Iron is required for life. The iron–sulfur clusters are pervasive and include nitrogenase, the enzymes responsible for biological nitrogen fixation. Iron-containing proteins participate in transport, storage and use of oxygen. Iron proteins are involved in electron transfer. The ubiquity of Iron in life has led to the Iron–sulfur world hypothesis that iron was a central component of the environment of early life.
Examples of iron-containing proteins in higher organisms include hemoglobin, cytochrome (see high-valent iron), and catalase. The average adult human contains about 0.005% body weight of iron, or about four grams, of which three quarters is in hemoglobin – a level that remains constant despite only about one milligram of iron being absorbed each day, because the human body recycles its hemoglobin for the iron content.
Microbial growth may be assisted by oxidation of iron(II) or by reduction of iron (III).