An essential role for sodium in the bicarbonate transporting system of the cyanobacterium <i>Anabaena variabilis</i>

Abstract

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 HCO 3 uptake, and Na + ‐HCO − 3 symport.