Factors affecting poly‐β‐hydroxybutyrate accumulation in cyanobacteria and in purple non‐sulfur bacteria

Spirulina maxima and Rhodopseudomonas palustris , which are known to be capable of synthesizing poly‐β‐hydroxybutyrate (PHB), were grown under different conditions in order to investigate the metabolic significance of PHB synthesis in phototrophic microorganisms. The intracellular concentration of PHB in S. maxima , growing photoautotrophically in batch cultures under either balanced or unbalanced (depletion of nitrogen or phosphorus in the mineral medium) conditions, was below 0.005% of cell dry weight. PHB was synthesized (up to 0.7% of dry weight) only after a prolonged period of N‐starvation (although no PHB synthesis occurred when N‐starvation was induced by azaserine addition) or when cells, after the exhaustion of intracellular phosphorus reserves, became P‐starved. Under the latter condition, the PHB concentration reached a value of 1.2% of cell dry weight, the same figure reached in the presence of the uncoupler carbonylcyanide‐ m ‐chlorophenylhydrazone (CCCP). When photosynthetic activity was enhanced by a sudden shift of the culture to higher light intensity or when S. maxima was grown at 18°C, no PHB synthesis was detectable. Under all the photoautotrophic growth conditions tested, glycogen was much more heavily accumulated than PHB. Batch cultures of R. palustris , growing photoheterotrophically on acetate with varying nitrogen sources and regimens of nitrogen supplementation, demonstrated that some competition for reducing equivalents exists between nitrogenase activity and PHB biosynthetic pathway. The results seem to suggest that, in phototrophic bacteria able to synthesize both PHB and glycogen, the polyester acts mainly as a regulator of the intracellular reduction charge.