METABOLIC ENERGY FROM SULFUR REDOX FOR HYPERTHERMOPHILES IN VULCANO HOT SPRINGS
H2 + S° ® H2S, (1)
is prevalent among the deepest branching Archaea and Bacteria and believed to be the sole energy-yielding process in numerous autotrophic hyperthermophiles. In addition, the aerobic oxidation of S° and H2S as well as the reduction with organic and inorganic electron donors of SO42- and S° are metabolic processes known to be carried out by hyperthermophiles. Here, chemical analyses of hydrothermal fluids and sediments of several Vulcano hot springs are combined with thermodynamic calculations at in situ temperatures to determine the overall Gibbs free energies of sulfur redox reactions. As an example, it can be shown that reaction (1), though exergonic, yields only a moderate amount of energy (10-20 kJ/mol e- transferred) in the hydrothermal system at Vulcano. The disequilibrium between hydrothermal solutions and the minerals sulfur, magnetite, and pyrite in this system can provide as much or more energy to hyperthermophiles than sulfidogenesis.