Fingerprinting molecular and isotopic biosignatures on different hydrothermal scenarios of Iceland, an acidic and sulfur-rich Mars analog

An astrobiological perspective on microbial biofilms: their importance for habitability and production of detectable and lasting biosignatures

The search for life elsewhere in the universe has remained one of the main goals of astrobiological exploration. In this quest, extreme environments on Earth have served as analogs to study the potential habitability of Mars and icy moons, which include but are not limited to hydrothermal vent systems, acid lakes, deserts, and polar ice, among others. Within the various forms that life manifests, biofilms constitute one of the most widespread phenotypes and are ubiquitous in extreme environments. Biofilms are structured communities of microorganisms enclosed in a matrix of extracellular polymeric substances (EPS) that protect against unfavorable and dynamic conditions. These concentrated structures and their associated chemistry may serve as unique and persistent signatures of life processes that may aid in their detection. Here we propose biofilms as a model system to understand the habitability of extraterrestrial systems and as sources of recognizable and persistent biosignatures for life detection. By testing these ideas in extreme analog environments on Earth, this approach could be used to guide and focus future exploration of samples encompassing the geologic record of early Earth as well as other planets and moons of our solar system.

Acidophilic sulphate-reducing bacteria: Diversity, ecophysiology, and applications

Acidophilic sulphate-reducing bacteria (aSRB) are widespread anaerobic microorganisms that perform dissimilatory sulphate reduction and have key adaptations to tolerate acidic environments (pH <5.0), such as proton impermeability and Donnan potential. This diverse prokaryotic group is of interest from physiological, ecological, and applicational viewpoints. In this review, we summarize the interactions between aSRB and other microbial guilds, such as syntrophy, and their roles in the biogeochemical cycling of sulphur, iron, carbon, and other elements. We discuss the biotechnological applications of aSRB in treating acid mine drainage (AMD, pH <3), focusing on their ability to produce biogenic sulphide and precipitate metals, particularly in the context of utilizing microbial consortia instead of pure isolates. Metal sulphide nanoparticles recovered after AMD treatment have multiple potential technological uses, including in electronics and biomedicine, contributing to a cost-effective circular economy. The products of aSRB metabolisms, such as biominerals and isotopes, could also serve as biosignatures to understand ancient and extant microbial life in the universe. Overall, aSRB are active components of the sulphur and carbon cycles under acidic conditions, with potential natural and technological implications for the world around us.

Lipid-based paleoecological and biogeochemical reconstruction of Store Saltsø, an extreme lacustrine system in SW Greenland

Polar lakes harbour a unique biogeochemistry that reflects the implications of climatic fluctuations against a susceptible yet extreme environment. In addition to polar, Store Saltsø (Kangerlussuaq, southwestern Greenland) is an endorheic lake with alkaline and oligotrophic waters that host a distinctive ecology adapted to live in such particular physico-chemical and environmental conditions. By exploring the sedimentary record of Store Saltsø at a molecular and compound-specific isotopic level, we were able to understand its ecology and biogeochemical evolution upon climate change. We employed lipid biomarkers to identify biological sources and metabolic traits in different environmental samples (shore terrace, sediment core, and white precipitates at the shore), and their succession over time to reconstruct the lake paleobiology. Different molecular ratios and geochemical proxies provided further insights toward the evolution of environmental conditions in the frame of the deglaciation history of Kangerlussuaq. The relative abundance of terrestrial (i.e., plant derived) biomarkers (odd long-chain n-alkanes, even long-chain n-alkanols, and phytosterols) in the upper half of the shore terrace versus the relatively more present aquatic biomarkers (botryococcenes and long-chain alkenones) in its lower half revealed higher lake water levels in the past. Moreover, the virtual absence of organics in the deepest section of the sediment core (32-29 cm depth) suggested that the lake did not yet exist at the northwestern shore of Store Saltsø ∼5100 years ago. According to the relative abundance of lipid biomarkers detected in the adjacent section above (29-25 cm depth), we hypothesize that the northwestern shore of Store Saltsø formed ∼4900 years ago. By combining the molecular and compound-specific isotopic analysis of lipids in a ∼360 cm sedimentary sequence, we recreated the paleobiology and evolution of an extreme lacustrine environment suitable for the study of the limits of life and the effects of climate warming.

