The stability and energetics of the citric acid cycle and a precursor prebiotic network in ocean world interiors

Icy ocean worlds in our solar system have attracted significant interest for their astrobiological and biogeochemical potential due to the predicted presence of global subsurface liquid water oceans, the presence of organics in Enceladus and Titan, and plausible sources of chemical energy available for life therein. A difficulty in placing quantitative constraints on the occurrence and effectiveness of biogeochemical reactions favorable for life and metabolism in ocean worlds is the paucity of thermodynamic data for the relevant reactions for pressure, temperature and compositional conditions pertaining to ocean worlds, in addition to uncertainties in the estimation of such conditions. Here, we quantify, for the first time, the viability and energetics of various reactions related to metabolism under abiotic conditions at pressures and temperatures relevant to ocean worlds Enceladus, Europa, Titan and Ganymede, and conditions relevant to the Lost City Hydrothermal Field for comparison. Specifically, we examine the tricarboxylic acid cycle (also known as TCA, Krebs cycle, or citric acid cycle) and plausible prebiotic precursor reactions leading to the TCA cycle. We use DEWPython, a program based on the Deep Earth Water (DEW) model (which is a high pressure and high temperature extension of the Helgeson-Kirkham-Flowers equation of state used to calculate thermodynamic properties of ions and complexes in aqueous solutions), to compute the equilibrium constants and the Gibbs free energy change for given reactions, as a function of pressure and temperature. We carry out similar calculations using the SUPCRT model for lower pressures and temperatures. Together, the two models span temperatures between 0 to 1200 C and pressures between 1 bar--60 kbar. We found that while some molecules involved in this cycle critical for aerobic organisms and prebiotic network remain stable across the wide temperature-pressure ranges of ocean worlds, others degrade and their availability changes at different depths. In the sequential reactions of the citric acid cycle, we have observed a periodically oscillating pattern. We found that across the majority of oceanic pressure-temperature profiles, certain TCA cycle species, such as citrate and isocitrate, accumulate, while others, including cis-aconitate and oxaloacetate, exhibit a diminishing trend. This observation suggests that the internal conditions of ocean worlds may not thermodynamically favor a unidirectional TCA cycle, thereby implying an additional source of energy, metabolites or catalysts to overcome energy bottlenecks. Notably, we find similar bottlenecks at the Lost City Hydrothermal Field, which is undoubtedly inhabited by organisms. In the prebiotic network, we found that pyruvate and its bound acetate exhibit remarkable stability and accumulate in substantial quantities, thereby feeding the TCA cycle through the production of citrate. In this case the oxaloacetate bottleneck within the TCA cycle is bypassed via the prebiotic pathway. We also found that the formation of all TCA cycle species from inorganic compounds (CO2 + H2) is highly favored throughout the geotherms of ocean worlds. Our thermodynamic equilibrium predictions may aid in the interpretation of data gathered by future missions. Specifically, spacecraft measurements of TCA cycle species in aqueous environments that deviate strongly from our predictions would imply non-equilibrium processes, and perhaps, life.