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 plants 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 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 O-2 tension: the growth yield decreased, and the specific O-2 consumption and CO2 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-,O- tension were estimated. These dependencies were interpreted in terms of the physiological concepts of respiratory protection and autoprotection.