Iron is the most abundant metal in the universe and the fourth most common element on earth. It is essential for all known life forms, with the possible exception of a few micro-organisms. Depending on the ligand it is linked to, iron has access to a wide range of redox potentials. It is involved in the redox reactions of the cell, especially in those reactions implying molecular oxygen formation and the release of chemical energy for use in the energy-requiring processes of life.