Metabolic rhythms of a diazotrophic cyanobacterium, Cyanothece sp strain ATCC 51142, heterotrophically grown in continuous dark

Aerobic unicellular cyanobacteria temporally separate nitrogen fixation and photosynthetic activity to protect oxygen-sensitive nitrogenase, Diurnal metabolic oscillations in the unicellular diazotroph, Cyanothece sp, str, CGD occurred even when cells were gown under continuous dark conditions, as determined in this study. The metabolic processes of nitrogen fixation, respiration, carbohydrate storage, and nitrogen storage all remained highly synchronized during heterotrophic growth, despite the absence of Light signals. This shows the importance of circadian regulation in this unicellular cyanobacterium, The nitrogenase complex is apparently tightly regulated at the transcriptional and post-translational levels. Carbohydrates accumulated during the subjective light phase were metabolized in a burst of respiration in the subjective dark phase. The utilization of carbohydrates occurred at the same time as the intense period of nitrogenase activity and downregulation of the capacity for photosynthetic oxygen evolution. Abundance of the D1 protein of photosystem II in Cyanothece sp, str, CGD remained relatively constant under continuous darkness, but the psbA transcript was more abundant in the subjective light phase. Accumulation of the psaAB gene products in photosystem I was highest in the subjective dark phase. The storage of fixed nitrogen as cyanophycin granules also exhibited periodic changes, with the amount of cyanophycin being greatest after the peak of nitrogenase activity. These results strongly implicate a circadian regulatory mechanism operating on these metabolic processes, independent of light cues, and stress the importance of circadian rhythms in global metabolic regulation in this unicellular cyanobacterium.