Rhizobium is a symbiotic bacteria present in the root nodules of leguminous plants.
The basic requirements for Rhizobium to carry out nitrogen fixation are as follows:

(a) Presence of the enzyme nitrogenase
(b) Presence of leg-hemoglobin
(c) Non-haem iron protein, ferredoxin as the electron-carrier
(d) Constant supply of ATP
(e) ${\mathrm{Mg}}^{2+}$ions as co-factors

Rhizobium contains the enzyme nitrogenase – a Mo-Fe protein – that helps in the conversion of atmospheric free nitrogen into ammonia.

The reaction is as follows:

${\mathrm{N}}_2 + 8{\mathrm{e}}^{-} + 8{\mathrm{H}}^{+} + 16 \mathrm{ATP}\to 2 {\mathrm{NH}}_3 + {\mathrm{H}}_2 + 16\mathrm{ADP} + 16\mathrm{Pi}$

The Rhizobium bacteria live as aerobes under free-living conditions, but require anaerobic conditions during nitrogen fixation. This is because the enzyme nitrogenase is highly sensitive to molecular oxygen. The nodules contain leghaemoglobin, which protects nitrogenase from oxygen.