Effect of cell water amount on photosynthetic yield in the cyanobacterium Nostoc flagelliforme and involvement of NADPH dehydrogenase-mediated cyclic electron transport

Nostoc flagelliforme is a terrestrial cyanobacterium, and water is one of the most important factors limiting its photosynthetic yield. The aims of the present study were to investigate the effect of cell water amount on photosynhetic yield and the role of NADPH dehydrogenase (NDH-1)-mediated cyclic electron transport in this effect. The role of NDH-1-mediated cyclic electron transport was assessed by measuring NDH-1 expression, several chlorophyll fluorescence parameters, and photosynthetic O₂ evolution at several time points after cell water had been redried. The results indicated that the highest rate of NDH-1-mediated cyclic electron transport, reflected by post-illumination increase in chlorophyll fluorescence and NDH-1 amount, was only obtained when the cells contained about 1.8 times water relative to dry weight. This was consistent with observed changes in photosynthetic yield, reflected by O₂ evolution. However, the highest photochemical activity of photosystem II, reflected by F _v/F _m and qP, could be maintained when N. flagelliforme cells included water in a broad range. This implies that the effect of cell water amount on photosynthetic yield is related to NDH-1-mediated cyclic electron transport. The possible mechanisms of this effect are discussed.