Experimental Identification of Potential Martian Biosignatures in Open and Closed Systems.

NASA's Perseverance and ESA's Rosalind Franklin rovers have the scientific goal of searching for evidence of ancient life on Mars. Geochemical biosignatures that form because of microbe-mineral interactions could play a key role in achieving this, as they can be preserved for millions of years on Earth, and the same could be true for Mars. Previous laboratory experiments have explored the formation of biosignatures under closed systems, but these do not represent the open systems that are found in natural martian environments, such as channels and lakes. In this study, we have conducted environmental simulation experiments using a global regolith simulant (OUCM-1), a thermochemically modelled groundwater, and an anaerobic microbial community to explore the formation of geochemical biosignatures within plausible open and closed systems on Mars. This initial investigation showed differences in the diversity of the microbial community developed after 28 days. In an open-system simulation (flow-through experiment), the acetogenic Acetobacterium (49% relative abundance) and the sulfate reducer Desulfosporomusa (43% relative abundance) were the dominant genera. Whereas in the batch experiment, the sulfate reducers Desulfovibrio, Desulfomicrobium, and Desulfuromonas (95% relative abundance in total) were dominant. We also found evidence of enhanced mineral dissolution within the flow-through experiment, but there was little evidence of secondary deposits in the presence of biota. In contrast, SiO2 and Fe deposits formed within the batch experiment with biota but not under abiotic conditions. The results from these initial experiments indicate that different geochemical biosignatures can be generated between open and closed systems, and therefore, biosignature formation in open systems warrants further investigation.

Habitability and Biosignature Formation in Simulated Martian Aqueous Environments.

Water present on early Mars is often assumed to have been habitable. In this study, experiments were performed to investigate the habitability of well-defined putative martian fluids and to identify the accompanying potential formation of biosignatures. Simulated martian environments were developed by combining martian fluid and regolith simulants based on the chemistry of the Rocknest sand shadow at Gale Crater. The simulated chemical environment was inoculated with terrestrial anoxic sediment from the Pyefleet mudflats (United Kingdom). These enrichments were cultured for 28 days and subsequently subcultured seven times to ensure that the microbial community was solely grown on the defined, simulated chemistry. The impact of the simulated chemistries on the microbial community was assessed by cell counts and sequencing of 16S rRNA gene profiles. Associated changes to the fluid and precipitate chemistries were established by using ICP-OES, IC, FTIR, and NIR. The fluids were confirmed as habitable, with the enriched microbial community showing a reduction in abundance and diversity over multiple subcultures relating to the selection of specific metabolic groups. The final community comprised sulfate-reducing, acetogenic, and other anaerobic and fermentative bacteria. Geochemical characterization and modeling of the simulant and fluid chemistries identified clear differences between the biotic and abiotic experiments. These differences included the elimination of sulfur owing to the presence of sulfate-reducing bacteria and more general changes in pH associated with actively respiring cells that impacted the mineral assemblages formed. This study confirmed that a system simulating the fluid chemistry of Gale Crater could support a microbial community and that variation in chemistries under biotic and abiotic conditions can be used to inform future life-detection missions.

Sulfur Cycling as a Viable Metabolism under Simulated Noachian/Hesperian Chemistries.

Water present on the surface of early Mars (>3.0 Ga) may have been habitable. Characterising analogue environments and investigating the aspects of their microbiome best suited for growth under simulated martian chemical conditions is key to understanding potential habitability. Experiments were conducted to investigate the viability of microbes from a Mars analogue environment, Colour Peak Springs (Axel Heiberg Island, Canadian High Arctic), under simulated martian chemistries. The fluid was designed to emulate waters thought to be typical of the late Noachian, in combination with regolith simulant material based on two distinct martian geologies. These experiments were performed with a microbial community from Colour Peak Springs sediment. The impact on the microbes was assessed by cell counting and 16S rRNA gene amplicon sequencing. Changes in fluid chemistries were tested using ICP-OES. Both chemistries were shown to be habitable, with growth in both chemistries. Microbial communities exhibited distinct growth dynamics and taxonomic composition, comprised of sulfur-cycling bacteria, represented by either sulfate-reducing or sulfur-oxidising bacteria, and additional heterotrophic halophiles. Our data support the identification of Colour Peak Springs as an analogue for former martian environments, with a specific subsection of the biota able to survive under more accurate proxies for martian chemistries.

Microbes from Brine Systems with Fluctuating Salinity Can Thrive under Simulated Martian Chemical Conditions.

The waters that were present on early Mars may have been habitable. Characterising environments analogous to these waters and investigating the viability of their microbes under simulated martian chemical conditions is key to developing hypotheses on this habitability and potential biosignature formation. In this study, we examined the viability of microbes from the Anderton Brine Springs (United Kingdom) under simulated martian chemistries designed to simulate the chemical conditions of water that may have existed during the Hesperian. Associated changes in the fluid chemistries were also tested using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The tested Hesperian fluid chemistries were shown to be habitable, supporting the growth of all of the Anderton Brine Spring isolates. However, inter and intra-generic variation was observed both in the ability of the isolates to tolerate more concentrated fluids and in their impact on the fluid chemistry. Therefore, whilst this study shows microbes from fluctuating brines can survive and grow in simulated martian water chemistry, further investigations are required to further define the potential habitability under past martian conditions.

Infrared Spectroscopic Detection of Biosignatures at Lake Tírez, Spain: Implications for Mars.

The detection of potential biosignatures with mineral matrices is part of a multifaceted approach in the search for life on other planetary bodies. The 2020 ExoMars Rosalind Franklin rover includes within its payload three IR spectrometers in the form of ISEM (Infrared Spectrometer for ExoMars), MicrOmega, and Ma-MISS (Mars Multispectral Imager for Subsurface Studies). The use of this technique in the detection and characterization of biosignatures is of great value. Organic materials are often co-deposited in terrestrial evaporites and as such have been proposed as relevant analogs in the search for life on Mars. This study focuses on Ca-sulfates collected from the hypersaline Tírez Lake in Spain. Mid infrared and visible near infrared analysis of soils, salt crusts, and crystals with green and red layering indicative of microbial colonization of the samples was acquired from across the lake and identified the main mineral to be gypsum with inputs of carbonate and silica. Organic functional groups that could be attributed to amides and carboxylic acids were identified as well as chlorophyll; however, due to the strong mineralogical absorptions observed, these were hard to unambiguously discern. Taxonomical assignment demonstrated that the archaeal community within the samples was dominated by the halophilic extremophile Halobacteriaceae while the bacterial community was dominated by the class Nocardiaceae. The results of this research highlight that sulfates on Mars are a mixed blessing, acting as an effective host for organic matter preservation but also a material that masks the presence of organic functional groups when analyzed with spectroscopic tools similar to those due to fly on the 2020 ExoMars rover. A suite of complementary analytical techniques therefore should be used to support the spectral identification of any candidate extraterrestrial biosignatures.