Effect of the intracellular inorganic carbon pool on chlorophyll a fluorescence quenching and O2 photoreduction in air-grown cells of the cyanobacterium Synechococcus UTEX 625
Simultaneous measurements were made of O2 exchange, inorganic carbon (Ci) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of Ci to cells at the CO2 compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2 fixation inhibitor, iodoacetamide. The magnitude of quenching was related to electron flow to terminal electron acceptors such as CO2 and O2. When photosynthetic CO2 fixation was allowed, the rate of electron transport, as expressed by (F*m – F)/F*m, was highly correlated with the onset of photosynthesis. When CO2 fixation was inhibited by the addition of iodoacetamide, the observed fluorescence quenching was consistent with the enhanced rate of O2 photoreduction that occurred when Ci was added. There was a close correlation (r = 0.98) between the magnitude of O2-dependent fluorescence quenching and the amount of O2 photoreduction. The degree of stimulation of electron flow to O2 photoreduction was dependent on the inorganic carbon concentration. The K1/2 (Ci) for extracellular Ci was 1.36 ± 0.13 μM (mean ± SD, n = 3) and K1/2 (Ci) for the intracellular Ci pool was 1.4 ± 0.18 mM (mean ± SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also stimulated by the addition of Ci, whereas the addition of Ci had no effect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. The results suggest that Ci stimulates electron flow in photosystem I. Key words: cyanobacteria, O2 photoreduction, fluorescence, Ci concentrating mechanism, inorganic carbon pool, linear electron transport, kinetic study.