Nitrogen Fixation and Hydrogen Metabolism in Relation to the Dissolved Oxygen Tension in Chemostat Cultures of the Wild Type and a Hydrogenase-Negative Mutant of Azorhizobium caulinodans

Both the wild type and an isogenic hydrogenase-negative mutant of Azorhizobium caulinodans growing ex planta on N(2) as the N source were studied in succinate-limited steady-state chemostat cultures under 0.2 to 3.0% dissolved O(2) tension. Production or consumption of O(2), H(2), and CO(2) was measured with an on-line-connected mass spectrometer. In the range of 0.2 to 3.0%, growth of both the wild type and the mutant was equally dependent on the dissolved O(2) tension: the growth yield decreased, and the specific O(2) consumption and CO(2) production increased. A similar dependency on the dissolved O(2) tension was found for the mutant with 2.5% H(2) in the influent gas. The H(2)/N(2) ratio (moles of H(2) evolved per mole of N(2) consumed via nitrogenase) of the mutant, growing with or without 2.5% H(2), increased with increasing dissolved O(2) tensions. This increase in the H(2)/N(2) ratio was small but significant. The dependencies of the ATP/N(2) ratio (moles of ATP consumed per mole of N(2) fixed) and the ATP/2e ratio [moles of ATP consumed per mole of electron pairs transferred from NAD(P)H to nitrogenase] on the dissolved O(2) tension were estimated. These dependencies were interpreted in terms of the physiological concepts of respiratory protection and autoprotection