Abstract: | Both the wild type and an isogenic hydrogenase-negative mutant of Azorhizobium caulinodans growing ex planta on N2 as the N source were studied in succinate-limited steady-state chemostat cultures under 0.2 to 3.0% dissolved O2 tension. Production or consumption of O2, H2, and CO2 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 O2 tension: the growth yield decreased, and the specific O2 consumption and CO2 production increased. A similar dependency on the dissolved O2 tension was found for the mutant with 2.5% H2 in the influent gas. The H2/N2 ratio (moles of H2 evolved per mole of N2 consumed via nitrogenase) of the mutant, growing with or without 2.5% H2, increased with increasing dissolved O2 tensions. This increase in the H2/N2 ratio was small but significant. The dependencies of the ATP/N2 ratio (moles of ATP consumed per mole of N2 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 O2 tension were estimated. These dependencies were interpreted in terms of the physiological concepts of respiratory protection and autoprotection. |