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

Two Clark-type polarographic electrodes were used to measure simultaneous H₂ and O₂ exchange from three species of the blue-green alga Anabaena. Maximum H₂ photoevolution from N₂-fixing cultures of Anabaena required only the removal of dissolved O₂ and N₂; no adaptation period was necessary. No correlation of H₂ photoproduction with photosynthetic O₂ evolution, beyond their mutual light requirement, was found. Hydrogen photoevolution has the following characteristics in common with N₂ fixation in these organisms: DCMU insensitivity; similar white light dependency with very low dark production rates; maximum efficiency in photosystem I light; inhibition by N₂, O₂ 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 CO₂ conditions have H₂ photoproduction rates proportional to their growth rates. Hydrogenase activity is inferred from H₂ uptake in the dark, but this activity apparently is independent of the photoevolution of H₂ which is ascribed strictly to the nitrogenase system.