Thermodynamic Constraints on the Citric Acid Cycle and Related Reactions 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 the thermodynamic viability and energetics of various
reactions of interest to metabolism 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 a plausible precursor
prebiotic network of 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 changes for given reactions, as a function
of pressure and temperature. Using instantaneous concentrations of
inorganics and organics from terrestrial microbial experiments and
those derived from the Cassini mission for Enceladus, we calculate
chemical affinities of reactions in the network. We carry out similar
calculations using the SUPCRT model for lower pressures and temperatures.
Together, the two models span temperatures between 0 and 1200 °C
and pressures between 1 bar and 60 kbar. We found that across the
majority of oceanic pressuretemperature profiles, certain
TCA cycle species, such as citrate and succinate, accumulate, while
others, including fumarate 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 (e.g., metabolites)
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
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. Although based on largely uncertain concentrations of chemical
species in ocean worlds, our nonequilibrium thermodynamic predictions
are rather insensitive to changes in the activities, and may aid in
the interpretation of data gathered by future missions, as compositional
data will become available. Specifically, spacecraft measurements
of TCA cycle species in aqueous environments that align with or deviate
strongly from our estimations would have a critical impact on the
search for life in ocean worlds.