Nitrogen cycle diagram:Enlarge (Image: Mike Jones)
The diagram at right outlines the main steps by which nitrogen is converted between its various chemical forms in the environment (click on the link beneath the diagram to enlarge it).
Nitrogen occurs naturally in many chemical compounds, the simplest of which are nitrogen gas (N₂), ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻).
|Adenine-thymine base pair|
|Guanine-cytosine base pair|
The nitrogen present in living things, and in the matter produced by the decomposition of living things, is known as "organic nitrogen." Nitrogen is intrinsic to life at its most basic level because nitrogenous bases make up the building blocks of DNA and RNA: adenine, cytosine, guanine, thymine and uracil (a nitrogenous base is a nitrogen-containing molecule with the same chemical properties as a base). The processes of the nitrogen cycle create these various molecules from each other by a straightforward recycling process.
The utilization of gaseous nitrogen, however, is not such a simple matter. Nitrogen in its gaseous form (N₂) makes up the great majority (~78%) of the earth's atmosphere. Nitrogen is also essential to all life since it is present in all amino acids. But atmospheric nitrogen cannot be assimilated by most organisms. It must first be "fixed." The fixation of nitrogen is a natural process of conversion by which certain prokaryotes (bacteria) convert gaseous nitrogen into forms that other organisms can use. In the diagram above, note that this is the essential first step that makes all of the subsequent steps possible. Without it, complex life could not exist on earth. Indeed, no life of any sort could exist since nitrogen is a component of DNA and RNA, which are essential even to the simplest forms of life.
The prokaryotes that fix atmospheric nitrogen as organic nitrogen (e.g., Azospirillum, Frankia, Klebsiella and Rhizobium) often live in association with legumes. But certain free-living prokaryotes can also carry out this conversion, for example, those in the genus Azotobacter. Denitrifying prokaryotes (shown at right in the diagram above) complete the cycle by converting organic nitrogen back into nitrogen gas.
Bacteria that fix nitrogen are known as diazotrophs. Two of the most studied diazotrophs are Klebsiella pneumoniae and Azotobacter vinelandii. The prefix diazo- specifically refers to the group N₂ joined to a carbon in a single organic radical. All diazotrophs contain iron-molybdenum or -vanadium nitrogenase systems.