Earth allows life, and therefore biochemistry. Imagining biochemistry as an outgrowth of geochemistry may yield novel perspectives on how life adapts to geologic change. But it is inconvenient to conduct studies over geologic timescales. Substituting space for time is a common technique used in the geological sciences, and can be applied to geobiochemistry as well. Environments that exhibit steep thermal, compressional, and compositional gradients offer opportunities for evaluating changes in the energetic costs of making biomolecules from small metabolites to large biopolymers. Extrapolation of standard state thermodynamic properties for all constituents in the citric acid cycle (CAC) from the Earth’s surface, through the crust, and into subduction zones allows evaluation of chemical affinities for the overall cycle throughout the deep biosphere. Assuming constant metabolite concentrations permits examination of how geochemical constraints on dissolved hydrogen, inorganic carbon, and phosphorous affect the pressures and temperatures where the forward or reverse CAC can provide energy, and reveals that the reverse CAC prevails at increasing depth. Taking advantage of gradients in hydrothermal ecosystems permits evaluation of energy costs and power demands of making lipids and proteins against geochemical backdrops. Constraints on the abundances of inorganic forms of S, P, O, N, C, and H in hot spring fluids from Yellowstone, together with independent estimates of lipid standard state thermodynamic properties, reveals that the changes in lipid compositions in hydrothermal microbiomes track local minimizations in the Gibbs function. At submarine hydrothermal systems, chemical energy emerges from disequilibria induced by fluid mixing and permits exergonic organic synthesis. Perhaps surprisingly, synthesis of proteins of thermophilic microbes from inorganic forms of C, H, N, O and S in the fluid mixture releases energy. These examples illustrate quantitative results of treating biochemistry as the intersection of evolution with geochemical processes.<br/>

Geobiochemistry and planetary habitability

2019 Spring National Meeting

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