Abstract

Microbial mats are self-sustained, functionally complex ecosystems that make goodmodels for the understanding of past and presentmicrobial ecosystems aswell as putative extraterrestrial ecosystems. Ecological theory suggests that the composition of these communitiesmight be affected bynutrient availability anddisturbance frequency.We characterized twomicrobial mats from two contrasting environments in the oligotrophic Cuatro Ciénegas Basin: a permanent green pool and a red desiccation pond. We analyzed their taxonomic structure and composition by means of 16S rRNA clone libraries andmetagenomics and inferred theirmetabolic role by the analysis of functional traits in themost abundant organisms. Bothmats showed a high diversitywithmetabolically diversemembers and strongly differed in structure and composition. The green mat had a higher species richness and evenness than the red mat, which was dominated by a lineage of Pseudomonas. Autotrophs were abundant in the green mat, and heterotrophs were abundant in the red mat. When comparing with other mats and stromatolites, we found that taxonomic compositionwas not shared at species level but at order level,which suggests environmental filtering for phylogenetically conserved functional traits with random selection of particular organisms. The highest diversity and composition similarity was observed among systems from stable environments, which suggests that distur-bance regimes might affect diversity more strongly than nutrient availability, since oligotrophy does not appear to prevent the establishment of complex and diverse microbial mat communities. These results are discussed in light of the search for extraterrestrial life. Key Words: Cuatro Ciénegas—Metagenomics—Microbial mats— Oligotrophic—Phosphorus limitation—Stromatolites. Astrobiology 12, 659–673. 1.