Cryogenic silicification of microorganisms in hydrothermal fluids

1,2Mark G. Fox-Powell, 3Alan Channing, 4Daniel Applin, 4Ed Cloutis, 5Louisa J. Preston, 1,2Claire R. Cousins
Earth and Planetary Science Letters 498, 1-8 Link to Article [https://doi.org/10.1016/j.epsl.2018.06.026]
1School of Earth and Environmental Sciences, University of St Andrews, Irvine Building, North Street, St Andrews, Fife, KY16 9AL, UK
2St Andrews Centre for Exoplanet Science, University of St Andrews, UK
3School of Earth and Ocean Sciences, Cardiff University, Cardiff, Wales, CF10 3AT, UK
4Department of Geography, University of Winnipeg, Winnipeg, MB, R3B 2E9, Canada
5Department of Earth and Planetary Science, Birkbeck, University of London, Malet St., Bloomsbury, London, UK
Copyright Elsevier

Silica-rich hydrothermal fluids that experience freezing temperatures precipitate cryogenic opal-A (COA) within ice-bound brine channels. We investigated cryogenic silicification as a novel preservation pathway for chemo- and photo-lithotrophic Bacteria and Archaea. We find that the co-partitioning of microbial cells and silica into brine channels causes microorganisms to become fossilised in COA. Rod- and coccoidal-form Bacteria and Archaea produce numerous cell casts on COA particle surfaces, while Chloroflexus filaments are preserved inside particle interiors. COA particles precipitated from natural Icelandic hot spring fluids possess similar biomorphic casts, including those containing intact microbial cells. Biomolecules and inorganic metabolic products are also captured by COA precipitation, and are detectable with a combination of visible – shortwave infrared reflectance, FTIR, and Raman spectroscopy. We identify cryogenic silicification as a newly described mechanism by which microbial biosignatures can be preserved within silica-rich hydrothermal environments. This work has implications for the interpretation of biosignatures in relic hydrothermal settings, and for life-detection on Mars and Enceladus, where opaline silica indicative of hydrothermal activity has been detected, and freezing surface conditions predominate.

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