Evaporitic Preservation of Modern Carotenoid Biomarkers and Halophilic Microorganisms in Mars Analog Hypersaline Environments

Our investigation in Mars-relevant terrestrial environments where biological material is entombed within rapidly precipitated evaporite crystals has given us the ability to evaluate the preservation potential of a hypersaline brine system in advance of interrogating similar environments on Mars.

These evaporite minerals, halite (NaCl) and gypsum (CaSO₄), have been found to host authigenic fluid inclusions over geologic time, with cellular life and carotenoid pigments that are understudied in the planetary context.

Great Salt Lake provides an excellent site to test the ability to detect organic matter in Mars-relevant evaporite crystals. DNA was extracted to determine which microbial clades were present and assess the attenuation of DNA preservation from the host fluid of the lake to the mineral.

Raman spectroscopy was used to investigate the presence of pigments that have longer preservation potential than DNA. Compared with the water column, evaporite minerals preserve higher volumes of DNA and associated biochemistry, whereas entombed fluid inclusions preserve even higher magnitudes of both biomarkers.

This indicates organic addition and continued preservation as the crystals precipitate from the fluid, which was later confirmed as micrometer-scale environments continued to maintain the ecology within closed-system fluid inclusions. Raman analyses of halite revealed the presence of β-carotene and bacterioruberin, consistent with the presence of carotenoid-generating bacteria and archaea in this hypersaline environment, which are characterized by pink coloration.

The continued preservation of these chemical biomarkers over time has led to the formation of physical biosignatures within the evaporite record. Given that these same minerals are present in ancient fluvial sites across Mars, halite and gypsum are ideal candidates for future in situ observation and should be considered high priority for sample return missions.

IMAGE

In situ evaporite minerals from the Great Salt Lake. (A) Halite terrace roughly 75 m near the shoreline of the north arm of Great Salt Lake at Rozel Bay. Excavated sections of the terrace revealed pink coloration due to carotenoid pigments, either entombed or associated with halophilic microorganisms. Height of terrace is ∼0.3 m. (B) Ripple-fanlike megastructure approximately 600 m from the start of the halite terrace formation containing vertically precipitated gypsum evaporite crystals. These crystals contain entombed intercrystalline clays making up <3–10% of the mineral matrix and intracrystalline insoluble sediment in smaller abundances. The tops of the protruding CaSO₄ crystals are ∼70–180 mm in height (C, D). Modern subsurface sections of Searles Lake that contain hypersaline brine pools in close proximity to precipitating evaporite mineral assemblages containing hanksite [Na₂₂K(SO₄)₉(CO₃)₂Cl], halite (NaCl), gypsum [CaSO₄, and can be precipitated as selenite (CaSO₄·2H₂O) alongside gypsum], mirabilite (Na₂2SO₄·10H²O), and major features of measured trona (Na₂CO₃·NaHCO₃·2H₂O). Astrobiology via Sage Journals

Astrobiology