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

Background: Rhizobium is a soil bacterium that lives freely but is only able to fix atmospheric nitrogen (N2) into a usable form when it enters into a symbiotic (mutualistic) association with the roots of leguminous plants, forming root nodules.

Conditions necessary for nitrogen fixation inside the nodule:

• A microaerobic (low-oxygen) environment: the enzyme responsible for nitrogen fixation, nitrogenase, is extremely sensitive to oxygen and is irreversibly destroyed in its presence, yet the bacteroids (the differentiated form of Rhizobium inside the nodule) still need some oxygen for aerobic respiration to generate energy. This conflicting requirement is resolved by leghaemoglobin, a red/pink oxygen-binding pigment synthesised jointly by the plant and bacterial genomes, which is present in the nodule cytoplasm and buffers the free oxygen concentration to a level low enough to protect nitrogenase but high enough to sustain respiration.
• Anaerobic/reducing conditions and adequate energy supply: nitrogen fixation is highly energy-intensive, consuming roughly 16 molecules of ATP for every molecule of N2 reduced. This ATP is supplied by the host plant, which channels photosynthetically-fixed carbohydrates to the nodule.
• Presence of the nitrogenase enzyme complex: nitrogenase is a molybdenum-iron (Mo-Fe) protein complex; Rhizobium must synthesise and activate this enzyme (using Mo and Fe as essential cofactors) for the reaction to proceed.

Role of Rhizobium in N2-fixation:

• Rhizobium provides the biochemical machinery (nitrogenase) that actually catalyses the reduction of inert atmospheric N2 to ammonia (NH3), a reaction that plants themselves cannot perform. …

List the three conditions nitrogen fixation needs inside a nodule (low-O2 environment via leghaemoglobin, ATP supply from the host, and the nitrogenase enzyme with its Mo/Fe cofactors), then state Rhi …

• Describing Rhizobium as fixing nitrogen freely in the soil, when it can only do so once established symbiotically inside a legume root nodule.
• Omitting leghaemoglobin's role in protecting the oxygen-sensitive nitrogenase enzyme while still permitting bacterial respiration. …