Metabolism and regulatory roles of glycogen in nitrogen fixation of marine unicellular cyanobacteria

When cyanobacteria developed O$\sb2$-evolving photosynthesis, they had to maintain the O$\sb2$-sensitive N$\sb2$ fixation, and then adapt to the increase of atmospheric PO$\sb2$ caused by their own photosynthesis.Several adaptive regulations of N$\sb2$ fixation which are elaborately organized in the cell cycle events of a marine unicellular cyanobacterium, Synechococcus sp. strain Miami BG 043511, were studied by the use of synchronous culture technique.By means of excessive accumulation of glycogen and its rapid degradation, this type of N$\sb2$-fixing marine unicellular cyanobacteria segregated photosynthesis and N$\sb2$ fixation temporally in the cell cycle under both aerobic and anaerobic conditions. They also overcome the problems of atmospheric O$\sb2$ and dark period successfully by modifying the manner of temporal separation and by operating oxidative phosphorylation.Since no CO$\sb2$ fixation is observed during the phase of N$\sb2$ fixation, an imbalance in the supplies of NADPH, which supports the nitrogenase reaction, and carbon skeletons for incorporating the fixed nitrogen (NH$\sb3$) occurs. The more NADPH is produced through the oxidative pentose-phosphate pathway with the release of CO$\sb2$, the fewer carbon skeletons remain for biosynthesis. Under aerobic conditions during glycogen degradation, the oxidative phosphorylation is forced to operate consuming NADPH, and more than 50% of the carbon was released as CO$\sb2$.Although the efficiency of glycogen utilization for N$\sb2$ fixation is found to be optimum under anaerobic illuminated conditions where cyanobacteria evolved, they compensate for their lower efficiency by accumulating more glycogen under the aerobic conditions, and maintain the fast growth rate which they can exhibit under anaerobic conditions.In addition to the glycogen utilization, enzymes responsible for NADPH and carbon skeleton supplies were shown to be actively synthesized just before the phase of N$\sb2$ fixation began, positively supporting the temporal operation of N$\sb2$ fixation.These high growth rates and their adaptive strategies under different culture conditions indicate a significant contribution of unicellular cyanobacteria to N$\sb2$ fixation carried out in the marine environment