Control of photosynthesis during nitrogen depletion and recovery in a non-nitrogen-fixing cyanobacterium

When cells of Synechocystis strain PCC 6308 are starved for nitrogen, the amount of stored carbohydrate increases, the phycocyanin to chlorophyll a ratio decreases, and the rates of oxygen evolution and of carbon dioxide fixation decrease. When nitrate-nitrogen is replenished, the amount of carbohydrate decreases, the rate of oxygen evolution increases immediately, preceeding the increase in phycocyanin or carbon dioxide fixation. The rate of respiration first increases and then decreases upon nitrogen addition. Nitrogen-starved cells show no variable fluorescence; variable fluorescence recovered in parallel with oxygen evolution. This suggests that photosystem II is inactive in nitrogen depleted cells and not blocked by a build up of metabolic endproducts. Since carbon dioxide fixation does not increase until two to four hours after nitrate is replenished to nitrogen starved cells, it is suggested that reducing power may first be needed within the cell for some other process than photosynthesis, such as nitrate reduction.