Astrobiology

On Mars, the amount of ultraviolet C (UVC) radiation that reaches the surface is sufficiently deleterious for life as we know it. However, it has been predicted that some ancient lakes on Mars had high concentrations of Fe³⁺, an ionic species known for a high absorption of UVC radiation. Some models of UV attenuation have already been established; however, there is a lack of reliable simulations that make the connection between this radiation absorption in an aqueous medium and its impact on the viability of microorganisms. This work proposes a simple model to estimate the viability of microorganisms irradiated in solution with different concentrations of Fe³⁺ and constrains the lethal UVC dose in these conditions. In experimental assays, the median lethal dose of Saccharomyces boulardii increased consistently with the model’s predictions, which thereby demonstrated the model’s predictive validity. This ability was then used in a case study to simulate the viability of life in a Fe³⁺-containing lake on ancient Mars. Although the actual conditions of this kind of environment are not known, the simulations showed that lakes with small water columns that contain Fe³⁺ should have been able to protect growing microorganisms. This model enhances the ability to assess potentially habitable conditions on ancient Mars. Key Words: Photoprotection—UVC radiation—Fe³⁺ ions—Mars—Astrobiology. Astrobiology 26, 319–330.

Lipids constitute resistant biomarkers that have been proposed as pacemakers in the search for possible microbial life-forms in Mars’ past. The extreme conditions on the surface of Mars—such as high levels of radiation and the presence of brines and/or perchlorates—must be taken into account when considering the preservation of hypothetical biomarkers. On Earth, alkaline lakes that are rich in microbialites and that have a high saturation index of carbonates have been proposed as analogs of the martian Noachian era. Here, we describe for the first time the lipid biomarker profile of stromatolites from the Alchichica soda lake in Mexico (n-alkanes, alkenes, fatty acids, and n-alkanols). Additionally, DNA sequencing was conducted to taxonomically characterize the Alchichica microbialites, while mineralogical analyses showed typical morphologies associated with aragonite. Microbialite samples were exposed to high doses of gamma (0.87 and 10.52 MGy) and UV (250 W/m² for 140 h) radiation. Another experiment used salt solutions {brine conditions (NaCl + Na₂SO₄) and perchlorates [Mg(ClO₄)₂]} to explore the resilience of biomarkers to simulated martian primitive and present-day conditions. Our results suggest that different types and doses of radiation significantly degrade indigenous lipid biomarkers in microbialites (94% for 0.87 MGy, 99.5% for 10.52 MGy, and 93.5% for UV), with n-alkanes and fatty acids the best-preserved compounds even at higher radiation doses. While the presence of salts such as chloride and sulfate brines did not modify the preservation of fatty acids and alcohols, the presence of perchlorates alone favored their degradation. Our findings support the idea that remnants of past or present life on Mars may be better preserved in ancient chloride or sulfate saline environments.

We analyzed the lipidome of the endolithic community in halite nodules from Salar Grande in the hyperarid Atacama Desert, which is considered an analog for martian subsurface environments. We tested the utility of complex functionalized lipids to trace possible extraterrestrial biosignatures under poly-extreme conditions (high osmolarity, desiccation, and irradiance) and evaluated their chemotaxonomic potential for space applications. We identified a total of 269 compounds in the Mars analog community. The complex functionalized lipid pool includes 150 bacterial bilayer-forming intact ester and ether lipids, 15 membrane-regulating bacteriohopanepolyols, and 9 heterocyte glycolipids, as well as 34 archaeal diether membrane lipids and 8 eukaryotic sterols. Thirteen pigments and quinones were also detected. The complex functionalized lipid pool carries the diagnostic fingerprints of cyanobacteria (including N₂-fixing cyanobacteria), Salinibacter, Planctomycetes, and archaeal Halobacteria; each showed core lipid and headgroup adaptation to the multistress environment. We also compared the complex functionalized lipid fingerprints with the lipid fingerprint obtained following base hydrolysis. Our results show that lipid assemblages differ at the compound class level (e.g., absence of hopanoids) as well as core lipid structure level (e.g., degree of unsaturation). Much of the diagnostic information contained in the complex functionalized lipid pool is lost when analyzing gas chromatography–amenable biomarkers only. These results should guide future analysis of Mars return samples as well as ongoing technological developments for in situ detection of mineral-associated lipids.

