INORGANIC CARBON-STIMULATED O-2 PHOTOREDUCTION IS SUPPRESSED BY NO2- ASSIMILATION IN AIR-GROWN CELLS OF SYNECHOCOCCUS UTEX-625

The effect of NO2- assimilation on O-2 exchange and CO2 fixation of the cyanobacterium, Synechococcos UTEX 625, was studied mass spectrometrically. Upon addition of 1 mM inorganic carbon to the medium, inorganic carbon pools developed and accelerated O-2 photoreduction 5-fold when CO2 fixation was inhibited. During steady-state photosynthesis at saturating light, O-2 uptake represented 32% of O-2 evolution and balanced that portion of O-2 evolution that could not be accounted for by CO2 fixation. Under these conditions, NO2- assimilation reduced O-2 uptake by 59% but had no influence on CO2 fixation. NO2- assimilation decreased both CO2 fixation and O-2 photoreduction at low light and and increased net O-2 evolution at all light intensities. The increase in net O-2 evolution observed during simultaneous assimilation of carbon and nitrogen over carbon alone was due to a suppression of O-2 photoreduction by NO2- assimilation. When CO2 fixation was precluded, NO2- assimilation inhibited O-2 photoreduction and stimulated O-2 evolution. When the electron supply was limiting (low light), competition among O-2, CO2, and NO2- for electrons could be observed, but when the electron supply was not limiting (saturating light), O-2 photoreduction and/or NO2- reduction caused electron transport that was additive to that for maximum CO2 fixation.