Energy Generation and Utilization in Hydrogen Bacteria

Studies on the relationship between cell synthesis and energy utilization in Hydrogenomonas eutropha have shown that the amount of oxidative energy required for synthetic reactions depends on the conditions of growth. The energy of hydrogen oxidation was most efficiently used when growth conditions were optimal (continuous culture, cells in exponential growth phase) and when the rate of growth was limited by H(2) or O(2) supply. Under these conditions, 2 to 2.5 atoms of oxygen were consumed by the oxyhydrogen reaction for the concomitant conversion of 1 mole of CO(2) to cell matter. This conversion efficiency, expressed as the O/C energyyield value, was observed with continuous cultures. A less efficient conversion was found with batch cultures. With limiting concentrations of CO(2) the rate of hydrogen oxidation was relatively high, and the O/C value was dependent on the growth rate. With nonlimiting concentrations of CO(2), the rate of hydrogen oxidation was strictly proportional to the rate of CO(2) fixation, and the O/C value was independent of growth rate. This proportionality between the rate of H(2) oxidation and the rate of CO(2) fixation suggested that energy supply regulates the (maximum) rate of growth. From the energy-yield measurements, we concluded that the oxidation of 1 mole of H(2) yields the equivalent of 2 moles of adenosine triphosphate for H. eutropha, and that at least 5 moles of this high-energy phosphate is required for the conversion of 1 mole of CO(2) into cellular constituents.