HR: 13:40h
AN: B23C-01 INVITED     [Abstracts]
TI: Geochemical Gradients and the Supply Side of Habitability
AU: * Shock, E L
EM: eshock@asu.edu
AF: Arizona State University, GEOPIG Depts. of Geological Sciences and Chemistry & Biochemistry, Tempe, AZ 85287 United States
AB: One way to quantify habitability is to evaluate the ratio of the power supply from an environment to the power demand of the organisms that live there. Conditions at which this ratio is maximized provide ample power for growth and reproduction, and can be considered to be highly habitable. In contrast, conditions at which this ratio approaches one qualify as extreme. Characteristics of extreme environments that meet this definition include a variety of stresses that can be assessed by studying the response of organisms to the factors that determine the power supply. It follows that quantifying habitability in chemosynthetic communities awaits the development of a geochemically-derived analysis of physiology. Meanwhile, it is presently possible to evaluate the supply side of the habitability ratio, which may help to identify those conditions where habitability can ultimately be quantified. Guidance may be provided by the hypothesis that there is likely to be a close positive correlation between the presence of strong gradients in an environment and the magnitude and variance of habitability. Microbial communities in ecosystems supported by hot springs undergo dramatic population changes as boiling water flows, cools, spreads out, becomes capable of supporting photosynthesis, and ultimately hosts grazers of photosynthetic mats and their predators. In the high temperature portions of these systems, where photosynthesis is inhibited, power supplies can be assessed from thermodynamic analysis of compositional data. In this analysis, power supply is evaluated by factoring the energy per mole of electrons transferred in metabolic reactions with the concentrations of the limiting constituents of those reactions and fluid flow rates. Chemical power supplies in hydrothermal ecosystems that are tapped by microbial communities at Yellowstone National Park tend to fall in the range of 0.1 to 100 watts. In general, reactions in which O₂ is the electron acceptor fall at the upper end of this range. Oxidation of CO, CH₄, H₂, H₂S, pyrite, sulfur and magnetite rank as relatively large power supplies regardless of whether the hot springs are acidic or basic. Power from reactions in which nitrite and ammonium are oxidized by O₂ is also largely independent of pH, but the supply tends to be considerably lower. In contrast, the power supply from the oxidation of Fe⁺² is strongly pH dependent, with the lowest values provided by acidic springs. Power supplies tend to diminish through a sequence of other electron acceptors including nitrate, nitrite, sulfur, sulfate, ferric iron and CO₂. Although the present analysis is limited to the analysis of the supply side, gradients in hot spring composition lead to several reactions that are promising candidates for habitability studies in these systems including iron oxyhydroxide reduction and the interconversion of pyrite and sulfur.
DE: 0448 Geomicrobiology
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 0456 Life in extreme environments
DE: 0463 Microbe/mineral interactions
DE: 0471 Oxidation/reduction reactions (4851)
SC: Biogeosciences [B]
MN: Fall Meeting 2005