The Irony of Iron — Life’s Major Trace Element

All organisms, from lowly microbes to higher forms of life (including humans), need iron. Yet there is irony to iron. Despite being the second most abundant element in the Earth, it is not readily available for consumption. Earth owes this irony to the combined effects of geodynamics and biology. The early segregation of iron into the Earth’s core relegated iron to “only” the fourth most abundant element in the crust. About 2.3 billion years ago, a complex interplay between photosynthesis and redox changes in Earth’s mantle allowed the buildup of free atmospheric oxygen. Today, there is a sufficient supply of photosynthetic oxygen to convert all iron at the Earth’s surface and in its surface waters, including seawater, into its ferric [Fe(III)] form. This ferric form is barely soluble, making it hard to access by organisms. Ironically, life itself made iron a ‘trace element’.

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