Like many elements, iron (chemical symbol Fe) can exist in more than one chemical form, or "oxidation state." The two most common forms for iron are Fe(II), in which the iron ion shares two of its electrons, and Fe(III), in which it shares three electrons.
The oxidation state of iron is interesting and important, because it dramatically effects the solubility of iron in sea water. Very long ago, in the earliest days of life on earth, there was little or no oxygen (O2). As a result, iron was most often found in the Fe(II) state. Fe(II) is quite soluble in water, so in those days iron was probably readily available. In those days cyanobacteria, photosynthetic microorganisms responsible for fixing much of the carbon in those early days and, concurrently, manufacturing the earth's oxygen-rich atmosphere, probably had no need for special machinery for harvesting the iron it needed grow, reproduce, and to do this important work. But as atmospheric oxygen increased, iron began to prefer the Fe(III) state, which is quite insoluble in sea water. Acquiring iron then became difficult, and microorganisms began to develop specialized machinery for iron acquisition.
This difference in solubility between Fe(II) and Fe(III) also means that iron acquisition tends to be much more of a problem for aerobic organisms than for anaerobic organisms, since anaerobic environments favor the more soluble Fe(II).