It is well established that various strains of Synechococcus, a genus of unicellular cyanobacteria, can photoreduce O2, at high rates even during concomitant photosynthetic CO2 fixation. This photoreduction probably involves photosystem I (PS1) and the so-called Mehler reaction. Although all photosynthetic organisms carry out this reaction to some extent, the mechanistic details remain unclear. Good candidates for the study of this reaction should be the various cyanobacteria that exhibit high rates of O2 photoreduction. Unfortunately, the strains of Synechococcus that have been examined so far in this context are obligate photoautotrophs, which precludes the dissection of the photoreduction process by mutational analysis of PS1. In the present study, we show that Synechocystis PCC 6803, a species capable of growth on glucose can photoreduce O2 at high rates. When grown photoautotrophically, the cells exhibited an O2-dependent quenching of chlorophyll a fluorescence that was 50–70% that of CO2-dependent quenching. The magnitude of this O2-dependent photochemical quenching could be related to the rate of linear photosynthetic electron flow. For cells grown at a photosynthetic photon flux density of 50 μmolm−2 ∙ s−1, the rate of O2, photoreduction was saturated at about 200 μmolm−2 ∙ s−1. Unlike Synechococcus UTEX 625 and PCC 7942, the rate of photoreduction of O2 was high even at the CO2 compensation point, when there would be very little active accumulation of inorganic carbon (Ci). The O2 concentration, or Km(O2), that supported the half-maximal rate of O2 photoreduction was in the order of 6 μM. This low value for the Km(O2) and also the ability of the cells to photoreduce O2 in the presence of glycolaldehyde, which inhibits ribulose bisphosphate (RuBP) regeneration, ruled out the involvement of RuBP oxygenase. Cells grown photomixotrophically, on Ci and glucose, were also capable of high rates of O2 photoreduction as manifest by high rates of O2-dependent quenching of chlorophyll fluorescence. The response to O2 of these cells was very similar to that observed with Synechococcus UTEX 625 and PCC 7942, in that O2 photoreduction was greatly stimulated by the accumulation of Ci by the active CO2 and HCO3− transport systems. The demonstration that the facultatively heterotrophic Synechocystis PCC 6803 photoreduces O2 at high rates opens up the possibility of using targeted genetic inactivation of PS1 to study the Mehler reaction. Key words: O2 photoreduction, cyanobacteria, chlorophyll a fluorescence.
CHLOROPHYLL A FLUORESCENCE AS A QUANTITATIVE PROBE OF PHOTOSYNTHESIS: EFFECTS OF CO2 CONCENTRATION DURING GAS TRANSIENTS ON CHLOROPHYLL FLUORESCENCE IN SPINACH LEAVES