Molecular Hydrogen

N₂-fixing organisms also produce a membrane bound enzyme called uptake-hydrogenase under N₂-fixing condition. The physiological significance of the presence of uptake hydrogenase lies in its ability to consume hydrogen produced during nitrogenase activity through aerobic electron transport chain generating ATP in the process. Such nitrogenase uptake-hydrogenase relationship in nitrogen fixing cells provides the following advantages:

1. By aerobic respiration it restores a major part of energy in the form of ATP
2. It offers protection of nitrogenase from oxygen.
3. It prevents inhibitory effect of H₂ on nitrogenase activity.

It has been observed that the nitrogenase enzyme catalyses exclusive production of hydrogen provided the N₂-fixer is in an atmosphere devoid of nitrogen. Efforts are being made to exploit this special property of the enzyme for photo production of H₂ by heterocysts of cyanobacteria on a commercial scale. This would require construction of special strains of cyanobacteria that would be genetically deficient in uptake hydrogenase enzyme. It. is expected that these strains would continuously generate hydrogen in photosynthetic and N₂-fixing condition.