Geomicrobiological characterization of the evaporitic ecosystem in the hypersaline lake Laguna Verde (Andean Puna, Northwestern Argentina)

Laguna Verde's dome-shaped structures are distinctive formations within the Central Andes, displaying unique geomicrobiological features. This study represents a pioneering investigation into these structures, assessing their formation, associated taxa, and ecological significance. Through a multifaceted approach that includes chemical analysis of the water body, multiscale characterization of the domes, and analysis of the associated microorganisms, we reveal the complex interplay between geology and biology in this extreme environment. The lake's alkaline waters that are rich in dissolved cations and anions such as chloride, sodium sulfate, and potassium, coupled with its location at the margin of the Antofalla salt flat, fed by alluvial fans and hydrothermal input, provide favorable conditions for mineral precipitation and support for the microorganism's activity. Laguna Verde's dome-shaped structures are mainly composed of gypsum and halite, displaying an internal heterogeneous mesostructure consisting of three zones: microcrystalline, organic (orange and green layers), and crystalline. The green layer of the organic zone is predominantly composed of Proteobacteria, Bacteroidetes, and Cyanobacteria, while the orange layer is mostly inhabited by Cyanobacteria. The results of the study suggest that oxygenic photosynthesis performed by Cyanobacteria is the main carbon fixation pathway in the microbial community, supported by carbon isotopic ratios of specific biomarkers. This finding highlights the important role played by Cyanobacteria in this ecosystem.

Biosignature Detection and MinION Sequencing of Antarctic Cryptoendoliths After Exposure to Mars Simulation Conditions

In the search for life in our Solar System, Mars remains a promising target based on its proximity and similarity to Earth. When Mars transitioned from a warmer, wetter climate to its current dry and freezing conditions, any putative extant life probably retreated into habitable refugia such as the subsurface or the interior of rocks. Terrestrial cryptoendolithic microorganisms (i.e., those inhabiting rock interiors) thus represent possible modern-day Mars analogs, particularly those from the hyperarid McMurdo Dry Valleys in Antarctica. As DNA is a strong definitive biosignature, given that there is no known abiotic chemistry that can polymerize nucleobases, we investigated DNA detection with MinION sequencing in Antarctic cryptoendoliths after an ∼58-sol exposure in MARTE, a Mars environmental chamber capable of simulating martian temperature, pressure, humidity, ultraviolet (UV) radiation, and atmospheric composition, in conjunction with protein and lipid detection. The MARTE conditions resulted in changes in community composition and DNA, proteins, and cell membrane-derived lipids remained detectable postexposure. Of the multitude of extreme environmental conditions on Mars, UV radiation (specifically UVC) is the most destructive to both cells and DNA. As such, we further investigated if a UVC exposure corresponding to ∼278 martian years would impede DNA detection via MinION sequencing. The MinION was able to successfully detect and sequence DNA after this UVC radiation exposure, suggesting its utility for life detection in future astrobiology missions focused on finding relatively recently exposed biomarkers inside possible martian refugia.

Multi-analytical characterization of an oncoid from a high altitude hypersaline lake using techniques employed in the Mars2020 and Rosalind Franklin missions on Mars

In this work, a geological sample of great astrobiological interest was studied through analytical techniques that are currently operating in situ on Mars and others that will operate in the near future. The sample analyzed consisted of an oncoid, which is a type of microbialite, collected in the Salar Carachi Pampa, Argentina. The main peculiarity of microbialites is that they are organo-sedimentary deposits formed by the in situ fixation and precipitation of calcium carbonate due to the growth and metabolic activities of microorganisms. For this reason, the Carachi Pampa oncoid was selected as a Martian analog for astrobiogeochemistry study. In this sense, the sample was characterized by means of the PIXL-like, SuperCam-like and SHERLOC-like instruments, which represent instruments on board the NASA Perseverance rover, and by means of RLS-like and MOMA-like instruments, which represent instruments on board the future ESA Rosalind Franklin rover. It was possible to verify that the most important conclusions and discoveries have been obtained from the combination of the results. Likewise, it was also shown that Perseverance rover-like remote-sensing instruments allowed a first detailed characterization of the biogeochemistry of the Martian surface. With this first characterization, areas of interest for in-depth analysis with Rosalind Franklin-like instruments could be identified. Therefore, from a first remote-sensing elemental identification (PIXL-like instrument), followed by a remote-sensing molecular characterization (SuperCam and SHERLOC-like instruments) and ending with an in-depth microscopic analysis (RLS and MOMA-like instruments), a wide variety of compounds were found. On the one hand, the expected minerals were carbonates, such as aragonite, calcite and high-magnesium calcite. On the other hand, unexpected compounds consisted of minerals related to the Martian/terrestrial surface (feldspars, pyroxenes, hematite) and organic compounds related to the past biological activity related to the oncoid (kerogen, lipid biomarkers and carotenes). Considering samples resembling microbialites have already been found on Mars and that one of the main objectives of the missions is to identify traces of past life, the study of microbialites is a potential way to find biosignatures protected from the inhospitable Martian environment. In addition, it should be noted that in this work, further conclusions have been obtained through the study of the results as a whole, which could also be carried out on Mars.

