Cryogenic silicification of microorganisms in hydrothermal fluids

Abstract

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 copartitioning 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 2 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.

Partitioning of Crystalline and Amorphous Phases During Freezing of Simulated Enceladus Ocean Fluids

“…After complete sublimation of ice this vein structure remains intact (Figures 1f and 1g). Ice‐free broken up cryogenic particles bear morphological features similar to those previously identified in terrestrial ice‐templated cryogenic opal‐A (COA), such as triple‐junctions, branched threads and sheets (Channing & Butler, 2007; Fox‐Powell et al., 2018; Figure ).…”

“…The resulting crystallization textures, expressed here at the 10 µm scale, are similar to eutectic solidification textures formed in igneous and metallic systems (Akamatsu & Plapp, 2016), thus we conclude they record the final solidification of the system at the eutectic. We also note that in pH 9 fluids, small filiform carbonates can morphologically resemble common terrestrial microorganisms (Figure 3d), with cautionary implications for interpreting potential microfossils in cryogenic particles (Fox-Powell et al, 2018).…”

Terrestrial Hot Spring Systems: Introduction

“…Hydrothermal activity in cold surface conditions can enhance fossil preservation (Fox-Powell, 2018). “Cryogenic opal-A” (COA) precipitates within ice-bound brine channels in Icelandic hot springs.…”

“…The surfaces of COA particles were populated with bacterial and archaeal rods and coccoids, and particle interiors retained Chloroflexus filaments. Fox-Powell et al (2018) also detected biomolecules and inorganic microbial metabolic products in COA. The authors concluded that fossil preservation in COA is highly relevant to the exploration of extraterrestrial environments where hydrothermal activity has occurred under freezing conditions, for example on Mars and Enceladus.…”

Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples

“…Geyserites and silica sinters could thus experience variable temperature of formation that could influence their NIR signatures. Also, some opals can form under cryogenic conditions in contact with ice grains and/or snow, like in Iceland or within the Yellowstone Park, during the winter (Channing and Butler, 2007;Jones and Renaut, 2010;Fox-powell et al, 2018). We hypothesize that some geyserites and silica sinters may exhibit CRC values consistent with continental weathering or low-temperature hydrothermal processes (below 50°C) even if formed in hydrothermal setting.…”