iron reduction forms a part of the respiratory pathway in some microorganisms.
Some bacteria are capable of producing magnetite.
Fe2+ is abiotically oxidized to Fe3+ in the presence of oxygen, and the reduction of Fe3+ to Fe2+ by iron-sulfide minerals in the abiotic process of the iron cycle
The biological cycling of Fe2+ is done by iron-oxidizing and reducing microbes.