SUMMARY: Nitrogenase activity of suspensions of the unicellular cyanobacterium sp. PCC 6909 plotted against the concentration of dissolved O (dO) resulted in a bell-shaped curve, both in the light and in the dark, with optima of 25 or 80 μ-O depending on the age of the culture. At the optimum dO, nitrogenase activity [typically 4 to 6 nmol CH (mg protein) min] 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)-1,1-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 dO 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 O concentration; in the presence of DCMU, activity increased twofold, independent of dO.

In the dark, the dependence of the rate of respiration on O concentration suggested the presence of three O-uptake systems with apparent values of 1 μ, 5 μ and 25 μ. The ADP/ATP ratio under anaerobic conditions was 0·47 and showed a marked decrease as dO was increased to 25 μ. A CN-insensitive respiratory activity, which neither supported nitrogenase activity nor was coupled to ATP synthesis, was associated with the system with the apparent of 5 μ. The dependence of the specific activity of nitrogenase on dO indicated that both the high affinity ( 1 μ) and low affinity ( 25 μ) O-uptake systems contributed ATP or reductant for N-fixation. KCN (2·5 m) completely inhibited nitrogenase activity in the dark and at moderate levels of illumination and dO. 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.


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