Nitrogen Fixation by Gloeothece sp. PCC 6909: Respiration and Not Photosynthesis Supports Nitrogenase Activity in the Light

Abstract

SUMMARY: Nitrogenase activity of suspensions of the unicellular cyanobacterium Gloeothece sp. PCC 6909 plotted against the concentration of dissolved O2 (dO2) resulted in a bell-shaped curve, both in the light and in the dark, with optima of 25 or 80 μm-O2 depending on the age of the culture. At the optimum dO2, nitrogenase activity [typically 4 to 6 nmol C2H4 (mg protein)−1 min−1] 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 dO2 was increased. However, a high level of illumination (30 klx) stimulated or inhibited nitrogenase activity, depending on the external dO2, presumably as a consequence of changes in the intracellular O2 concentration; in the presence of DCMU, activity increased twofold, independent of dO2. In the dark, the dependence of the rate of respiration on O2 concentration suggested the presence of three O2-uptake systems with apparent K m values of 1 μm, 5 μm and 25 μm. The ADP/ATP ratio under anaerobic conditions was 0∙47 and showed a marked decrease as dO2 was increased to 25 μm. 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 m of 5 μm. The dependence of the specific activity of nitrogenase on dO2 indicated that both the high affinity (K m 1 μm) and low affinity (K m 25 μm) O2-uptake systems contributed ATP or reductant for N2-fixation. KCN (2∙5 mm) completely inhibited nitrogenase activity in the dark and at moderate levels of illumination and dO2. 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.