Sodium-Dependent Growth of Azotobacter chroococcum

The majority of Azotobacter chroococcum strains in a collection obtained from Alberta soils were absolutely dependent on Na for growth. Two strains from the American Type Culture Collection also were either Na dependent or were stimulated by Na. Optimal growth required 0.8 mM Na and was limited at 0.2 to 0.35 mM Na. Growth promoted by 0.8 mM Na was inhibited by Rb > K >> NH(4) but was not affected by Li. Growth inhibition by Rb and K was overcome by increasing the Na concentration present in the medium. Excellent growth in media containing limiting Na was obtained when 25 mM Li or 25 mM Mg was added. Li was significantly more effective in replacing Na than was Mg. Na was required for growth on all C sources (glucose, sucrose, melibiose, and mannitol) and N sources (N(2), NH(4), and NO(3)) and was required throughout the pH range of growth. Cells suspended in Na -deficient medium or in distilled water did not appear to lyse or lose viability.