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Metabolism and Enzymology

Photosynthetic Assimilation of NO₃^– by Intact Cells of the Cyanobacterium Anacystis nidulans¹

Influence of NO₃^– and NH₄⁺ Assimilation on CO₂ Fixation

Departamento de Bioquímica, Facultad de Biología y Consejo Superior de Investigaciones Científicas, Apdo. 1095, 41080-Sevilla, Spain

Illuminated suspensions of Anacystis nidulans, supplied with saturating concentrations of CO₂ evolved O₂ 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 CO₂ fixation at limiting but not at saturating light, this depression reflecting the competition between both processes for assimilatory power. In contrast, ammonium stimulated CO₂ fixation at any light intensity assayed, the stimulation being dependent on the incorporation of ammonium to carbon skeletons. The positive effect of ammonium on CO₂ 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 CO₂ fixation are discussed.