Temporal Changes in Fluid Chemistry and Energy Profiles in the Vulcano Island Hydrothermal System

In June 2003, the geochemical composition of geothermal fluids was determined at 9 sites in the Vulcano hydrothermal system, including sediment seeps, geothermal wells, and submarine vents. Compositional data were combined with standard state reaction properties to determine the overall Gibbs free energy (ΔG_r ) for 120 potential lithotrophic and heterotrophic reactions. Lithotrophic reactions in the H-O-N-S-C-Fe system were considered, and exergonic reactions yielded up to 120 kJ per mole of electrons transferred. The potential for heterotrophy was characterized by energy yields from the complete oxidation of 6 carboxylic acids— formic, acetic, propanoic, lactic, pyruvic, and succinic—with the following redox pairs: O₂/H₂O, SO₄ ²⁻/H₂S, NO₃ ⁻/NH₄ ⁺, S⁰/H₂S, and Fe₃O₄/Fe²⁺. Heterotrophic reactions yielded 6–111 kJ/mol e⁻. Energy yields from both lithotrophic and heterotrophic reactions were highly dependent on the terminal electron acceptor (TEA); reactions with O₂ yielded the most energy, followed by those with NO₃ ⁻, Fe(III), SO₄ ²⁻, and S⁰. When only reactions with complete TEA reduction were included, the exergonic lithotrophic reactions followed a similar electron tower. Spatial variability in ΔG_r was significant for iron redox reactions, owing largely to the wide range in Fe²⁺ and H⁺ concentrations. Energy yields were compared to those obtained for samples collected in June 2001. The temporal variations in geochemical composition and energy yields observed in the Vulcano hydrothermal system between 2001 and 2003 were moderate. The largest differences in ΔG_r over the 2 years were from iron redox reactions, due to temporal changes in the Fe²⁺ and H⁺ concentrations. The observed variations in fluid composition across the Vulcano hydrothermal system have the potential to influence not only microbial diversity but also the metabolic strategies of the resident microbial communities.