Nitrogenase activity of suspensions of the unicellular cyanobacterium Gloeothece sp. PCC 6909 plotted against the concentration of dissolved 0, (do,) resulted in a bell-shaped curve, both in the light and in the dark, with optima of 25 or 80 ~M -O , depending on the age of the culture, At the optimum d o 2 , nitrogenase activity [typically 4 to 6 nmol C2H, (mg protein)-' min-l] was similar in the light or in the dark. Alteration of light intensity from zero to 2 klx, or addition of 3-(3,4-dichlorophenyl)-l, 1-dimethylurea (DCMU), had no effect on nitrogenase activity. At 1 klx the ADP/ATP ratio was 0.2 and showed only a marginal increase as the d o 2 was increased. However, a high level of illumination (30 klx) stimulated or inhibited nitrogenase activity, depending on the external do,, presumably as a consequence of changes in the intracellular O2 concentration; in the presence of DCMU, activity increased twofold, independent of do,.In the dark, the dependence of the rate of respiration on O2 concentration suggested the presence of three 0,-uptake systems with apparent K, values of 1 PM, 5 p~ and 25 p~. The ADP/ATP ratio under anaerobic conditions was 0.47 and showed a marked decrease as d o 2 was increased to 25 VM. A CN-insensitive respiratory activity, which neither supported nitrogenase activity nor was coupled to ATP synthesis, was associated with the system with the apparent K, of 5 p~. The dependence of the specific activity of nitrogenase on d o 2 indicated that both the high affinity ( K , 1 WM) and low affinity ( K , 25 p~) O2-uptake systems contributed ATP or reductant for N,-fixation. KCN (2.5 mM) completely inhibited nitrogenase activity in the dark and at moderate levels of illumination and d o 2 . We conclude that respiration is the major source of reductant and ATP for nitrogenase activity both in the dark and in the light, but that photosystem I can contribute ATP at very high levels of illumination.