An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration

Lipid molecules are organic compounds, insoluble in water, and based on carbon-carbon chains that form an integral part of biological cell membranes. As such, lipids are ubiquitous in life on Earth, which is why they are considered useful biomarkers for life detection in terrestrial environments. These molecules display effective membrane-forming properties even under geochemically hostile conditions that challenge most of microbial life, which grants lipids a universal biomarker character suitable for life detection beyond Earth, where a putative biological membrane would also be required. What discriminates lipids from nucleic acids or proteins is their capacity to retain diagnostic information about their biological source in their recalcitrant hydrocarbon skeletons for thousands of millions of years, which is indispensable in the field of astrobiology given the time span that the geological ages of planetary bodies encompass. This work gathers studies that have employed lipid biomarker approaches for paleoenvironmental surveys and life detection purposes in terrestrial environments with extreme conditions: hydrothermal, hyperarid, hypersaline, and highly acidic, among others; all of which are analogous to current or past conditions on Mars. Although some of the compounds discussed in this review may be abiotically synthesized, we focus on those with a biological origin, namely lipid biomarkers. Therefore, along with appropriate complementary techniques such as bulk and compound-specific stable carbon isotope analysis, this work recapitulates and reevaluates the potential of lipid biomarkers as an additional, powerful tool to interrogate whether there is life on Mars, or if there ever was.

Immunoanalytical Approach for Detecting and Identifying Ancestral Peptide Biomarkers in Early Earth Analogue Environments

Several mass spectrometry and spectroscopic techniques have been used in the search for molecular biomarkers on Mars. A major constraint is their capability to detect and identify large and complex compounds such as peptides or other biopolymers. Multiplex immunoassays can detect these compounds, but antibodies must be produced for a large number of sequence-dependent molecular targets. Ancestral Sequence Reconstruction (ASR) followed by protein "resurrection" in the lab can help to narrow the selection of targets. Herein, we propose an immunoanalytical method to identify ancient and universally conserved protein/peptide sequences as targets for identifying ancestral biomarkers in nature. We have developed, tested, and validated this approach by producing antibodies to eight previously described ancestral resurrected proteins (three β-lactamases, three thioredoxins, one Elongation Factor Tu, and one RuBisCO, all of them theoretically dated as Precambrian), and used them as a proxy to search for any potential feature of them that could be present in current natural environments. By fluorescent sandwich microarray immunoassays (FSMI), we have detected positive immunoreactions with antibodies to the oldest β-lactamase and thioredoxin proteins (ca. 4 Ga) in samples from a hydrothermal environment. Fine epitope mapping and inhibitory immunoassays allowed the identification of well-conserved epitope peptide sequences that resulted from ASR and were present in the sample. We corroborated these results by metagenomic sequencing and found several genes encoding analogue proteins with significant matches to the peptide epitopes identified with the antibodies. The results demonstrated that peptides inferred from ASR studies have true counterpart analogues in Nature, which validates and strengthens the well-known ASR/protein resurrection technique and our immunoanalytical approach for investigating ancient environments and metabolisms on Earth and elsewhere.

Biospeleothems Formed by Fungal Activity During the Early Holocene in the “Salar de Uyuni”

The Chiquini and Galaxias caves contain speleothems that are templated by long fungal structures. They have been associated with the carbonate lacustrine deposits in the margins of the Coipasa and Uyuni Salar basins. During a wetter episode, such carbonates formed at the end of the last glaciation raising the lake level to more than 100 m in the Tauca events (15–12 ky). Such an event flooded the caves that eventually became a cryptic habitat in the lake. The caves show bizarre speleothems framed by large (>1 m) fungal buildings covering the older algal mineralized structures. Although the origin of the caves is not fully understood, the occurrence of two carbonatic units with very distinctive fabric suggests that they formed in two separated humid events. In this regard, the mineralized algal structures, showing the same features as the lacustrine carbonates, likely formed during the Tauca flooding events in the terminal Pleistocene that inundated older caves. The different caves were exposed to the atmosphere after a drop in the lake level that promoted alluvial erosion by <12–10 ky (Ticaña episode) under arid conditions. A last humid episode rising the lake surface 10 m above the Salar level, which was not enough to inundate the caves a second time, drove the formation of the biospeleothems by fungi biomineralization. The abundance and size of the preserved fungal structures suggest that they were sustained by a stable hydrological activity plus a constant organic supply. While nutrients could have been primarily sourced from the vegetal communities that occupied the exhumated lake margins, they might have also been released from the lacustrine carbonatic unit. The combination of hydrology and biological activities were likely determinants for a fast rock dissolution and mineralization ending in the construction of the fungal biospeleothems.

