Carbon dioxide fixation by a hydrogen-oxidizing bacterium: Biomass yield, reversal respiratory quotient, stoichiometric equations and bioenergetics

A strain of Cupriavidus necator was grown on a gas mixture of hydrogen, oxygen and carbon dioxide to measure the cell mass formation and uptake rates of individual gases. The cell mass yield on consumed hydrogen and reversal respiratory quotient (RRQ) changed with cell density, indicating dynamic growth of the autotrophic microbes. A general stoichiometric equation was formulated by combining three half reactions of electron donor, electron accepter and cell mass synthesis. The only unknown parameter was the mole fraction of electrons (free energy) used in cell mass synthesis and determined with the measured cell mass yield or RRQ. The stoichiometric coefficients depended on the time duration of the measurements because of maintenance requirements. The energy efficiency of biomass formation from CO2 declined from 25% to 14% with increased cell density. The efficiency decline was not caused by wasteful oxygenase activity of Rubisco, but other mechanisms that decoupled the proton pump and ATP synthesis. Under the conditions of fast autotrophic growth with a low RRQ, the coupling was quite efficient, generating ca. 3 mol ATP per mole H-2 oxidation (P/O approximate to 3), and the energy cost of CO2 reduction to carbohydrate via the CBB cycle was ca. 263 kJ (C-mol CO2)(-1).