High rates of O-2 photoreduction by the unicellular cyanobacterium Synechocystis PCC 6803 as determined by the quenching of chlorophyll fluorescence

It is well established that various strains of Synechococcus, a genus of unicellular cyanobacteria, can photoreduce O-2 at high rates even during concomitant photosynthetic CO2 fixation. This photoreduction probably involves photosystem 1 (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 O-2 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 O-2 at high rates. When grown photoautotrophically, the cells exhibited an O-2-dependent quenching of chlorophyll a fluorescence that was 50-70% that of CO2-dependent quenching. The magnitude of this O-2-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 mu mol.m(-2).s(-1), the rate of O-2 photoreduction was saturated at about 200 mu mol.m(-2) Unlike Synechococcus UTEX 625 and PCC 7942, the rate of photoreduction of O-2 was high even at the CO2 compensation point, when there would be very little active accumulation of inorganic carbon (C-i). The O-2 concentration, or K-m(O-2), that supported the half-maximal rate of O-2 photoreduction was in the order of 6 mu M. This low value for the K-m(O-2) and also the ability of the cells to photoreduce 0, in the presence of glycolaldehyde, which inhibits ribulose bisphosphate (RuBP) regeneration, ruled out the involvement of RuBP oxygenase. Cells grown photomixotrophically, on C-i and glucose, were also capable of high rates of O-2 photoreduction as manifest by high rates of O-2-dependent quenching of chlorophyll fluorescence. The response to O-2 of these cells was very similar to that observed with Synechococcus UTEX 625 and PCC 7942, in that O-2 photoreduction was greatly stimulated by the accumulation of C-i by the active CO2 and HCO3- transport systems. The demonstration that the facultatively heterotrophic Synechocystis PCC 6803 photoreduces O-2 at high rates opens up the possibility of using targeted genetic inactivation of PS1 to study the Mehler reaction.