Functional potential of microbial communities within the Western Sahara salt plains and implications for life on early Mars

Ilieva, Velislava; Goodall, Tim; Read, Daniel; Pearson, Victoria; Olsson-Francis, Karen and Macey, Michael (2023). Functional potential of microbial communities within the Western Sahara salt plains and implications for life on early Mars. In: Astrobiology Society of Britain 9 (ASB9), 07-08 Sep 2023, Birbeck, University of London, London.


Mars is the most studied astrobiological target, with extensive geochemical and morphological data collected via satellite and lander/rover missions. These data indicate that liquid water, bio-essential elements, and possible energy sources such as compounds in different oxidation states (e.g., sulfates and sulfides) were present on the surface of early Mars, thereby rendering it potentially habitable for terrestrial-like life. An understanding of relevant geochemical and biological processes on early Mars can be developed through chemically-relevant analogue environments on Earth. For example, the Western Sahara salt plains are an analogue for salt-rich Noachian/Hesperian-aged terrain, representing a period when atmospheric loss resulted in widespread surface water evaporation and concentration of salt phases within fluids.
In this study, salt crystals, water and sediment were collected from the Western Sahara salt plains. To identify the suitability of the study site as an analogue for early Mars, the chemistry of the samples was studied by ion chromatography (IC) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The microbiomes of the samples were characterised through 16S rRNA gene amplicon sequencing and shotgun metagenomic sequencing of extracted DNA. Enrichments with liquid Postgate media were set up to identify metabolically-active sulfate reducing microbes.
The chemical analyses showed that sodium and chloride ions were most prevalent in the water and salt crystal samples, suggesting that halite was the dominant salt phase. Notably, there was a high concentration of sulfate ions, ~1.5 – 2 g/L, supporting the relevance of the site as a Mars analogue for sulfate-rich salt deposits on Mars. 16S rRNA gene amplicon sequencing showed that moderately halophilic and extremely halophilic Bacteria and Archaea were most abundant across all sample types. Sequences belonging to sulfate reducing bacteria (SRB) and sulfur oxidising bacteria (SOB) were identified in the sediment, suggesting that active microbial sulfur cycling is present. Metagenomic analysis revealed that the functional capacity of the salt microbiome is different from the functional capacity of the sediment microbiome. Specifically, genes associated with nitrogen cycling were highly abundant in the salt crystal samples, whereas genes associated with sulfur cycling and methanogenesis/methanotrophy were abundant in the sediment samples. Moreover, high-quality metagenome assembled genomes (MAGs) of the dominant taxa and sulfur cycling microbes were assembled from the salt and sediment samples, allowing the reconstruction of metabolic pathways that might be active at the study site. Finally, microbial enrichments from the environmental samples successfully isolated sulfate reducing bacteria from the sediment, indicating that microbial sulfur cycling is active in the site.
The results of this study add to the existing knowledge of microbial life in hypersaline environments and showcase a halite-rich Mars analogue environment with high sulfate content and the potential for biogeochemical sulfur cycling. Future work will focus on full genome sequencing of the isolated sulfate reducing species and isolation of the sulfur oxidising species identified in the sequencing data.

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