Iron is a considerably larger atom than even P. It is as much abundant in the universe as C and N, but it is more abundant than C and N on Earth. It is the first or second most abundant element in the whole of the earth. A majority of the Fe in the Earth's crust is in the form Fe2+ (ferrous) but this is quickly oxidized at the oxygen-

rich surface to Fe3+ (ferric). Though making up 30% of the mass of the whole Earth (a very large fraction), concentrations of Fe in oxic waters can be extremely low due to the low solubility of ferric iron. Low iron concentrations have been shown to limit primary production rates, biomass accumulation, and ecosystem structure in a variety of open-ocean environments, including the equatorial Pacific, the subarctic Pacific, and the Southern Ocean, and even in some coastal areas. Binding of Fe with other molecules is very important to governing its overall solubility. Organically bound Fe may be the dominant chemical form in aqueous solutions.

Iron is chemically versatile. It can coordinate with O, N, or sulfur (S), and it can bind to small molecules. Iron-containing proteins are key features of many energy-transducing biological reactions in processes such as photosynthesis, respiration, and others. Iron serves no major structural role in biological systems. Instead, it is a renewable reaction center. Thus, the soft tissue of living organisms is generally <1% Fe by dry mass.

Molecules containing Fe include hemoglobin, cyto-chrome c, and ferridoxin.

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