Ecological variability based on lipid biomarkers in astrobiologically interesting wetlands from the Argentinian central Andes

Andean wetlands hold extremophilic communities adapted to live in harsh conditions. Here, we investigated the microbial ecology of three high-altitude hypersaline ponds from La Puna region (Argentina) showing an increasing extent of desiccation by analyzing their lipid sedimentary record. We recreated the microbial community structure and the carbon metabolisms in each lacustrine system based on the molecular distribution of lipid biomarkers and their compound-specific carbon and hydrogen isotopic signatures. We detected lipid compounds considered to be biomarkers of cyanobacteria, sulfate-reducing bacteria, purple sulfur bacteria, and archaea in the three Andean ponds, as well as diatoms in the intermediate salinity system. The relative abundance of purple sulfur and sulfate-reducing bacteria decreased with salinity, whereas cyanobacteria and archaea decreased their relative abundance in the mid-saline pond to increase it again and became both prevailing at the highest salinity. Carbon fixation in the three ponds was driven by a combination of the reductive tricarboxylic acid cycle, the reductive pentose phosphate cycle, and the reductive acetyl-CoA pathway. This work is the first to describe molecular and isotopic lipid fingerprints in wetlands from the central Andean Puna, and serves as a basis for further biogeochemical studies in the area.

Raman spectroscopic peculiarities of Icelandic poorly crystalline minerals and their implications for Mars exploration

In this work, we have analyzed natural samples collected at three hydrothermal areas of Iceland by Raman spectroscopy. The studied high-latitude regions are considered environmentally and mineralogically appropriate Martian analogues since they are rich in weathered basalts that have been altered by hydrothermalism to mineral phases such as silica, clay minerals, sulfates, oxides, and sulfur. The main objective of this work was to assess the relation of the spectroscopic signatures of alteration to hydrothermal processes and biomediation, considering previous studies focused on the detection of lipid biomarkers in the same samples. The recorded Raman spectra, taken with optical parameters similar to the ExoMars 2022 Raman spectrometer, showed structural modifications in all secondary minerals in the form of peak shifts (in the case of sulfur and clay minerals), changes in the relative ratio intensity (in anatase) and/or shape broadening (in sulfates and hematite). These results reveal the suitability of Raman spectroscopy to examine areas rich in water-altered minerals, where a mixture of crystalline and amorphous phases can co-exist. The detection of silica is singularly interesting since, on the one hand, it can imply the past existence of hydrothermal hot springs rich in nutrient and redox gradients and, on the other hand, provides excellent matrix for biosignature preservation. The data can be helpful as an astrobiological database for the forthcoming missions to Mars, where potential upwelling groundwater systems could have altered the mineral phases in a similar way to that observed in this work.

Rolling Ironstones from Earth and Mars: Terrestrial Hydrothermal Ooids as a Potential Analogue of Martian Spherules

High-resolution images of Mars from National Aeronautics and Space Administration (NASA) rovers revealed mm-size loose haematite spherulitic deposits (nicknamed “blueberries”) similar to terrestrial iron-ooids, for which both abiotic and biotic genetic hypotheses have been proposed. Understanding the formation mechanism of these haematite spherules can thus improve our knowledge on the possible geologic evolution and links to life development on Mars. Here, we show that shape, size, fabric and mineralogical composition of the Martian spherules share similarities with corresponding iron spherules currently forming on the Earth over an active submarine hydrothermal system located off Panarea Island (Aeolian Islands, Mediterranean Sea). Hydrothermal fluids associated with volcanic activity enable these terrestrial spheroidal grains to form and grow. The recent exceptional discovery of a still working iron-ooid source on the Earth provides indications that past hydrothermal activity on the Red Planet is a possible scenario to be considered as the cause of formation of these enigmatic iron grains.