Effect of Light and Oxygen on Nitrogenase Activity and Dinitrogenase Reductase (Fe-protein) Content in AzollaAnabaena Association

Nitrogen fixation in Azolla is regulated by light, in vivo and in vitro, and occurs only upon illumination. After long incubation in the dark, illumination of Azolla caused a slow recovery of nitrogenase to maximal activity after 3 h without correlation with either photosynthesis or the content of soluble sugars. Cyanobiont heterocyst extracts immunobloted with anti-Klebsiella or anti-Rhodospirillum Fe-protein antibodies showed two reacting polypeptides, one of 30 and the other of 36 kDa. The content of the 30 kDa polypeptide was not affected by transition of Azolla to 18 h in the dark while the 36 kDa polypeptide completely disappeared in the dark and reappeared within 10 to 30 min of reillumination. The cyanobiont nitrogenase was active only in the presence of a detectable amount of the 36 kDa polypeptide, suggesting that it is the active form of the Fe-protein. Illumination of Azolla with high light intensity after a short dark period caused inhibition of nitrogenase activity and reduced the content of the 36 kDa polypeptide. Nitrogenase was inactivated by high O₂ only in the dark while in the light nitrogenase was not inhibited and the content of the 36 kDa Fe-protein polypeptide was retained.