Evidence for nitrogenase-catalyzed hydrogen uptake in nitrogen-fixing filamentous blue-green algae

Two new strains of (heterocystous) Anabaena, one of them devoid of hydrogenase activity, were compared with respect to hydrogen‐uptake activities. Evidence was obtained for a hydrogen‐recycling pathway in the dark independent of hydrogenase, because a similar recycling activity is measured in both strains. This hydrogen uptake is dependent on substrates for nitrogenase (molecular nitrogen or acetylene) and can be abolished by oxygen treatment of filaments or by uncouplers. The findings are interpreted as nitrogenase itself being directly involved in the hydrogen‐uptake mechanism.