In microbial nitrogen fixation, atmospheric nitrogen is converted to ammonia by nitrogenases. Microorganisms that use nitrogen fixation include cyanobacteria, green sulfur bacteria, rhizobia, and diazotrophs. Nitrogen fixation is coupled with hydrolysis and results in the formation of H2 along with ammonia. Nitrogen fixation in incredibly diverse in the microbial world. Diazotrophs use nitrogenase generated ammonium and assimilate it into glutamate by the glutamate synthase pathway. Of note, nitrogen fixation is susceptible to oxygen and often occurs in oxygen free environments.

Rhizobia can also be found in plant nodules and facilitates nitrogen fixation to produce nitrogen compounds that help in plant growth. This relationship is also important beyond just the plant and bacterial organism in that fixed nitrogen released from dead plants help fertilize the soil. Artificial fertilization by nitrogen fixation has also been industrialized in processes such as the Haber process.