Energetics of chemolithoautotrophy in the hydrothermal system of Vulcano Island, southern Italy

The hydrothermal system at Vulcano, Aeolian Islands (Italy), is home to a wide variety of thermophilic, chemolithoautotrophic archaea and bacteria. As observed in laboratory growth studies, these organisms may use an array of terminal electron acceptors (TEAs), including O₂, (Figure presented.), Fe(III), (Figure presented.), elemental sulphur and CO₂; electron donors include H₂, (Figure presented.), Fe²⁺, H₂S and CH₄. Concentrations of inorganic aqueous species and gases were measured in 10 hydrothermal fluids from seeps, wells and vents on Vulcano. These data were combined with standard Gibbs free energies ((Figure presented.)) to calculate overall Gibbs free energies (ΔG_r) of 90 redox reactions that involve 16 inorganic N-, S-, C-, Fe-, H- and O-bearing compounds. It is shown that oxidation reactions with O₂ as the TEA release significantly more energy (normalized per electron transferred) than most anaerobic oxidation reactions, but the energy yield is comparable or even higher for several reactions in which (Figure presented.), (Figure presented.) or Fe(III) serves as the TEA. For example, the oxidation of CH₄ to CO₂ coupled to the reduction of Fe(III) in magnetite to Fe²⁺ releases between 94 and 123 kJ/mol e⁻, depending on the site. By comparison, the aerobic oxidation of H₂ or reduced inorganic N-, S-, C- and Fe-bearing compounds generally yields between 70 and 100 kJ/mol e⁻. It is further shown that the energy yield from the reduction of elemental sulphur to H₂S is relatively low (8–19 kJ/mol e⁻) despite being a very common metabolism among thermophiles. In addition, for many of the 90 reactions evaluated at each of the 10 sites, values of ΔG_r tend to cluster with differences < 20 kJ/mol e⁻. However, large differences in ΔG_r (up to 60 kJ/mol e⁻) are observed in Fe redox reactions, due largely to considerable variations in Fe²⁺, H⁺ and H₂ concentrations. In fact, at the sites investigated, most variations in ΔG_r arise from differences in composition and not in temperature.