Cyanobacteria are important model organisms for studying the process of photosynthesis
and the effects of environmental stress factors. This study aimed to identify the
inhibitory sites of NaCl in the whole photosynthetic electron transport in Synechocysti
s sp. PCC 6803 WT cells by using multiple biophysical tools. Exposure of cells to
various NaCl concentrations (200 mM to 1 M) revealed the inhibition of Photosystem
II (PSII) activity at the water oxidizing complex and between the Q A and Q B electron
acceptors. In contrast to the inhibition of PSII, electron flow through Photosystem
I (PSI) was accelerated, indicating enhanced cyclic electron flow. The oxygen‐evolving
capacity of the cells was inhibited to a larger extent when only CO 2 was the final
electron acceptor in the Calvin‐Benson‐Bassham (CBB) cycle than in the presence of
the PSII electron acceptor DMBQ, suggesting important NaCl inhibitory site(s) downstream
of PSI. Measurements of NADPH kinetics revealed NaCl‐induced inhibition of light‐induced
production of NADPH as well as retardation of NADPH consumption both in the light
and in the initial dark period after switching off the light. Chlorophyll fluorescence
kinetics, measured in parallel with NADPH fluorescence, showed the enhancement of
post‐illumination fluorescence rise up to 500 mM NaCl, which was however inhibited
at higher NaCl concentrations. Our results show, for the first time, that NaCl inhibits
the activity of the CBB cycle at least at two different sites, and confirm earlier
results about the NaCl‐induced inhibition of the PSII donor and acceptor side and
the enhancement of electron flow through PSI.
