Since the activity of the bidirectional hydrogenase is not directly dependent on ATP, use of this enzyme may be energetically more efficient than the use of nitrogenase. Nevertheless, the hydrogenase enzyme may be inhibited by the oxygen generated at photosystem II, and since it operates close to chemical equilibrium, H2-production is inhibited above a certain H2 partial pressure. Therefore, continuous removal of both O2 and H2 is necessary to obtain appreciable turnover rates. Furthermore, also accumulation of ATP might impede its turnover, since ATP is produced by photosynthetic electron transport, but not consumed by the hydrogenase.