Nitrogen-fixing Growth in Continuous Culture of a Strain of Rhizobium sp. Isolated from Stem Nodules on Sesbania rostrata

Summary: A strain of Rhizobium sp. (ORS571), isolated from stem nodules on Sesbania rostrata, was grown as a N₂-fixing continuous culture for up to five weeks (> 80 culture replacements). Under optimum conditions of O₂ supply [< 9 μm dissolved O₂, 100–160 nmol O₂ (mg dry wt)⁻¹ min⁻¹], N₂ fixation rates were 300–400 nmol N₂ (mg dry wt)⁻¹ h⁻¹ and nitrogenase activity in culture samples reached 2000 nmol C₂H₄ (mg dry wt)⁻¹ h⁻¹, the highest values yet recorded for any Rhizobium species. Other properties of the continuous cultures are described and a comparison is made with previous reports of N₂ fixation by cultures of Rhizobium spp. Significant features were the greater tolerance to O₂ of ORS571 and the retention of fixed N in the cells of this strain.