Sodium Requirement for Photosynthesis and Its Relationship with Dinitrogen Fixation and the External CO(2) Concentration in Cyanobacteria

Cells of Anabaena PCC 7119 and of a mutant strain of Nostoc muscorum unable to fix dinitrogen, grown at pH 8 and under low CO₂ tension (air), showed a reduced capacity for photosynthesis when cultured in the absence of sodium, this inhibition being followed by symptoms of photooxidation, such as chlorosis, oxygen consumption in the light, and decrease of superoxide dismutase activity. The impairment of photosynthesis preceded that of nitrogenase activity, indicating that the requirement for sodium in photosynthesis was independent of its effects on nitrogen metabolism. However, when cyanobacteria were grown at pH 6.3 or under high CO₂ tensions, sodium was not required for photosynthesis and no symptoms of photooxidation were observed.