Simultaneous measurement of oxygen and hydrogen exchange from the blue-green alga anabaena

Two Clark-type polarographic electrodes were used to measure simultaneous H(2) and O(2) exchange from three species of the blue-green alga Anabaena. Maximum H(2) photoevolution from N(2)-fixing cultures of Anabaena required only the removal of dissolved O(2) and N(2); no adaptation period was necessary. No correlation of H(2) photoproduction with photosynthetic O(2) evolution, beyond their mutual light requirement, was found. Hydrogen photoevolution has the following characteristics in common with N(2) fixation in these organisms: DCMU insensitivity; similar white light dependency with very low dark production rates; maximum efficiency in photosystem I light; inhibition by N(2), O(2) and acetylene; and an apparent requirement for the presence of heterocysts. Growth on nitrate medium reduces, and on ammonium medium obliterates, both reactions. Cultures grown under limiting CO(2) conditions have H(2) photoproduction rates proportional to their growth rates. Hydrogenase activity is inferred from H(2) uptake in the dark, but this activity apparently is independent of the photoevolution of H(2) which is ascribed strictly to the nitrogenase system.