The effects of inorganic carbon and oxygen on fluorescence in the cyanobacterium Synechococcus UTEX 625

The active transport of inorganic carbon and the accumulation of the internal pool caused quenching of chlorophyll a fluorescence both when CO2 fixation was allowed or when CO2 fixation was inhibited. Upon the addition of inorganic carbon in the presence of 240 μM oxygen the rate of change in fluorescence (or quenching) was correlated (r = 0.98) with the rate of active CO2 uptake, and the extent of quenching was correlated (r = 0.99) with the size of the internal inorganic carbon pool. Fluorescence was quenched by the fixation of inorganic carbon in the absence of oxygen but the reoxidation of QA following a flash of light was slow. In the presence of inorganic carbon, with or without the inhibition of CO2 fixation, oxygen quenched fluorescence. If CO2 fixation was inhibited, the degree of quenching depended upon the oxygen concentration with a K1/2 (O2) of about 42 μM. Below 60 μM oxygen there was a further reduction of QA following a flash of light and the reoxidation of QA was slow. Rapid reoxidation of QA following a flash of light required about 240 μM oxygen. From the response to added 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, the quenching by oxygen was photochemical quenching and nonphotochemical quenching did not seem to be present. For reasons that are unknown, however, only about 80% of the quenching could be reversed with high intensity flashes of light. The photoreduction of oxygen was regulated by the presence of inorganic carbon, although fixation of CO2 was not required. The mechanism of this regulation is not known but it may be due to bicarbonate relief of electron transfer between QA and QB. Some results on measuring Fo, F′o, Fm, and F′m, in Synechococcus UTEX 625 are presented. Key words: cyanobacteria, fluorescence, oxygen photoreduction, active inorganic carbon transport.