Hydrogen Gas Using Nitrogenase - In cyanobacteria (blue green algae), 'nitrogenase' enzyme is the chief hydrogen producing enzyme. This is considered better, because hydrogenase enzyme is sensitive to oxygen which is produced during the photolysis of water, while nitrogenase enzyme is relatively less sensitive to oxygen. Further, in the heterocyst of cyanobacteria, oxygen concentration is low or completely absent, so that hydrogen production becomes more efficient. The enzyme nitrogenase reduces nitrogen to NH3 with associated release of hydrogen gas. Hydrogen evolution depends on light, which provides electron donors and also activates nitrogenase and ATP formation in heterocysts. It has been shown that light dependent hydrogen evolution can continue for several days in some strains of Anabaena (nitrogen fixing cyanobacteria).