Na+-Stimulation of Photosynthesis in the Cyanobacterium Synechococcus UTEX 625 Grown on High Levels of Inorganic Carbon

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Abstract
Photosynthesis of washed cells of Synechococcus UTEX 625 grown on 5% CO2 was markedly stimulated (647 .+-. 50%) at pH 8.0 by the addition of low concentrations of NaCl (concentration required for half-maximal response, K1/2 = 18 micromolar). Studies with KCl and Na2SO4 showed that the stimulation was due to Na+. Photosynthesis at pH 6.1 was only slightly stimulated by Na+. The response of photosynthesis at pH 8.0 to [Na+] was strongly sigmoidal for dissolved inorganic carbon ([DIC] .ltoreq. 500 micromolar). Cells grown with high total [DIC], but air-levels of CO2, at pH 9.6 showed the same response to low [Na+]. The absence of Na+ could be partially, but not completely overcome, by higher [DIC]. Various methods for examining CO2 or HCO3- use (K1/2CO2 determinations; isotopic disequilibrium; and consideration of HCO3- dehydration rate) were consistent with CO2 use by the cells, but HCO3- use could not be ruled out. Isotopic disequilibrium studies showed that CO2 use was stimulated by Na+. Cells grown on 5% CO2 accumulated DIC against a concentration gradient by a process (or processes) dependent on Na+. No evidence for uptake of Na+ concomitant with DIC uptake could be formed. The lack of O2 evolution during the initial and most rapid period of DIC accumulation suggested that the required energy was obtained from cyclic photophosphorylation.