Electron Flow from NAD(P)H Dehydrogenase to Photosystem I is Required for Adaptation to Salt Shock in the Cyanobacterium Synechocystis sp. PCC 6803

The role of the NAD(P)H-dehydrogenase complex in adaptation to salt stress was examined in an ndhB-inactivated mutant of the cyanobacterium Synechocystis sp. PCC 6803. Wild-type cells and ndAfl-inactivated mutant cells grew at similar rates under conditions of low salinity (<0.6M NaCl) and high CO2 (3%). However, when the concentration of NaCl in the culture medium was higher than 0.6 M, the mutant cells grew much more slowly than the wild-type cells. Upon addition of high concentrations of NaCl, the oxygen-evolving activity was rapidly inhibited but then it recovered, with the rate of recovery depending on the concentration of NaCl. The recovery of the mutant cells was significantly delayed when the concentration of NaCl was above 0.3 M. At 0.9 M NaCl, wild-type cells recovered with a half time of about 40 min, while mutant cells did not recover. The kinetics of changes in Chi fluorescence confirmed these results. In wild-type cells, input of electrons from the cytosol to PSI via the NAD(P)H-dehydrogenase complex increased upon salt shock. It appears, therefore, that the electron flow from the cytosol to PSI via NAD(P)H-dehydrogenase is essential for the adaptation of cyanobacteria to salt shock.