Brasilense Strain Cd Research Articles

Effect of Dissolved Oxygen Tension on Production of Carotenoids, poly- -hydroxybutyrate, Succinate Oxidase and Superoxide Dismutase by Azospirillum brasilense Cd Grown in Continuous Culture

SUMMARY: Azospirillum brasilense strain Cd was grown in a medium containing NH4 + in a chemostat at a range of constant dissolved oxygen tensions (d.o.t.) (0·007--0·18 atm). Poly-β-hydroxybutyrate (up to 12% of the cell dry weight) increased under oxygen limitation and moderate dilution rate (D = 0·14 h-1). The highest carotenoid content was observed at high d.o.t. and dilution rates up to 0·12 h-1. The amount of protein varied with d.o.t. from 0·29 mg protein (mg dry wt)-1 at 0·007 atm to 0·54 mg at 0·18 atm. The yield efficiency and respiration rate were highest at low d.o.t. and decreased significantly at a d.o.t. of 0·18 atm. Succinate dehydrogenase and malate dehydrogenase activities increased 2·5-fold at 0·10-0·18 atm O2, whereas succinate oxidase and NADH oxidase activities increased consistently with increasing d.o.t. Azospirillum brasilense showed a low specific activity for catalase; the specific activity of superoxide dismutase increased sharply above 0·16 atm O2.

Carotenoid Composition and Function in Nitrogen-fixing Bacteria of the Genus Azospirillum

Four different carotenoids were isolated and purified from membranes of the N2-fixing bacterium Azospirillum brasilense strain Cd, grown under aerobic conditions, and one from strain Cd-1. Carotenoid synthesis did not occur under microaerobic conditions. Cells of these strains formed aggregates (>0·1 mm in diameter) when subjected to aerobic conditions. Neither carotenoids nor cell aggregation were observed in A. brasilense strains Sp 7, Sp81 and Sp51e. All of the differently pigmented Azospirillum strains tested contained similar amounts of soluble cytochrome c. In the presence of diphenylamine, which specifically inhibits carotenoid synthesis, the rate of acetylene reduction (N2 fixation) in strain Cd decreased to 50% of the control. Carotenoid synthesis was inhibited in cells grown in the presence of NH4Cl at concentrations higher than 10 mm. Carotenoid synthesis started in liquid cultures of suain Cd only after the concentration of NH4Cl in the medium decreased, and N2 fixation became evident at the same time. In comparison, strain Sp81 did not grow or fix nitrogen after NH4Cl was depleted. Carotenoids appear to protect the nitrogenase of A. brasilense strains Cd and Cd-1 from oxidative damage.