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.

Active microbial ecosystem in glacier basal ice fuelled by iron and silicate comminution-derived hydrogen.

The basal zone of glaciers is characterized by physicochemical properties that are distinct from firnified ice due to strong interactions with underlying substrate and bedrock. Basal ice (BI) ecology and the roles that the microbiota play in biogeochemical cycling, weathering, and proglacial soil formation remain poorly described. We report on basal ice geochemistry, bacterial diversity (16S rRNA gene phylogeny), and inferred ecological roles at three temperate Icelandic glaciers. We sampled three physically distinct basal ice facies (stratified, dispersed, and debris bands) and found facies dependent on biological similarities and differences; basal ice character is therefore an important sampling consideration in future studies. Based on a high abundance of silicates and Fe-containing minerals and, compared to earlier BI literature, total C was detected that could sustain the basal ice ecosystem. It was hypothesized that C-fixing chemolithotrophic bacteria, especially Fe-oxidisers and hydrogenotrophs, mutualistically support associated heterotrophic communities. Basal ice-derived rRNA gene sequences corresponding to genera known to harbor hydrogenotrophic methanogens suggest that silicate comminution-derived hydrogen can also be utilized for methanogenesis. PICRUSt-predicted metabolism suggests that methane metabolism and C-fixation pathways could be highly relevant in BI, indicating the importance of these metabolic routes. The nutrients and microbial communities release from melting basal ice may play an important role in promoting pioneering communities establishment and soil development in deglaciating forelands.