The response of Synechococcus PCC7942 (Cyanophyta) to changes in CO2 supply in relation to the acclimation of the CO2-concentrating mechanism. I: physiological study

Cultures of Synechococcus PCC7942 growing in BG11 medium, Hepes 20 mmol/L, pH 7.8, 100 μmol · m−2 · s−1 continuous light, experienced inorganic carbon (Ci) limitation (as indicated by a decrease in growth rate) when shifted from high-CO2 to ordinary air under conditions of active bubbling. The high-affinity state of the carbon-concentrating mechanism (CCM), evaluated as K0.5(Ci), was induced to the same extent under conditions of Ci limitation imposed by air or CO2-depleted air (20 μL · L−1), though it required a lag of 3 h in the first case and was immediate in the second. Dissolved inorganic carbon (DIC) in cultures transferred to air was 1.24 mmol/L (corresponding to [HCO3−] = 1.19 mmol/L and [CO2] = 36 μmol/L at pH 7.8) at the time induction started. Values of the initial rate and total photosynthetic O2 evolution during the compensation period, and the time required to reach compensation, differed in non-induced and induced cells: O2 evolution and time required to reach compensation were high in non-induced cells and low in induced cells, while V0 was low in non-induced and high in induced cells. The coincident time courses of growth rate, K0.5(Ci), V0 of compensation, and total O2 during compensation in response to changes in CO2 supply suggested that these variables were directly related, and that an imbalance between metabolic/energetic capacity for CO2 fixation and internal CO2 availability might be involved in CCM induction under the experimental conditions used in this study.