Presentation Information
[U05-P06]Thermodynamic Habitability and the abiotic baseline in the Deep Continental Subsurface
*Zohra Zahir1, Alero Gure1,2, Oliver Warr1,3, Barbara Sherwood Lollar1,4 (1.Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada, 2.Geosyntec Consultants Inc., Burnaby, British Columbia, Canada , 3.Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada, 4.Institut de Physique du Globe de Paris (IPGP), Universite Paris Cite, Paris, France)
Keywords:
chemolithoautotrophy,methanogenesis,habitability,deep biosphere,subsurface
As the search for life expands beyond Earth, efforts to define and redefine habitability leading to novel biogeochemical models which challenge long-held assumptions and highlight the uncertain boundary between what is energetically possible in the theoretical sense and what is biologically realized. In that regard, the deep continental subsurface of our planet, in places that are isolated from sunlight and characterized by low biomass and extreme geochemical conditions provides a natural laboratory for investigating the limits of life and habitability under chemosynthetic conditions which may be found elsewhere in our solar system. Within this lens this study investigated the thermodynamic habitability of supersaline fracture fluids from the Kidd Creek Observatory in Ontario, Canada that have been isolated for hundreds of millions to over a billion years. Thermodynamic calculations for multiple chemolithoautotrophic metabolisms indicate that these fluids are energetically capable of sustaining microbial life, with sulfate reduction providing the highest energy yields and theoretical power supply sufficient to support ~102–103 cells/L. However, despite favorable energetics and abundant electron donors, isotopic, geochemical, and microbiological evidence indicates extremely low biological activity allowing for the preservation of an abiotic baseline. These results highlight and allow estimation of the fundamental gap that may exist between energetic habitability and realized biology. Specifically, in low-energy, transport-limited systems, thermodynamic potential instead should be viewed as what could be theoretically sustained rather than what is actively maintained. The Kidd Creek system in this study therefore demonstrates that habitability does not necessarily imply extant life—an insight with important implications for interpreting subsurface ecosystems on Earth and for assessing habitability beyond our planet.