Thermodynamic Constraints On The Citric Acid Cycle And Related Reactions In Ocean World Interiors

Icy ocean worlds attract significant interest for their astrobiological potential due to subsurface oceans, organics, and chemical energy sources.

We quantify thermodynamic viability of metabolism-relevant reactions at pressure-temperature conditions of Enceladus, Europa, Titan, Ganymede, and the Lost City Hydrothermal Field for comparison.

We examine the tricarboxylic acid (TCA) cycle and a plausible prebiotic reaction network leading to it, using DEWPython (based on the Deep Earth Water model) and SUPCRT to compute equilibrium constants and Gibbs free energy changes across temperatures from 0-1200°C and pressures between 1 bar-60 kbar.

Pipeline diagram for all the calculations and analysis throughout the paper. Colors denote various object classes, indicated in the legend. Relevant figures, tables, and method sections are also referenced. –astro-ph.EP

We found that across most oceanic P-T profiles, certain TCA cycle species accumulate (citrate, succinate) while others diminish (fumarate, oxaloacetate), suggesting ocean worlds’ conditions may not thermodynamically favor a unidirectional TCA cycle, requiring additional energy to overcome bottlenecks.

Similar bottlenecks exist at Lost City, which is inhabited. In the prebiotic network, pyruvate and acetate show remarkable stability, feeding the TCA cycle through citrate production, bypassing the oxaloacetate bottleneck. Formation of most TCA cycle species from inorganic compounds (CO₂ + H₂) is highly favored throughout ocean world geotherms, except for oxaloacetate.

While based on uncertain chemical concentrations, our non-equilibrium thermodynamic predictions are relatively insensitive to activity changes and may aid interpretation of future mission data. [ABRIDGED]

Affinity (left panels) and Gibbs free energy change (right panels) of the reactions involved in the prebiotic network. The curves show the PT curves of the studied moons and the depths near the Lost City Hydrothermal Field. The dashed curve indicates the critical affinity (A=0; left) or the transition between forced-endergonic and spontaneous-exergonic reaction states in equilibrium (right). — astro-ph.EP

Seda Işık, Mohit Melwani Daswani, Emre Işık, Jessica Weber, Nazlı Olgun Kıyak

Comments: 36 pages, 13 figures, accepted for publication in ACS Earth and Space Chemistry
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2504.19369 [astro-ph.EP] (or arXiv:2504.19369v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2504.19369
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Submission history
From: Emre Işık
[v1] Sun, 27 Apr 2025 22:17:11 UTC (3,289 KB)
https://arxiv.org/abs/2504.19369
Astrobiology, Biochemistry,