Driving Forces for Bicarbonate Transport in the Cyanobacterium Synechococcus R-2 (PCC 7942)

Abstract

Air-grown Synechococcus R-2 (PCC 7942) cultures grown in BC-11 medium are very alkaline (outside pH is 10.0) and use HC0,-as their inorganic carbon source. The cells showed a dependente on Na+ for photosynthesis, but low Na+ conditions (1 mo1 m-3) were sufficient to support saturating photosynthesis. The intracellular dissolved inorganic carbon in the light was greater than 20 mo1 m-3 in both low-Na+ conditions and in BC-11 medium containing the usual [Na+] (24 mo1 m-,, designated high-Na+ conditions). l h e electrochemical potential for HC0,-in the light was in excess of 25 kJ mol-', even in high-Na+ conditions. The Na+-motive force was greater than -12 kJ mol-' under both Na+ conditions. On thermodynamic grounds, an Na+-driven co-port process would need to have a stoichiometry of 2 or greater (22Na+i,,/HC03-'i,), but we show that Nat or K+ fluxes cannot be linked to HC0,-transport. Na+ and K+ fluxes were unaffected by the presence or absence of dissolved inorganic carbon. In low-Na+ conditions, Na+ fluxes are too low to support the observed net C-carbon fixation rate. Active transport of HC0,-hyperpolarizes (not depolarizes) the membrane potential. 14

Genes Essential to Sodium-dependent Bicarbonate Transport in Cyanobacteria

“…5). These data are consistent with the suggestion that SbtA is a component of a Na ϩ /HCO 3 Ϫ symporter that drives the HCO 3 Ϫ transport secondary to a primary Na ϩ pump (7,9,10 (21) concluded that the Na ϩ flux was not sufficient to support the rate of photosynthesis (thought to be supported solely by HCO 3 Ϫ transport). However, photosynthesis in both Synechocystis 6803 and Synechococcus 7942 is largely supported by CO 2 uptake, even at high external pH.…”

“…SbtA-mediated HCO 3 Ϫ transport activity was highest at pH 9.0 and lowest at pH 7.0, whereas the ⌬H ϩ in cyanobacteria declines with rising pH. At alkaline pH such as 9.0, ⌬H ϩ would not suffice to drive HCO 3 Ϫ uptake (8,21). We cannot dismiss the possibility that Na ϩ binds to the HCO 3 Ϫ carrier and alters its kinetic parameters (7,10).…”

Two Members of a Network of Putative Na + /H + Antiporters Are Involved in Salt and pH Tolerance of the Freshwater Cyanobacterium Synechococcus elongatus

“…Additionally, disruption of the nha2 gene resulted in a lowNa ϩ requirement that was essential at alkaline pH. A threshold concentration of Na ϩ in growth media is essential for cyanobacteria to establish the necessary Na ϩ gradient across their membranes in order to take up nutrients (34,49,51,61) and to survive under alkaline conditions (3,13). Although the Nha2 protein is unable to complement the salt-sensitive phenotype of TO114 cells and despite the low transcript levels of the nha2 gene, our results suggest that Nha2 is involved in low-Na ϩ -dependent pH regulation of cyanobacterial cells under alkaline conditions.…”

Evolved physiological responses of phytoplankton to their integrated growth environment

“…Prochlorococcus; Badger & Price 2003). Bicarbonate accumulation in the cytoplasm is thought to have an ATP : HCO K 3 or NADH : CO 2 cost ratio of 1 (but see Ritchie et al 1996). This ATP demand for CCM operation can be supplied directly by thylakoid pathways described in §5 (figure 6), including the pseudocyclic pathway (Mehler; Miller et al 1988;Sü ltemeyer et al 1993).…”