Alteration of nitrogenase activity of Anabaena CH1 and CH2 during light-dark transition.

Nitrogenase activity decreased as cultures transferred from light to the dark. In anabaena CH1 nitrogenase activity ceased to near zero in two hours; but it took longer in Anabaena CH2. When darkened culture was placed again in the light, nitrogenase activity recovered in two hours. Hydrogen gas could protect this dark declination of nitrogenase activity. It is suggested that dark declination of nitrogenase activity is due to irreversible inhibition by oxygen which can not be omitted through oxidative respiration and/or oxyhydrogen reaction in the dark.