Abstract: Gloeocapsa sp., a species of anicellular blue-green alga, fixes dinitrogen mostly under light. The energy (ATP and reductant) needed for nitrogen fixation may be provided by photoreaction and aerobic catabolism. The nitrogenase activity (acetylene reduction) in vivo was decreased under the conditions of dark and inhibition of photo-phosphorylation or oxidative phosphorylation in the light. When photosystem Ⅱ was inhibited by the presence of DCMU, nitrogenase activities in both reactions of acetylene reduction and hydrogen evolution may be muchenhanced probably due to eliminating of the damage caused by the oxygen produced in the photolysis of water. The effects of the oxygen present in the atmosphere of the reaction systemand produced by the cells are different. It is shown that some trace oxygen seems to be required for nitrogen fixation by the energy supply of aerobic actabolism and oxidative phosphorylation. While the fixation of dinitrogen was inhibited by CO or no any reducible substrate was present, 70-100% of the energy accepted by nitrogenase was evolved as hydrogen. The algal cells also showed hydrogen uptake reaction, but no enhancement of nitrogen fixation by the hydrogen uptake was found.

Key words: Gloeocapsa sp., Nitrogen fixation, Energy metabolism