The effect of different growth conditions on the glycogen and poly-β-hydroxybutyrate (PHB) content of the cyanobacterium is described. Under photoautotrophic growth conditions without any nutrient limitation, exhibited a glycogen content of between 7.1 and 10.7% of cell dry wt, whereas PHB was undetectable. When was grown under mixotrophic conditions in the presence of acetate, the intracellular PHB concentration increased to more than 3% of dry wt, while glycogen content remained within the range of 5 to 6% of cell dry wt. Nitrogen starvation favoured glycogen accumulation (up to 60 to 70% of dry wt), while the PHB content remained low (up to 0.7% of dry wt), even after prolonged nitrogen starvation. Inhibition of protein synthesis, induced by addition of azaserine, led to the accumulation of glycogen (up to 52% of cell dry wt) but did not stimulate PHB synthesis. Under phosphorus-limited growth conditions, glycogen and PHB accumulated (up to 23% and 1.2% of cell dry wt, respectively) only after the exhaustion of intracellular phosphorus reserves. Shifting the culture from low to high light irradiance induced a rapid accumulation of glycogen (up to 34% of cell dry wt after 9 h) but did not induce PHB synthesis. Results are discussed in terms of the metabolic significance of PHB synthesis in cyanobacteria, and suggest that this polymer acts exclusively as a disposal mechanism to eliminate excess reducing equivalents.


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