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 (C_i) 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 (C_i) of the C_i transporting system and to a lesser extent increased V_max. It did not affect photosynthetic rate expressed as a function of intracellular C_i. We infer an effect of Na⁺ on the C_i 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.