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

Abstract

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.