On the energetics of chemolithotrophy in nonequilibrium systems: case studies of geothermal springs in Yellowstone National Park

Chemolithotrophic micro-organisms are important primary producers in high-temperature geothermal environments and may catalyse a number of different energetically favourable redox reactions as a primary energy source. Analysis of geochemical constituents followed by chemical speciation and subsequent calculation of reaction free energies (Delta G(rxn)) is a useful tool for evaluating the thermodynamic favourability and potential energy available for microbial metabolism. The primary goal of this study was to examine relationships among geochemical gradients and microbial population distribution, and to evaluate the utility of energetic approaches for predicting microbial metabolism from free-energy calculations, utilizing as examples, several geothermal habitats in Yellowstone National Park where thorough geochemical and phylogenetic analyses have been performed. Acidic ( pH similar to 3) and near-neutral ( pH similar to 6-7) geothermal springs were chosen for their range in geochemical properties. Aqueous and solid phase samples obtained from the source pools and the outflow channels of each spring were characterized for all major chemical constituents using laboratory and field methods to accurately measure the concentrations of predominant oxidized and reduced species. Reaction free energies (Delta G(rxn)) for 33 oxidation-reduction reactions potentially important to chemolithotrophic micro-organisms were calculated at relevant spring temperatures after calculating ion activities using an aqueous equilibrium model. Free-energy values exhibit significant variation among sites for reactions with pH dependence. For example, free-energy values for reactions involving Fe(3+) are especially variable across sites due in large part to the pH dependence of Fe(3+) activity, and exhibit changes of up to 40 kJ mol(-1) electron from acidic to near neutral geothermal springs. Many of the detected 16S rRNA gene sequences represent organisms whose metabolisms are consistent with exergonic processes. However, sensitivity analyses demonstrated that reaction free energies do not generally represent the steep gradients in local geochemical conditions resulting from air-water gas exchange and solid phase deposition that are important in defining microbial habitats and 16S rRNA gene sequence distribution within geothermal outflow channels.