First insights into the chemistry and microbial community composition of the Western Sahara salt plains, a potential Mars analogue

Ilieva, Velislava; Stephens, Ben; Goodall, Tim; Ori, Gian Gabriele; Read, Daniel; Pearson, Victoria; Olsson-Francis, Karen and Macey, Michael (2022). First insights into the chemistry and microbial community composition of the Western Sahara salt plains, a potential Mars analogue. In: Geomicrobiology Research In Progress Meeting, 30 Jun - 1 Jul 2022, University of Huddersfield.

Abstract

Microorganisms that use inorganic sulfur compounds for energy conservation are dissimilatory sulfur cyclers. There is a great diversity among the metabolic strategies and biochemical pathways that these microbes use, which results in the production of sulfur compounds in varied chemical forms and oxidation states. The high abundance of sulfur cycling microbes in nature has made them an important contributor to the evolution of the biogeochemical sulfur cycle on Earth. The identification of mineral assemblages on the surface of Mars and within martian meteorites that contain sulfur species in different redox states has made dissimilatory sulfur cycling metabolisms of interest for possible extinct or extant life on Mars. A viable strategy to understand whether putative martian life could have been sustained through dissimilatory sulfur cycling is to study the prevalence of sulfur cycling microbes in Mars analogue environments. One such analogue environment is the Western Sahara salt plains, which are hypersaline, arid environments with high levels of UV exposure, which we propose as a suitable analogue for the Noachian-Hesperian transition period on Mars, characterised with thinning atmosphere and evaporation of surface water.

In this study, molecular and geochemical techniques were used to give the first insights into the Western Sahara salt plains. The microbiology was investigated through cultivation-independent and culture-dependent analyses of salt crystals, sediment and water samples obtained at three sites near Llaayoune. The chemical nature of the samples was analysed through ion chromatography (IC) and inductively coupled plasma - optical emission spectrometry (ICP-OES).

The geochemical characterisation confirmed the high salinity of the samples and identified that sodium, potassium, magnesium and sulfur were the most enriched elements within all samples. 16S rRNA gene amplicon sequencing of the samples identified a high relative abundance of sulfate reducing bacteria (SRB), Cyanobacteria, and Bacillus. To complement the 16S rRNA gene amplicon sequencing, enrichments were established to isolate aerobic heterotrophs, phototrophs and SRBs. The enrichments from the salt were dominated by strains of Bacillus, whereas sulfate-reducing strains of Clostridium were isolated from the sediment samples. Microscopic analysis of phototroph-selective media also indicated that algae and Cyanobacteria were successfully enriched from the samples.

The preliminary analysis has confirmed that there is active sulfur-cycling occurring in the environment. Future work will involve metagenomic analysis of the samples and genome sequencing of the isolates to identify the key metabolisms underpinning the survival and viability of the microbial community. Comparative studies with other Mars analogue environments will then be undertaken to identify metabolisms that may have been thermodynamically viable in ancient martian aqueous environments.