Unique traits of an ultra low-diversity bacterial and fungal high-elevation (> 6000 m) cold desert mineral soil community

High-elevation, plant and glacier free soils rank among the most physiochemically extreme and oligotrophic terrestrial environments.  The mountain topography and low atmospheric pressure drive daily temperature fluctuations across the freezing point (with an amplitude of up to 70°C), and generate periodic snowfall.  Building upon our initial microbial community and biogeochemical assays of the mineral soils of Volcán Llullaillaco (6739 m elevation) and Volcán Socompa (6051 m elevation), which are stratovolcanos that rise above the hyperarid Atacama desert, we now report our initial results from both deep-coverage metagenomic sequencing efforts and related physiological experiments. 
 
Our results confirm that like many arid desert, polar, and alpine settings, Actinobacteria dominate both bacterial and overall microbial abundance.  Specifically, soils from 6030 m elevation on Volcán Llullaillaco are dominated by a highly divergent spore-forming filamentous Pseudonocardia sp.  Kilobase scale high coverage contigs reveal extensive levels of horizontal recombination from both other Actinobacteria, as well as other bacterial phyla.  At the community level we also reconfirm the absence of any complete photosynthetic pathways, yet find an abundance of genes that code for the both the complete Calvin-Benson and C4-dicarboxylic acid CO₂ fixation cycles.  The presence of complete gene sets for the aerobic oxidation of carbon monoxide, hydrogen, methane, and nitrate suggest these atmospheric substrates may provide energy for chemoautotrophy in this system.  Overall, we hypothesize that the majority of biomass resides below the top centimeters of soil that light penetrates.   Here microbial life finds a limited buffer from the hyperarid and solar scorched surface, and relies on trace gases, aerosols and other Aeolian inputs to survive during periodic episodes of sufficient snowmelt derived water availability.  Our freeze-thaw cycle isolation and growth experiments provide further evidence for the selectivity of this environment, and suggests how the resident microbes have adapted to this niche.  All isolates are psychrotolerant yeasts and bacteria capable of strong growth under a daily freeze (-4 °C) thaw (40 °C) cycle that mimics temperature data collected at 4 cm soil depth from 6000 m elevation.