Oxygen photoreduction and its effect on CO2 accumulation and assimilation in air-grown cells of Synechococcus UTEX 625

Mass spectrometric measurements of 16O2, 18O2, and 13CO2 were used to measure the rates of gross O2 evolution, O2 uptake, and CO2 assimilation in relation to light intensity, temperature, pH, and O2 concentration by air-grown cells of the cyanobacterium Synechococcus UTEX 625. CO2 fixation and O2 photoreduction increased with increased light intensity and, although CO2 fixation was saturated at 250 μmol ∙ m−2 ∙ s−1, O2 photoreduction was not saturated until about 550 μmol ∙ m−2 ∙ s−1. At high light intensity addition of inorganic carbon to the cells stimulated O2 photoreduction 2-fold when CO2, fixation was allowed and 5-fold when CO2, fixation was inhibited with iodoacetamide. The ability of O2, to act as an acceptor of photosynthetically generated reducing power was dependent upon the O2 concentration, and the substrate concentration required for half maximum rate (K½(O2)) was 53.2 ± 4.2 μM (mean ± SD, n = 3). The Q10 for oxygen photoreduction was about 2. A certain amount (10%) of O2 appeared to be required for maximum photosynthesis, as photosynthesis was inhibited under anaerobic conditions, especially at high light intensity. The point of inhibition is unknown but it seemed unlikely to be on CO2 transport or the concentration of intracellular dissolved inorganic carbon (Ci), as the rate of initial CO2 transport was enhanced and the intracellular Q1 pool increased in size under anaerobic conditions. Key words: cyanobacteria, photosynthesis, Ci concentrating mechanism, inorganic carbon pool, O2 photoreduction, electron transport, temperature.