Photosynthetic Assimilation of NO(3) by Intact Cells of the Cyanobacterium Anacystis nidulans: Influence of NO(3) and NH(4) Assimilation on CO(2) Fixation.

Illuminated suspensions of Anacystis nidulans, supplied with saturating concentrations of CO(2) evolved O(2) at a greater rate when nitrate was simultaneously present. The extent of the stimulation of noncyclic electron flow induced by nitrate was dependent on light intensity, being maximal under light saturating conditions. Accordingly, nitrate depressed the rate of CO(2) fixation at limiting but not at saturating light, this depression reflecting the competition between both processes for assimilatory power. In contrast, ammonium stimulated CO(2) fixation at any light intensity assayed, the stimulation being dependent on the incorporation of ammonium to carbon skeletons. The positive effect of ammonium on CO(2) fixation also appeared to occur when nitrate was the nitrogen source, since with either nitrogen source an increase in the incorporation of newly fixed carbon into acid-soluble metabolites took place. From these results, the in vivo partitioning of assimilatory power between photosynthetic nitrogen and carbon assimilation and the quantitative and qualitative effects of inorganic nitrogen assimilation on CO(2) fixation are discussed.