The Geochemical Potential for Metabolic Processes on the Sub-Neptune Exoplanet K2-18b

Quantifying disequilibria is important to understand whether an environment could be habitable. It has been proposed that the exoplanet K2-18b has a hydrogen-rich atmosphere and a water ocean, making it a "hycean world". The James Webb Space Telescope recently made measurements of methane, CO$_2$, and possibly dimethyl sulfide (DMS) in the atmosphere of this planet. The initial interpretation of these data is that they may support the occurrence of hycean conditions. Here, I attempt to take a next step in exploring the prospects for habitability. I use constraints on the abundances of atmospheric gases to calculate how much chemical disequilibrium there could be, assuming K2-18b is a hycean world. I find that the presence of oxidized carbon species coexisting with abundant H$_2$ (1-1000 bar) at cool to warm (25-120{\deg}C) conditions creates a strong thermodynamic drive for methanogenesis. More than ~75 kJ (mol C)$^{-1}$ of free energy can be released from CO$_2$ hydrogenation. Partially oxidized carbon compounds such as DMS (if present) also have potential to provide metabolic energy, albeit in smaller quantities. Because of the thermodynamic instability of CO$_2$ under hycean conditions, other reductive reactions of CO$_2$ are likely to be favored, including the synthesis of amino acids. Glycine and alanine synthesis can be energy-releasing or at least much less costly on K2-18b than in Earth's ocean, even when NH$_3$ is scarce but not totally absent. These first bioenergetic calculations for a proposed ocean-bearing exoplanet lay new groundwork for assessing exoplanetary habitability.