The role of oxygen concentration in the synthesis of carotenoids, poly-/3-hydroxybutyrate and succinate oxidase in continuous culture of azospirillum¹

The nitrogen fixing bacterium Azospirillum brasilense Cd was grown in ammonium- mineral salts medium in a chemostat, with malate as a growth-limiting carbon and energy source under various dissolved oxygen tensions (d.o.t.). Maximum poly-0-hydroxy- butyrate content (12% w/w of the biomass) was observed under microaerobic conditions (d.o.t. = 0.007 atm) and intermediate dilution rates (D = 0.14 h^-1). In carbon-limited cultures (D = 0.008 h^-1) cell yield and respiration in situ ranged from 48 to 62 g dry wt-(mole malate)”1 and from 75 to 325 mI O, '(mg protein)^-1, respectively. Highest cell yield and respiration rate were obtained at relatively low concentration of oxygen (d.o.t. = 0.007 atm). Protein content of cultures was significantly higher at high oxygen levels. However, a rapid decrease was observed under lower oxygen levels. Succinate oxidase and succinate dehydrogenase activities were especially high under high oxygen concentration (d.o.t. > 0.1 atm). Four different carotenoids were isolated and purified from membranes of the Cd strain. In the presence of diphenylamine which specifically inhibits carotenoid synthesis, the rate of acetylene reduction in strain Cd decreased to 50% of the control. In liquid cultures of strain Cd, carotenoid synthesis was initiated only after the depletion of NH₄C1 in the growth medium when N₂ fixing activity became evident. Carotenoids appeared to protect nitrogenase of A, brasilense strain Cd from oxidative damage.