Net O
2 evolution, gross CO
2 uptake and net HCO
inf3
su–
uptake during steady-state photosynthesis were investigated by a recently developed mass-spectrometric technique for disequilibrium flux analysis with cells of the marine cyanobacterium
Synechococcus PCC7002 grown at different CO
2 concentrations. Regardless of the CO
2 concentration during growth, all cells had the capacity to transport both CO
2 and HCO
inf3
su–
; however, the activity of HCO
inf3
su–
transport was more than twofold higher than CO
2 transport even in cyanobacteria grown at high concentration of inorganic carbon (C
i = CO
2 + HCO
inf3
su–
). In low-C
i cells, the affinities of CO
2 and HCO
inf3
su–
transport for their substrates were about 5 (CO
2 uptake) and 10 (HCO
inf3
su–
uptake) times higher than in high-C
i cells, while air-grown cells formed an intermediate state. For the same cells, the intracellular accumulated C
i pool reached 18, 32 and 55 mM in high-C
i, air-grown and low-C
i cells, respectively, when measured at 1 mM external C
i. Photosynthetic O
2 evolution, maximal CO
2 and HCO
inf3
su–
transport activities, and consequently their relative contribution to photosynthesis, were largely unaffected by the CO
2 provided during growth. When the cells were adapted to freshwater medium, results similar to those for artificial seawater were obtained for all CO
2 concentrations. Transport studies with high-C
i cells revealed that CO
2 and HCO
inf3
su–
uptake were equally inhibited when CO
2 fixation was reduced by the addition of glycolaldehyde. In contrast, in low-C
i cells steady-state CO
2 transport was preferably reduced by the same inhibitor. The inhibitor of carbonic anhydrase ethoxyzolamide inhibited both CO
2 and HCO
inf3
su–
uptake as well as O
2 evolution in both cell types. In high-C
i cells, the degree of inhibition was similar for HCO
inf3
su–
transport and O
2 evolution with 50% inhibition occurring at around 1 mM ethoxyzolamide. However, the uptake of CO
2 was much more sensitive to the inhibitor than HCO
inf3
su–
transport, with an apparent I
50 value of around 250 M ethoxyzolamide for CO
2 uptake. The implications of our results are discussed with respect to C
i utilisation in the marine
Synechococcus strain.Abbreviations Chl
chlorophyll
- C
i
inorganic carbon (CO
2 + HCO
inf3
su–
)
- CA
carbonic anhydrase
- CCM
CO
2-concentrating mechanism
- EZA
ethoxyzolamide
- GA
glycolaldehyde
- K
1/2
concentration required for half-maximal response
- Rubisco
ribulose-1,5,-bisphosphate carboxylase-oxygenase
D.S. is a recipient of a research fellowship from the Deutsche Forschungsgemeinschaft (D.F.G.). In addition, we are grateful to Donald A. Bryant, Department of Molecular and Cell Biology and Center of Biomolecular Structure Function, Pennsylvania State University, USA, for sending us the wild-type strain of
Synechococcus PCC7002.
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