Quantifying inorganic sources of geochemical energy in hydrothermal ecosystems, Yellowstone National Park, USA

Combining analytical data from hot spring samples with thermodynamic calculations permits a quantitative assessment of the availability and ranking of various potential sources of inorganic chemical energy that may support microbial life in hydrothermal ecosystems. Yellowstone hot springs of diverse geochemical composition, ranging in pH from <2 to >9 were chosen for this study, and dozens of samples were collected during three field seasons. Field measurements of dissolved oxygen, nitrate, nitrite, total ammonia, total sulfide, alkalinity, and ferrous iron were combined with laboratory analyses of sulfate and other major ions from water samples, and carbon dioxide, hydrogen, methane, and carbon monoxide in gas samples to evaluate activity products for ~300 coupled oxidation-reduction reactions. Comparison of activity products and independently calculated equilibrium constants leads to values of the chemical affinity for each of the reactions, which quantifies how far each reaction is from equilibrium. Affinities, in turn, show systematic behavior that is independent of temperature but can be correlated with pH of the hot springs as a proxy for the full spectrum of geochemical variability. Many affinities are slightly to somewhat dependent on pH, while others are dramatically influenced by changes in chemical composition. All reactions involving dissolved oxygen as the electron acceptor are potential energy sources in all hot spring samples collected, but the ranking of dominant electron donors changes from ferrous iron, and sulfur at high pH to carbon monoxide, hydrogen, and magnetite as pH decreases. There is a general trend of decreasing energy yields depending on electron acceptors that follows the sequence: O₂(aq)>NO₃^-≈NO₂^-≈S>pyrite≈SO₄^-2≈CO(g)≈CO₂(g) at high pH, and O₂(aq)≈magnetite>hematite≈goethite>NO₃^-≈NO₂^-≈S≈pyrite≈SO₄^-2 at low pH. Many reactions that are favorable sources of chemical energy at one set of geochemical conditions fail to provide energy at other conditions, and vice versa. This results in energy profiles supplied by geochemical processes that provide fundamentally different foundations for chemotrophic microbial communities as composition changes.