Amend, J. P.

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 (deltaGr) 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 deltaGr 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 deltaGr 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.

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, NO-3,Fe(III),elemental sulphur and CO2; electron donors include H2,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 ( ) to calculate overall Gibbs free energies (DGr) 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, 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 H2 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 DGr tend to cluster with differences < 20 kJ/mol e–. However, large differences in DGr up to ~ 60 kJ/mol e–) are observed in Fe redox reactions, due largely to considerable variations in Fe 2+, H + and H2 concentrations. In fact, at the sites investigated, most variations in DGr arise from differences in composition and not in temperature.

Hydrothermal fluids and sediments from subaerial and shallow submarine sites at Vulcano Island, Italy were investigated for relations between the thermophilic microbial communities, as analysed by fluorescence in situ hybridization, and their geochemical environment, as assessed by photometry, chromatography, and in situ microsensor measurements. Mixing between hydrothermal fluids and seawater in the sediment pore space was reflected in the chemical composition of the emitted fluids, in depth profiles of pore water oxygen and sulfide concentrations, and in the structure of the benthic microbial community. Organic compounds did not accumulate in the vent fluids (b10 AM fatty acids) or in the sediments (b0.1% Corg), suggesting that efficient utilization supported microbial populations on the order of 104 cells per ml fluid and 108 cells per cm3 sediment. Groups of thermophiles that typically gain metabolic energy from the fermentation of organic matter (Thermococcales, Thermotoga/Thermosipho spp., and Bacillus sp.)were detected in significant abundances at all study sites. Also abundant were thermophiles capable of oxidizing organic acids with oxygen, nitrate, or sulfate. Aerobic thermophiles (Aquificales and Thermus sp.) were more abundant at oxic sites than at anoxic sites. Increasingly oxygenated habitats were associated with decreasing abundance of anaerobic (hyper)thermophiles belonging to the order Archaeoglobales.