Na<sup>+</sup>-dependent HCO<sub>3</sub><sup>-</sup> transport in the cyanobacterium <I>Synechocystis</I> PCC6803

The effect of Na + on HCO 3 - transport, inorganic carbon (C i ) accumulation, and photosynthesis was investigated in the unicellular cyanobacterium Synechocystis sp. PCC6803 using the silicone fluid filtering centrifugation technique. Unlike other cyanobacteria, Synechocystis cells grown at low C i in standing culture had little capacity for Na + -independent HCO 3 - transport, when assayed at pH 9.6. However, 25 mM NaCl, but not KCl, strongly promoted HCO 3 - transport and accumulation. Kinetic analysis indicated that the HCO 3 - concentration required for one half the maximum rate of transport, K 0.5 (HCO 3 - ), decreased in the presence of Na + while the maximum rate of transport, V MAX , increased by up to 15-fold. Na + -dependent HCO 3 - transport occurred against an electrochemical potential of up to 24 kJ ·mol -1 , indicating the involvement of carrier-mediated active transport. Li + (1-3 mM) partially substituted for Na + in that K 0.5 ( HCO 3 - ) values were similar (38 vs. 50 µM), but V MAX was reduced by twofold. At higher concentrations, Li + counteracted the effects of Na + . Monensin reversibly inhibited Na + -dependent HCO 3 - transport and acted by reducing V MAX without affecting K 0.5 (HCO 3 - ). Monensin inhibition suggested that the electrochemical potential for Na + may play a role in Na + -dependent HCO 3 - transport, possibly through an involvement in intracellular pH regulation during transport. Na + also stimulated photosynthetic C fixation and O 2 evolution and these effects were correlated with the Na + -dependent increase in intracellular C i accumulation. The Na + -requirement for photosynthesis could be relieved by the provision of CA to the cell suspension, in agreement with the proposal that Na + is required for transport and not directly involved in the photosynthetic process.Key words: active transport, CO 2 -concentrating mechanism, cyanobacteria, Na + -dependent HCO 3 - transport, photosynthesis, Synechocystis PCC6803.