Active microbial sulfur cycling in 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). Active microbial sulfur cycling in the Western Sahara salt plains and implications for life on early Mars. In: European Astrobiology Network Association EANA 2023, 19-22 Sep 2023, Madrid, Spain.


The Western Sahara salt plains are a potential analogue of early Mars. Within astrobiology, defining the physicochemical limits of terrestrial life is essential for precisely constraining the possibility of finding evidence of extant or extinct life in the Solar System. 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 source such as sulfur compounds in different oxidation states (sulfides and sulfates) were present on the surface of early Mars, thereby rendering it plausibly 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.
In this study, samples of 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 were set up to identify metabolically-active microbes within the collected samples.
The chemical analyses showed that sodium and chloride ions were most prevalent in the water and salt crystal samples, confirming that halite was the dominant salt phase. Notably, there was a high concentration of sulfate ions, supporting the relevance of the site as a Mars analogue. The taxonomic diversity of the microbiomes determined through 16S rRNA gene amplicon sequencing showed that moderately halophilic and extremely halophilic Bacteria and Archaea were most abundant across all sample types. High-quality metagenome assembled genomes (MAGs) of the dominant taxa and a sulfur-cycling microbe 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 a range of halophilic heterotrophs from all sample types and 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 Mars analogue environment with active biogeochemical sulfur cycling. Future work will focus on isolation and genome sequencing of sulfur cycling microbes identified in the sequencing data

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