Dinitrogen (N₂), which is a triple bond molecule of two nitrogen (N) atoms, cannot be easily dissociated. This is a reason why N₂ consists of 78% of the Earth’s atmosphere. The materials that make up the Earth originally contain N, and once N becomes N₂, it accumulates in the atmosphere because of its inertness.

Although N is an element necessary for the formation of proteins and DNAs, the abundant N₂ cannot be used directly because of its inert nature. However, some microorganisms have acquired the ability to produce ammonia from N₂ at room temperature and pressure, i.e., biological N fixation. An enzyme called nitrogenase (also a protein) separates the strong triple bond of N₂ and attaches hydrogen created from water to the free N atom to form ammonia. The ammonia produced is converted to glutamic acid, which is further converted to other amino acids that can be used by living things. Earth’s ecosystems originally rely on the reactive N produced through biological N fixation.