Abstract EANA2024-92

Evidence within the rock record of Gale Crater on Mars has shown that during the Noachian-Hesperian transition (~4.2 billion years ago), Gale Carter had a dynamic hydrological environment that underwent cycles of wetting, drying, and rewetting before transitioning to more euxinic conditions. Moreover, with the discovery of all of the bioessential elements required to support life, questions have arisen around the habitability of these ancient environments and how fluctuations in conditions may have impacted a hypothetical martian microbiome within crater lake sediments. Studies of terrestrial analogue sites have demonstrated that environmental perturbations affect the taxonomic profile and functional potential of microbiomes, with these shifts having implications for the formation and preservation of biosignatures. Simulation experiments that replicate the geochemical environment of the extraterrestrial site of interest allow us to elucidate how the microbial community would have been affected, with results informing biosignature target selection.

In this study, a novel geochemical fluid and regolith simulant were produced to replicate the Gillespie Lake Member's former aqueous environment and allow exploration of the functional potential of the lake sediment microbiome. The Gillespie Lake Member groundwater composition was thermochemically modelled at a water-rock ratio of 100:1. It was compared to 44 other Mars-relevant fluids using an unsupervised machine-learning approach. Results identified that groundwater within the Gillespie Lake Member sediments was most similar to other groundwater fluid chemistries predicted for geological units from Gale Crater and Paso Robles regolith from Gusev Crater. The groundwater and regolith simulant was mixed at a 2:1 ratio with 15 g of regolith and 30 ml of groundwater simulant in 100 ml serum vials to create the microcosm used for the simulation experiment. A reducing H2/CO2 headspace was then supplied. Microcosms were then inoculated with a microbial culture enriched from a high-altitude lake Mars analogue site. Every 14 days, for 56 days, 1 ml of fluid simulant was sub-cultured from the microcosms, the cells were washed to minimise the transfer of compounds from the initial inoculum, which could potentially impact microbial community composition, and the cell concentration was assessed. The impact of the simulated chemistry on the abundance and diversity of the enriched microbiome was assessed via cell counts and the generation of metagenomes from DNA extracted from the enriched microbiome. Cell counts indicated exponential growth until the termination of the experiment with a cell density at the final time point at a concentration of 5.90 x 10-7/ml-1. Metagenome data showed that microbes from the genera Desulfovibrio, Humidesulfovibrio, and Sulfurospirillum dominated the community after 56 days, with sulfur oxidation and reduction, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification being the most abundant metabolisms within the endpoint community. These results indicate that the Gillespie Lake Member would have been habitable during diagenesis for anaerobic dissimilatory metabolisms and highlight the plausibility of microbially mediated biogeochemical cycles.