Unique Features of the Arid Soil Microbiome.

Deserts are oligotrophic ecosystems covering one third of Earth’s surface.  They are characterized by low levels of biotic productivity and have soil organic carbon (SOC) levels ranging from 0.02 – 1%. In these fragile ecosystems, the desert soil microbiome provides critical nutrient-cycling and carbon sequestration capacity, however the microbial diversity and environmental controls governing that diversity are poorly understood. This study characterized soils along a hyperarid to arid gradient traversing the Atacama Desert, Chile (0.02 – 0.4% SOC) and revealed that microbial communities were phylogenetically distinct from globally distributed non-desert soils, but similar to communities from the Chihuahuan and Mohave Deserts and the McMurdo Dry Valleys of Antarctica. Results revealed that average soil relative humidity and temperature best described the variance in the community richness and phylogenetic composition of the arid soil microbiome. Using a phylogenetic distance metric based on 16S rRNA gene profiling, communities of the barren, hyperarid regions were more similar to each other than to communities of the arid vegetated regions and were heavily dominated by Actinobacteria (78±7% relative abundance) followed by Proteobacteria (6.9±5.5%), Chloroflexi (6.6±2.4%) and Gemmatimonadetes (3.0±3.0%). Unique to these hyperarid microbial communities was the fact that single OTUs represented 22-45% of the entire community and the dominant OTU varied between sites. Phylogenetic analysis reveals the potential for non-phototrophic primary production in these communities by chemolithoautotrophic taxa that obtain energy through iron oxidation (Acidimicrobiales), nitrite oxidation (Nitrospiraceae), and ammonia oxidation (Thaumarchaeota, Candidatus Nitrososphaera). The results indicate that these hyperarid Atacama soils support specific microbial communities uniquely adapted for survival under extreme arid conditions.