Preservation of Cell Surface Biosignatures of Halophilic Microorganisms Exposed to Space Radiation on Exocube – ExocubeHalo

If life exists or has existed elsewhere in the solar system it was likely as microorganisms. Exobiology studies to date have therefore focussed primarily on the habitable range of conditions tolerated by terrestrial microorganisms using binary measures of survival (live/dead). Now a new era of research seeks to identify the biosignatures of past and present life, both on Earth and on other planetary bodies. This necessitates moving beyond these binary measures to more detailed biochemical models of precisely how microorganisms are able to survive under different conditions, and what 'biosignatures' are left behind by dead cells.
Refining the search for life requires certain conditions to be met: (i) widely recognizable basic biological structures essential to cell function with a high preservation potential, (ii) geological context compatible with protecting viable microorganisms and preserving their biosignatures, and (iii) ubiquitous environmental conditions common to many different planetary bodies including Earth to focus the search on areas of highest potential.
High-salt environments provide the perfect geobiological context for protecting extant life and preserving biosignatures of past life. High salt environments are ubiquitous in the solar system, with evaporite salts such as halite (NaCl) found everywhere from Earth to Mars to meteorites. Halite also provides some protection against solar radiation. Microscopic fluid inclusions within halite are known to host and preserve not only organic molecules but also viable microorganisms (halophilic archaea) over geological timescales. In addition to survival as “living fossils” within the fluid inclusions of halite, the cell envelopes (cell wall and membrane) of halophilic archaea have a particularly high potential for geobiological preservation. However, little is currently understood about the effects of solar radiation on the biochemistry of microbial cell envelopes under high salt conditions, including both the potential protective effects of halite for living halophiles and the potential preserving effects on biosignatures of dead microbial cells.
The objectives of the ExocubeHALO project are to determine the effects of terrestrial and space solar radiation regimes on (i) the molecular mechanisms permitting the long-term survival of halophilic “living fossil” microorganisms encased within the closed geobiological system of halite fluid inclusions, and (ii) the preservation of their cell envelope as a preserved biosignature. ExocubeHALO is based on a modular, step-wise approach to test the stability of haloarchaeal cell surfaces, first during containment within halite via evaporation, and then subsequent exposure to dark, terrestrial light, and finally full-spectrum solar irradiation. The final step is sample exposure outside the International Space Station, integrated with other samples as part of the European Space Agency Exocube (Exposure of organics/organisms cube) mission. Despite its links to space the Exocube space experiment, ExocubeHalo is an independent study focusing on fundamental research questions in regard to biosignature preservation of halophilic microorganisms. The step-wise approach, complemented by a large-scale ground-based analyses (with a greater sample sizes and greater levels of analytical resolution) will permit a full characterization of the effects of radiation on haloarchaeal cell envelopes and their constituents. A secondary benefit of this project will be the development of an adaptable framework for the study of extant life and biosignatures in high-salt conditions that can be extended to future studies that reflect a wide range of conditions, on Earth or other planetary bodies (e.g. Mars).