Nitrogen constitutes almost 80% ofthe atmosphere, but is metabolically inaccessible to plants due to the exceptional stability of the triple covalent bond (NxN). The ability to catalyze enzymatic reduction of N2 to NH3 is limited to a variety of prokaryotes defined as nitrogen-fixing or diazo-trophic microorganisms, which are widely distributed in all ecosystems as either free-living organisms or in symbiotic association with a number of different plants. These N2-fixing prokaryotes can be anaerobic, facultative aerobic, aerobic, photosynthetic, or nonphotosynthetic (Table 1). All carry out N2 reduction by an enzymatic complex termed nitrogenase.
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