Characterisation of CO₂ and HCO₃- Uptake during Steady-state Photosynthesis in the Cyanobacterium Synechococcus PCC7942 Steady-state Photosynthesis in the Cyanobacterium Synechococcus PCC7942

Abstract

We have recently developed a mass spectrometric disequilibrium technique to measure CO₂ and HCO₃^- uptake during steady-state photosynthesis in cyanobacteria and microalgae (Badger et al. 1993). Using this technique, we have found differences between the characteristics of C_i uptake in Synechococcus PCC7942 under steady- and non-steady state photosynthesis. Both low- and high-C_i cells have the capacity to transport CO₂ and HCO₃^-. In contrast to reported results obtained under non-steady-state conditions, the activity of HCO₃^- uptake is higher than that of CO₂ uptake, even in high-C_i cells. Dramatic increases in the affinity of the C^I transport system for CO₂ and HCO₃^- but not in maximal transport activities were found when high-C_i cells were induced under low-C_i conditions. These changes in affinity occurred within about 4 h. The affinity for HCO₃^- but not CO₂ also increased when uptake activities were measured at a lower pH (pH 7.5 compared to pH 8.5), especially in high-C_i cells. HCO₃^- uptake was also more sensitive to LiCl inhibition than CO₂ uptake. LiCl inhibition was partially relieved in the presence of NaCI. The carbonic anhydrase inhibitor ethoxyzolamide inhibited both CO₂ and HCO₃^- uptake to about the same extent, consistent with the view that both transports might share a common 'carbonic anhydrase-like' component. The results are discussed in relation to previous results obtained under non-steady-state conditions.