Influx and efflux of inorganic carbon during steady-state photosynthesis of air-grownAnabaena variabilis

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The inward and outward fluxes of inorganic carbon in illuminated cell suspensions of air-grown Anabaena variabilis were measured by mass spectrometry under conditions of inorganic carbon disequilibrium. The inclusion of 25 mM NaCl significantly enhanced both inward inorganic carbon influx during CO2fixation and outward CO2efflux when CO2fixation was blocked by the Calvin cycle inhibitor, iodoacetamide. At low, steady-state concentrations of inorganic carbon (< 100μM), CO2fixation was nearly entirely supported by HCO3−transport in the presence of 25 mM NaCl. At approximately 150 μM inorganic carbon, the contributions of CO2and HCO3−transport to CO2fixation were about equal. Above this, CO2transport provided most of the substrate for CO2fixation. The affinity (K0.5) of photosynthesizing cells for CO2, HCO3−and total inorganic carbon was determined and mean values of 1.7, 9.5, and 8.2 μM, respectively, were determined. Maximum rates of inward CO2and HCO3−transport and CO2fixation during steady state were 255.7, 307.3, and 329.1 μmol∙mg−1Chl∙h−1, respectively. Permeability coefficients for CO2of 9.8 × 10−8 m∙s−1and 2.8 × 10−7 m∙s−1were calculated for the plasma membrane and carboxysomal surface areas, respectively, from the dark efflux rates assuming an internal pH of 7.2. A permeability coefficient for HCO3−across the plasma membrane of 7.6 × 10−9 m∙s−1was calculated from the dark inorganic carbon efflux corrected for the corresponding dark CO2efflux. Sodium sulphide (Na2S, 200 μM) blocked CO2transport. In the presence of 25 mM NaCl, net CO2efflux was approximately seven times greater than in its absence, when CO2transport and fixation were both blocked, indicating greater CO2leakage as a result of larger internal inorganic carbon pools in the presence of NaCl. The rapidity and amount of C16O2generated from the exchange of18O from18O-enriched HCO3−with water in cell suspensions suggested that the internal inorganic carbon pool may be rapidly equilibrated. Key words: Anabaena variabilis, CO2-concentrating mechanism, CO2transport, HCO3−transport, CO2efflux, permeability coefficient.