Investigation of the effect of salt stress on photosynthetic electron transport pathways in the Synechocystis PCC 6803 cyanobacterium.

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 Synechocystis 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 CO₂ 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.