Iron is the fourth-most abundant element in the Earth 's crust .
Solubility of biologically relevant iron compounds is exceedingly low , and is generally insufficient for the needs of most ( but not all ) microorganisms .
Although iron is the fourth-most abundant element in the Earth 's crust , solubility of biologically relevant iron compounds is exceedingly low , and is generally insufficient for the needs of most ( but not all ) microorganisms .