Probing the impact of microbial CH4 in deep-sea hydrothermal fluids: isotopic bio-signatures at the warm limits of life – DEEPSIGN

Methane (CH4), the simplest organic carbon molecule, is key to carbon cycling in deep-sea hydrothermal systems. Methane-producing micro-organisms (Archaea) are found widespread in hydrothermal settings, from cold basaltic substrates to warm hydrothermal chimneys. However, distinguishing their contribution from abiotic synthesis, both driven by H2 from serpentinization, to the CH4 budget remains challenging. While high temperatures fluids (>300°C) likely inhibit microbial activity, colder ‘diffuse’ fluids offer a thermodynamic potential for microbial methanogenesis. As diffuse fluids could account for 50–90% of hydrothermal output, understanding such subsurface biosphere's contribution to the CH4 budget is crucial for accurately assessing carbon fluxes into the deep ocean.
DEEPSIGN aims to clarify the deep biosphere's influence on CH4 hydrothermal inputs into the oceans using carbon and hydrogen isotope systematics, including methane clumped isotopologues ?13CH3D and ?12CH2D2. It focuses on (1) a thorough evaluation of CH4 isotope compositions and carbon precursors across a temperature gradient from high-temperature endmember to low-temperature diffuse fluids, and (2) experimental constraints to validate isotope fractionation factors linked to microbial methanogenesis under hydrothermal conditions (P > 300 bars, T > 90°C). This interdisciplinary project combines geochemistry, microbiology, and thermodynamics, tackling challenges through deep-sea sampling, hyperbaric cultivation, and advanced isotope analysis. DEEPSIGN will provide unprecedented methane biosignatures at the warm limits of life, shedding light on the transition between abiotic and biotic processes at the lithosphere/ocean interface. These insights will also have implications for astrobiology, as methane plays is a central target for the search for extraterrestrial life