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

Abstract

ABSTRACT 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 O 2 , , Fe(III), , elemental sulphur and CO 2 ; electron donors include H 2 , , Fe 2+ , H 2 S and CH 4 . 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 ( ) to calculate overall Gibbs free energies (Δ G 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 O 2 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 , or Fe(III) serves as the TEA. For example, the oxidation of CH 4 to CO 2 coupled to the reduction of Fe(III) in magnetite to Fe 2+ releases between 94 and 123 kJ/mol e − , depending on the site. By comparison, the aerobic oxidation of H 2 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 H 2 S 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 Δ G r tend to cluster with differences < 20 kJ/mol e − . However, large differences in Δ G r (up to ∼ 60 kJ/mol e − ) are observed in Fe redox reactions, due largely to considerable variations in Fe 2+ , H + and H 2 concentrations. In fact, at the sites investigated, most variations in Δ G r arise from differences in composition and not in temperature.