Thermodynamics of formate-oxidizing metabolism and implications for H2 production

Abstract

Formate-dependent proton reduction to H2 (HCOO-+H2O→HCO3-+H2) has been reported for hyperthermophilic Thermococcus strains. In this study, a hyperthermophilic archaeon, Thermococcus onnurineus strain NA1, yielded H2 accumulation to a partial pressure of 1×105 to 7×105 Pa until the values of Gibbs free energy change (ΔG) reached near thermodynamic equilibrium (-1 to-3 kJ mol-1). The bioenergetic requirement for the metabolism to conserve energy was demonstrated by ΔG 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 H2 production potential of the strain was assessed. The volumetric H2 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 H2 productivity indicates the great potential of T. onnurineus strain NA1 for practical application in comparison with H2-producing microbes. Our result demonstrates that T. onnurineus strain NA1 has a highly efficient metabolic system to thrive on formate in hydrothermal systems. © 2012, American Society for Microbiology.