ABSTRACT
Only a very small proportion of the nitrogen present on earth is in a usable form at any one time. More than 99.9% of the nitrogen is present as the dinitrogen molecule (N2). Most organisms cannot metabolize this abundant, but relatively inert N2 and so they must assimilate nitrogen in a ‘‘fixed’’ form, such as ammonia (NH3) or nitrate (NO
3 ). As part of the geo-
biochemical nitrogen cycle, nitrogen fixation drives the conversion of atmospheric N2 to ammonia. However, the complementary processes of nitrification and denitrification convert ammonia back to dinitrogen, which is returned to the atmosphere. This constant cycling means that the pool of fixed nitrogen within the biosphere must be constantly replenished because ammonia is necessary for the formation of biologically essential, nitrogen-containing compounds, such as amino acids and nucleic acids. Dinitrogen is fixed naturally by both nonbiological processes, like lightning and combustion, and by biological processes. Natural nonbiological processes contribute 10% and biological processes contribute about 65% of the total annual fixation rate. Industrial ammonia synthesis contributes the other 25% (1, 2). Thus, biological nitrogen fixation is a pivotal life-supporting process that provides the majority of the fixed nitrogen source necessary to sustain life on earth.
