Na. Mir et al., INORGANIC CARBON-STIMULATED O-2 PHOTOREDUCTION IS SUPPRESSED BY NO2- ASSIMILATION IN AIR-GROWN CELLS OF SYNECHOCOCCUS UTEX-625, Plant physiology, 109(4), 1995, pp. 1295-1300
The effect of NO2- assimilation on O-2 exchange and CO2 fixation of th
e cyanobacterium, Synechococcos UTEX 625, was studied mass spectrometr
ically. Upon addition of 1 mM inorganic carbon to the medium, inorgani
c carbon pools developed and accelerated O-2 photoreduction 5-fold whe
n CO2 fixation was inhibited. During steady-state photosynthesis at sa
turating light, O-2 uptake represented 32% of O-2 evolution and balanc
ed that portion of O-2 evolution that could not be accounted for by CO
2 fixation. Under these conditions, NO2- assimilation reduced O-2 upta
ke by 59% but had no influence on CO2 fixation. NO2- assimilation decr
eased both CO2 fixation and O-2 photoreduction at low light and and in
creased net O-2 evolution at all light intensities. The increase in ne
t O-2 evolution observed during simultaneous assimilation of carbon an
d nitrogen over carbon alone was due to a suppression of O-2 photoredu
ction by NO2- assimilation. When CO2 fixation was precluded, NO2- assi
milation inhibited O-2 photoreduction and stimulated O-2 evolution. Wh
en 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 fo
r maximum CO2 fixation.