Chiral molecules are ubiquitous and necessary for life as we know it. In extant life, the amino acid and sugar constituents of proteins and biopolymers consist of only one enantiomer. This “homochirality” is plausibly generic to any biochemical life and therefore may represent a pure, agnostic biosignature. The ultraviolet (UV) circular polarization spectrum offers a practical method for detecting chirality for generic biomass and hence can be a powerful tool for detecting such biosignatures in an extraterrestrial setting. Here, we demonstrate distinctive UV chiral signatures from astrobiology mission-relevant chemoautotrophic microbes when measured in transmission spectroscopy. Polarization and spectral absorption features are strongly evident at wavelengths expected for protein secondary structures and other chiral molecules, even though the microbial samples are not expected to have survived the study environment. When measured with reflection spectropolarimetry, which would be required for a remote sensing observation, we obtained only null results. Following 10 keV electron irradiation in the Minos chamber at the Jet Propulsion Laboratory Ocean Worlds Laboratory, which is equivalent to exposure of approximately 1 month on the Europan surface in a region where radiation is most intense, chiral signatures persisted to a degree dependent on sample thickness and composition. A control measurement of a Murchison meteorite sample was also undertaken.

Enceladus and Europa are compelling targets for astrobiology investigations due to their potentially habitable subsurface oceans connected to the icy surface by geological processes. Both moons emit ice grains either ejected via micrometeoroid surface impacts or erupted from their interiors through plume activity. These grains can be sampled by spacecraft flybys, and their composition can be analyzed by impact ionization mass spectrometers, such as the SUrface Dust Analyzer (SUDA) onboard Europa Clipper, or similar instruments proposed for future Enceladus missions. These instruments can identify potential biomolecules, such as amino acids, down to nanomolar concentrations, as demonstrated through previous laboratory experiments. However, the identical masses of isomeric compounds could hinder the mass spectrometric identification and assignment of molecular biosignatures. Here, we investigate the general capability of impact ionization mass spectrometry to distinguish between isomeric compounds, validated with a test case of eight amino acid isomers with an identical molecular mass of 131.173 u and formula C₆H₁₃NO₂, using quantum chemical calculations. We show that the amino acid isomers (including diastereoisomers) can be uniquely identified due to their distinct mass spectral features and fragmentation patterns, explained through intramolecular hydrogen bonding and other structural specificities of the individual isomers. Importantly, α-amino acids can be clearly differentiated from non-α-amino acids owing to several major mass spectral features. We show that SUDA-type instruments have sufficient capabilities to differentiate certain isomers and to identify biosignatures from ocean worlds with high confidence.

Extreme environments on Earth are often studied as analog environments on other planetary bodies, since other planetary bodies in our solar system have extreme conditions for life as we know it. Extremophiles are commonly studied in astrobiology given that these microorganisms can survive in extreme conditions (e.g., pressure, temperature, pH). Omics aims to characterize and quantify biological molecules that regulate the structure, function, and dynamics of organisms; hence these methods can improve our understanding of their adaption strategies. The properties of the membranes of extremophiles, for example, which are amphiphilic molecules like lipids and fatty acids, play a key role in their adaptation to extreme conditions. Lipidomics of contemporary extremophiles offer a way to study the composition of their lipids exposed to a variety of stress conditions. Lipids are geostable biomolecules that can retain information about their biological origin for more than a billion years. Therefore, the ability of these molecular fossils to become preserved in extreme environments and assist in the reconstruction of early life on Earth indicate that they are likely to survive if preserved in extreme environments elsewhere. This review article highlights the importance of lipidomics in astrobiology and connects contemporary extremophilic lipids with the lipid fossils to outline approaches to detect extraterrestrial microbial life.