Thermodynamics of Formate-Oxidizing Metabolism and Implications for H-2 Production

Abstract

Formate-dependent proton reduction to H-2 (HCOO- + H2O -> HCO3- + H-2) has been reported for hyperthermophilic Thermococcus strains. In this study, a hyperthermophilic archaeon, Thermococcus onnurineus strain NA1, yielded H-2 accumulation to a partial pressure of 1 x 10(5) to 7 x 10(5) Pa until the values of Gibbs free energy change (Delta G) reached near thermodynamic equilibrium (-1 to 3 kJ mol(-1)). The bioenergetic requirement for the metabolism to conserve energy was demonstrated by Delta G values as small as 5 kJ mol(-1), which are less than the biological minimum energy quantum, -20 kJ mol(-1), as calculated by Schink (B. Schink, Microbiol. Mol. Biol. Rev. 61:262-280, 1997). Considering formate as a possible H2 storage material, the H-2 production potential of the strain was assessed. The volumetric H-2 production rate increased linearly with increasing cell density, leading to 2,820 mmol liter(-1) h(-1) at an optical density at 600 nm (OD600) of 18.6, and resulted in the high specific H2 production rates of 404 +/- 6 mmol g(-1) h(-1). The H, productivity indicates the great potential of T. onnurineus strain NA1 for practical application in comparison with H-2-producing microbes. Our result demonstrates that T. onnurineus strain NA1 has a highly efficient metabolic system to thrive on formate in hydrothermal systems.