We evaluated the microbial diversity and metabolome profile of an uncommon hypersaline elastic microbial mat from Cuatro Ciénegas Basin (CCB) in the Chihuahuan Desert of Coahuila, México. We collected ten samples on a small scale transect (1.5-m) and described its microbial diversity through NGS-based ITS and 16S rDNA gene sequencing. A very low number of taxa comprised a considerable proportion of the mat and were shared across all sampling points, whereas the rare biosphere was more phylogenetically diverse (Faith’s Phylogenetic Diversity (FPD) index) and phylogenetically disperse (using a null model distribution of Phylogenetic Species Clustering (nmdPSC)) than the abundant (high read count) taxa for both analyzed libraries. We also found a distinctive metabolome profile for each sample and were able to tentatively annotate several classes of compounds with relevant biological properties.

23Herein we describe the Archaea diversity in a shallow pond in the Cuatro Ciénegas Basin (CCB), 24Northeast Mexico, with fluctuating hypersaline conditions containing elastic microbial mats that 25 can form small domes where their anoxic inside reminds us of the characteristics of the Archaean 26Eon, rich in methane and sulfur gases; thus, we named this site the Archaean Domes (AD). These 27 domes only form after heavy rains that are rare in the Chihuahuan desert. CCB is a unique oasis 28 with hundreds of ponds, containing endemic species of animals, plants and highly diverse and 29 unique microbial communities, despite its very biased stoichiometry, due mostly to extreme low 30 phosphorus content (soils, water columns and sediments). This extreme oligotrophy has favored 31 survival of ancestral microorganisms. Whole metagenome sequencing approach was performed for 32 this unusual site in three different seasons to assess the extent of the Archaea biodiversity, with a 33 focus on extremophiles, since members of the Archaea had been underrepresented in different study 34 sites within the oasis. We found a highly diverse Archaea community compassing ~5% of the 35 metagenomes. The archaeal portion in all three metagenomes maintained its abundance and most of 36 the strains showed to form a resilient core during three seasonal samplings (2016-2017), despite 37 environmental fluctuations. However, relative abundances of all 230 archaeal OTUs (defined using 38 a 97% cutoff) were low enough (<0.1%) to be considered part of the rare biosphere. AD finding and 39 their description within CCB confirms that this particular pond is the most diverse for Archaea that 40we are aware of and opens new paths for understanding the forces that once drove and keep shaping 41 microbial community assemblage. 42 43 44 45 . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/766709 doi: bioRxiv preprint Archaea domain remains largely unexplored [14] along with a diverse metabolism [15, 16], 64 unveiling new processes and key features involving microbes and community structure.65 Cuatro Cienegas Basin (CCB) is an endangered oasis within the northern zone of the 66 Chihuahuan Desert in Mexico, characterized by an extremely unbalanced nutrient stoichiometry of 67 the area (N:P = 159:1), similar to the conditions of the Precambrian sea [17-21]. Strikingly, despite 68 . CC-BY-NC-ND 4.0 International license is made available under aThe copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/766709 doi: bioRxiv preprint this nutrient deficiency CCB is considered a biodiversity hotspot for macroorganisms [22] and one 69 of the most diverse sites for microorganisms in the world [23][24][25][26][27]; this microbial biodiversity is 70 mirrored by its extreme diversity in virus [28]. CCB biodiversity is believed to have evolved as a 71result of a long time en...

We evaluated the microbial diversity and metabolomic signatures of a hypersaline elastic microbial mat from Cuatro Cienegas Basin (CCB) in the Chihuahuan Desert of Coahuila, Mexico. We collected ten samples within a small scale (1.5-meters transect) and found a high microbial diversity through NGS-based ITS and 16S rRNA gene sequencing. A very low number of taxa were abundant and shared between all sites, whereas the rare biosphere was more phylogenetically diverse (FPD index) and phylogenetically disperse (PSC index) than the abundant taxa for both analyzed libraries. In regard to potential biotic interactions (Pearson analysis), there were more positive correlations than negative. We also found a distinctive metabolomic signature for each sample and were able to tentatively annotate several classes of compounds with relevant biological properties. Together, these results reveal cohesive, diverse and fluctuating microbial communities, where abundant and rare taxa appear to have different ecological roles.