Habitability and Biosignature Formation in Simulated Martian Aqueous Environments

Abstract

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.

Mineral Indicators of Geologically Recent Past Habitability on Mars

“…Modeling and experimental and analog investigations of low-temperature rock alteration applied to Martian environments yield similar results to the mineral assemblages produced here (e.g., [6,64,66,76,86,87,89,129,[163][164][165][166][167][168][169][170][171]). For example, Griffith and Shock [87] modeled the reaction of Icelandic igneous rocks with CO 2 -charged water as an analog for Martian meteorite alteration, using the EQ3/6 software package [version 7]; temperatures were elevated compared to the current study, by 50 • C (the lowest temperature modeled was 423.15 K), yet produced similar minerals-antigorite, annite, phlogopite, daphnite, prehnite, tremolite, andradite, calcite, diopside, hedenbergite, magnetite, albite, quartz, and troilite [87].…”

Geochemical biosignature formation in experimental Martian fluvio‐lacustrine and simulated evaporitic settings

“…2À concentration between the biotic test group and the abiotic test group (Macey et al, 2023). However, such differences in fluid chemistries have been considered ambiguous biosignatures and are not sufficient alone to indicate the presence of a biogenic signal (Macey et al, 2023).…”

“…Phyllosilicates such as nontronite and kaolinite would be fundamental for this, as they have a high potential to preserve biosignatures due to their long crustal residence time, low porosity and permeability, and a sheet-like structure that enables adsorption, protection, and preservation of organic matter and organic compounds from oxidizing fluids and biological activity over TABLE 3. Comparison between element concentration in the experimental fluid (data from Macey et al, 2023) and in the biotic model at (W/R) M interval of between 2000 and 278 equivalent to (W/R) D .…”

“…To better characterize short-term (1 month) microbial weathering processes in the early Gale crater fluvio-lacustrine system and to increase the accuracy of identified inorganic biosignatures that may have formed in this environment, Macey et al (2023) performed abiotic and microbially mediated simulation experiments using a chemically accurate simulant regolith (OUCM-1, Ramkissoon et al, 2019) and thermochemically modeled Martian groundwater fluids. These were inoculated with chemolithoautotrophic and chemoorganoheterotrophic anaerobic microbes with metabolisms that are models for potential microbial life that may have existed within aqueous environments at Gale crater in the Noachian (Macey et al, 2023). The study concluded that sulfate reducing, acetogenic, and other anaerobic and fermentative bacteria were able to survive across multiple subculturing steps under Martian conditions, producing shifts in chemistry that could be considered putative biosignatures.…”

“…We compared our model results and the experimental results from Macey et al. (2023) to identify chemical and mineralogical signatures that are uniquely produced during microbial‐mediated basalt weathering and that may be used as inorganic biosignatures for life detection on Mars. Thermochemical modeling was further used to perform evaporation simulations and identify secondary minerals and fluid chemistries that may have evolved in evaporitic environments during seasonal dry episodes or in the late Hesperian, when Mars experienced more arid hydrological regimes.…”