An essential role for sodium in the bicarbonate transporting system of the cyanobacterium Anabaena variabilis

The apparent photosynthetic affinity of Anabaena variabilis for extracellular inorganic carbon (Ci) was strikingly increased by Na+. The effect was highly specific for Na+ and was maximal at 40 mM Na+. Na+ supply decreased the apparent K m (Ci) of the Ci transporting system and to a lesser extent increased V max. It did not affect photosynthetic rate expressed as a function of intracellular Ci. We infer an effect of Na+ on the Ci transporting system rather than on the photosynthetic machinery itself. We propose several possible models, including Na+‐H+ antiport for maintenance of intracellular pH during HCO3 uptake, and Na+‐HCO− 3 symport.