N₂-Dependent Growth of Azospirillum Spp. in Batch Cultures at Low Concentrations of Oxygen

Summary

Azospirillum halopraeferens Au7 and A. amazonense Y1 were grown in batch cultures under controlled conditions on N₂ as substrate at about 1 μM dissolved O₂ concentration, both strains showed exponential growth up to an optical density of 13 for Y1 on 10 g/l of fructose (2.9 g bacterial dry weight per liter) and of 3 for Au7 on 5 g/l of L-malate (900 mg bacterial dry weight per liter). The generation time of Au7 was 4.2 h, and for Y1 13.5 h. Although the cells grew strictly exponentially, physiological changes could be seen during this growth phase. Protein contents of cells increased from 43 to 70% for Au7 and from 22 to 33% for Y1, efficiency of nitrogen fixation increased from 4 to 35 mg N per g L-malate for Au7 and from 14 to 27 mg N per g fructose for Y1. At the time of highest protein contents 10% of fixed nitrogen was found in the culture filtrate. Since no soluble protein was observed in the supernatant, the combined nitrogen was likely to be due to nitrogen export and not to cell lysis. This may give some evidence for the potential of Azospirillum to excrete nitrogen.