More than two dozen genera and approximately 70 species of hyperthermophiles (optimum growth temperature ³80°C) have been isolated from terrestrial and marine hydrothermal ecosystems (Amend & Shock, 2001, FEMS Microbiol. Rev., v. 25, 175-243; Huber & Stetter, 2001, Methods Enzymol., v. 330, 11-24). The shallow vents and heated sediments of the Baia di Levante on Vulcano Island (Italy) have yielded perhaps the greatest diversity of cultured hyperthermophiles, including members of the Bacteria Aquifex and Thermotoga and the Archaea Archaeoglobus, Ferroglobus, Palaeococcus, Pyrococcus, Pyrodictium, Staphylothermus, Thermococcus, and Thermodiscus. The vast majority of hyperthermophiles, excluding the methanogens, takes advantage of electron transfer among sulfur-bearing compounds. Sulfidogenesis owing to the reduction of elemental sulfur (S°) to sulfide, represented by

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 SO₄^2- 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.