Bio-signatures play a central role in determining whether life existed on early Mars. In this study, we used a combination of experimental microbiology and thermochemical modelling techniques to identify potential geochemical bio-signatures for life on early Mars. Laboratory experiments were used to determine the short-time effects that biota had on basalt dissolution and the formation of secondary alteration minerals; whilst the thermochemical modelling (code CHIM-XPT, M. H. Reed, University of Oregon) was used to predict growth of secondary alteration minerals, which can be used as bio-signatures, over a geological timescale. For the biotic experiments heterotrophic bacterium, Burkholderia sp. strain B_33, was grown in a minimal growth medium with basalt as the sole nutrient source. No growth was detected in the absence of the basalt. During exponential growth, the pH decreased rapidly from pH 7.0 to 3.6 and then gradually increased to a steady-state of equilibrium of between 6.8 and 7.1. Microbial growth coincided with an increase of key elements in the growth medium (Si, K, Ca, Mg and Fe). We modelled the dissolution of the basalt in very dilute brine at 25 °C, 1 bar; the pH was buffered by the mineral dissolution and precipitation reactions. In the model, we assumed complete dissolution of the rock into solution. Preliminary results suggested that at the water to rock ratio of 1 x 107; zeolite, hematite, chlorite, kaolinite and apatite formed abiotically. The biotic weathering processes were modelled by varying the pH conditions within the model to adjust for biologic influence. This study suggests that for a basaltic system the microbial mediated dissolution of basalt would result in ‘simpler’ secondary alteration, consisting of Fe-hydroxide and kaolinite, under conditions where the abiotic system would form chlorite in addition to Fe-hydroxide and kaolinite. The results from this study demonstrate that, by using an experimental and theoretical approach, it is possible to identify secondary alteration minerals that could potentially be used to distinguish between abiotic and biotic weathering processes on early Mars. This work is crucial for interpreting data from past, present and future life detection missions to Mars.

Evidence indicates that Gale crater on Mars harboured a fluvio-lacustrine environment that was subjected to physio-chemical variations such as changes in redox conditions and evaporation with salinity changes, over time. Microbial communities from terrestrial environmental analogues sites are important for studying such potential habitability environments on early Mars, especially in laboratory-based simulation experiments. Traditionally, such studies have predominantly focused on microorganisms from extreme terrestrial environments. These are applicable to a range of Martian environments; however, they lack relevance to the lacustrine systems. In this study, we characterise an anoxic inter-tidal zone as a terrestrial analogue for the Gale crater lake system according to its chemical and physical properties, and its microbiological community. The sub-surface inter-tidal environment of the River Dee estuary, United Kingdom (53°21′15.40″ N, 3°10′24.95″ W) was selected and compared with available data from Early Hesperian-time Gale crater, and temperature, redox, and pH were similar. Compared to subsurface ‘groundwater’-type fluids invoked for the Gale subsurface, salinity was higher at the River Dee site, which are more comparable to increases in salinity that likely occurred as the Gale crater lake evolved. Similarities in clay abundance indicated similar access to, specifically, the bio-essential elements Mg, Fe and K. The River Dee microbial community consisted of taxa that were known to have members that could utilise chemolithoautotrophic and chemoorganoheterotrophic metabolism and such a mixed metabolic capability would potentially have been feasible on Mars. Microorganisms isolated from the site were able to grow under environment conditions that, based on mineralogical data, were similar to that of the Gale crater’s aqueous environment at Yellowknife Bay. Thus, the results from this study suggest that the microbial community from an anoxic inter-tidal zone is a plausible terrestrial analogue for studying habitability of fluvio-lacustrine systems on early Mars, using laboratory-based simulation experiments.

Background: Hypersaline environments are dynamic ecosystems, the chemistry of which is significantly influenced by climate change, which in turn impacts the microbiota and biogeochemical processes. This study investigates the microbiome of Lake Karum, a hypersaline lake in the Danakil Depression, Ethiopia, with a particular focus on genome-based potential of climate-relevant biogeochemical processes. Results: The microbiomes of Lake Karum sediments and waters were dominated by halophilic Archaea ( Halobacteriota ) and Bacteria ( Bacteroidota , Pseudomonadota , and Cyanobacteriota ), with significant variation in community composition among sites, reflecting geochemical heterogeneity. Despite these taxonomic differences, sediment and water metagenomes exhibited broadly overlapping functional gene profiles. Genes involved in denitrification, carbon monoxide oxidation, osmotic stress tolerance, and utilisation of osmolytes were widespread and predominantly affiliated with Halobacteriales, indicating their pivotal role in nitrogen and carbon cycling. Genome‑resolved analyses revealed substantial intrageneric variation in metabolic potential within dominant halobacterial lineages as well as bacterial candidate phyla, including Candidatus Bipolaricaulota and Candidatus Salsurabacteriota, which encode genes linked to trace-gas metabolism and nitrogen cycling. Notably, a high‑quality metagenome‑assembled genome assigned to the Candidatus Salsurabacteriota was recovered that possesses novel combinations of functional genes not previously reported for this lineage. Conclusion: This study provides a comprehensive genome‑resolved assessment of the taxonomic and functional diversity of the Lake Karum microbiome and identifies microbial taxa with the potential to drive key carbon, nitrogen, and sulfur cycling processes in a hypersaline lake. By revealing previously unrecognised metabolic capabilities within bacterial candidate phyla and highlighting intrageneric functional heterogeneity among dominant halophilic Archaea , this work advances understanding of how hypersaline microbial communities contribute to biogeochemical cycling in extreme environments.