Question

What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N₂ -fixation?

Answer 1

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 leg-haemoglobin, which protects nitrogenase from oxygen.

Answer 2

The conditions necessary for nitrogen fixation by Rhizobium are :

(i) Presence of enzyme nitrogenase.

(ii)A protective mechanism for the enzyme nitrogenase against O2

(iii)A non-heme iron protein-ferrodoxin as an electron carrier.

(iv)The hydrogen donating system (viz, pyruvate, hydrogen, sucrose, glucoseetc).

(v) A constant supply of ATP.

(vi)Presence of thiamine pyrophosphate (TPP), coenzyme-A, inorganic phosphate and Mg++ as co-factors.

(vii)Presence of cobalt and molybdenum,

(viii) A carbon compound for trapping released ammonia.

In the process of biological nitrogen fixation by free living and symbiotic nitrogen fixers, the dinitrogen molecule is reduced step by step to ammonia (NH₃) by the addition of pairs of hydrogen atoms. The pyruvic acid mainly serves as an electron donor but in some cases hydrogen, sucrose, glucose, etc., have also been shown to operate. In leguminous plants, the glucose-6-phosphate molecule probably acts as a substrate for donating hydrogen. The overall process occurs in presence of enzyme nitrogenase, which is active in anaerobic condition. The enzyme nitrogenase consists of two sub-units – a non-heme iron protein (or dinitrogen reductase) and an iron molybdenum protein (Mo-Fe protein or dinitrogenase).

The Fe-protein component reacts with ATP and reduces Mo-Fe protein which then converts N₂ to ammonia. The ammonia is either directly taken by host or is converted to nitrates with the help of nitrifying bacteria (e.g., Nitrosomonas)