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-haemoglobin
(c) Non-haem iron protein, ferrodoxin as the electron-carrier
(d) Constant supply of ATP
(e) Mg2+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:
N2 + 8e– + 8H+ + 16 ATP→ 2 NH3 + H2 + 16ADP + 16Pi
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.