Effect of Oxygen and Nitrogen Limitation on Poly- -hydroxybutyrate Biosynthesis in Ammonium-grown Azotobacter beijerinckii

Azotobacter beijerinckii synthesized up to 70% of its dry weight as poly-β-hydroxybutyrate when grown in batch or oxygen-limited chemostat cultures on a glucose/ammonium salts medium. In a series of steady states during transition from oxygen to ammonium limitation and at different dilution rates in the chemostat, the poly-β-hydroxybutyrate content of the organism decreased to a minimum of 5 to 10 % of the dry weight at an oxygen inflow rate of 1·25 % (v/v) in 400 ml argon min−1. At higher dissolved oxygen tensions the polymer content increased to a new maximum of 20 to 40 % of the dry weight, depending upon the dilution rate, before declining to a negligible value. In contrast, nitrogen-grown organisms displayed a steady decrease in polymer content with increasing oxygen concentration. This difference in behaviour is attributed to the greater demand for reducing power and ATP by nitrogen-fixing cultures preventing the operation of respiratory control which, it is suggested, occurs in ammonium-grown cultures over a limited range of oxygen supply rates until respiratory protection and uncoupled electron transport intervene.