Biological fixation - Nitrogen Fixation

Approximately 63% of all nitrogen fixed is through biological fixation. Nitrogen fixing organisms are primarily prokaryotes; bacteria and blue green algae. Nitrogen fixation requires activation of molecular nitrogen by splitting nitrogen into two atoms of free nitrogen N₂ → 2N.

This is an energy requiring step, which in biological fixation requires 160 kcal/mole. The actual fixation step, in which two atoms of nitrogen combine with three molecules of H₂ to form two molecules of ammonia (NH₃) releases 13 kcal/mole. Therefore, the net energy requirement for nitrogen fixation is 147 kcal/mole.