EFFECT OF THE INTRACELLULAR INORGANIC CARBON POOL ON CHLOROPHYLL-A FLUORESCENCE QUENCHING AND O-2 PHOTOREDUCTION IN AIR-GROWN CELLS OF THE CYANOBACTERIUM SYNECHOCOCCUS UTEX-625

Simultaneous measurements were made of O-2 exchange, inorganic carbon (C-i) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of C-i to cel ls at the CO2 compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2 fixation inhibito r, iodoacetamide. The magnitude of quenching was related to electron f low to terminal electron accepters such as CO2 and O-2. When photosynt hetic CO2 fixation was allowed, the rate of electron transport, as exp ressed by (F-m() - F)/F-m(*), was highly correlated with the onset of photosynthesis. When CO2 fixation was inhibited by the addition of io doacetamide, the observed fluorescence quenching was consistent with t he enhanced rate of O-2 photoreduction that occurred when C-i was adde d. There was a close correlation (r = 0.98) between the magnitude of O -2-dependent fluorescence quenching and the amount of O-2 photoreducti on. The degree of stimulation of electron flow to O-2 photoreduction w as dependent on the inorganic carbon concentration. The K-1/2 (C-i) fo r extracellular C-i was 1.36 +/- 0.13 mu M (mean +/- SD, n = 3) and K- 1/2 (C-i) for the intracellular C-i pool was 1.4 +/- 0.18 mM (mean +/- SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also s timulated by the addition of C-i, whereas the addition of C-i had no e ffect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. T he results suggest that C-i stimulates electron flow in photosystem I.