NA-DEPENDENT HCO3-TRANSPORT IN THE CYANOBACTERIUM SYNECHOCYSTIS PCC6803()

The effect of Na+ on HCO3- transport, inorganic carbon (C-i) accumulat ion, and photosynthesis was investigated in the unicellular cyanobacte rium Synechocystis sp. PCC6803 using the silicone fluid filtering cent rifugation technique. Unlike other cyanobacteria, Synechocystis cells grown at low C-i in standing culture had little capacity for Na+-indep endent HCO3- transport, when assayed at pH 9.6. However, 25 mM NaCl, b ut not KCl, strongly promoted HCO3- transport and accumulation. Kineti c analysis indicated that the HCO3- concentration required for one hal f the maximum rate of transport, K(0.)5(HCO3-), decreased in the prese nce of Na+ while the maximum rare of transport, V-MAX, increased by up to 15-fold. Na+-dependent HCO3- transport occurred against an electro chemical potential of up to 24 kJ.mol(-1), indicating the involvement of carrier-mediated active transport. Li+ (1-3 mM) partially substitut ed for Na+ in that K-0.5(HCO3-) values were similar (38 vs. 50 mu M), but V-MAX was reduced by twofold. At higher concentrations, Li+ counte racted the effects of Na+. Monensin reversibly inhibited Na+-dependent HCO3- transport and acted by reducing V-MAX without affecting K-0.5(H CO3-). Monensin inhibition suggested that the electrochemical potentia l for Na+ may play a role in Na+-dependent HCO3- transport, possibly t hrough an involvement in intracellular pH regulation during transport. Na+ also stimulated photosynthetic C fixation and O-2 evolution and t hese effects were correlated with the Na+-dependene increase in intrac ellular C-i accumulation. The Na+-requirement for photosynthesis could be relieved by the provision of CA to the cell suspension, in agreeme nt with the proposal that Na+ is required for transport and not direct ly involved in the photosynthetic process.