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Nitrogen fixation is the basis of the global N cycle. Therefore it is not surprising that the ability to fix atmospheric N2 evolved in the ‘‘primeval soup’’and is deeply rooted in the evolutionary tree of life. Despite this, nitrogenase remains an enzyme exclusive to prokaryotes; no eukaryote has been described that can fix N2 except through a symbiotic relationship. Members of both the Archaea and Eubacteria can fix N2, and, even within the Eubacteria, nitrogenase is widely distributed among the various bacterial divisions.[1]