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 d uring concomitant photosynthetic CO2 fixation. This photoreduction pro bably 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 th e study of this reaction should be the various cyanobacteria that exhi bit high rates of O-2 photoreduction. Unfortunately, the strains of Sy nechococcus that have been examined so far in this context are obligat e photoautotrophs, which precludes the dissection of the photoreductio n process by mutational analysis of PS1. In the present study, we show that Synechocystis PCC 6803, a species capable of growth on glucose c an photoreduce O-2 at high rates. When grown photoautotrophically, the cells exhibited an O-2-dependent quenching of chlorophyll a fluoresce nce that was 50-70% that of CO2-dependent quenching. The magnitude of this O-2-dependent photochemical quenching could be related to the rat e of linear photosynthetic electron flow. For cells grown at a photosy nthetic 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 Synechoc occus UTEX 625 and PCC 7942, the rate of photoreduction of O-2 was hig h 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 photoreductio n 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 glycola ldehyde, which inhibits ribulose bisphosphate (RuBP) regeneration, rul ed out the involvement of RuBP oxygenase. Cells grown photomixotrophic ally, on C-i and glucose, were also capable of high rates of O-2 photo reduction as manifest by high rates of O-2-dependent quenching of chlo rophyll fluorescence. The response to O-2 of these cells was very simi lar 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 ina ctivation of PS1 to study the Mehler reaction.