Colonization patterns of soil microbial communities in the Atacama Desert
The Atacama Desert is one of the driest deserts in the world and its soil characterized by extremely low moisture, organic carbon content, and oxidizing conditions has been considered to be at the dry limit for life. We collected soil samples from 3 geographic locations in the hyper-arid zone of the Atacama Desert, and from 3 locations along a North-South transect of increasing rainfall, to identify what factors might shape the diversity of its soil microbiome. Analyses of non-culture based, high throughput DNA sequence data revealed that communities from the 6 geographic locations were structurally and phylogenetically distinct (ANOVA test for observed OTU0.03, p<0.001; UniFrac distances) and that communities from locations in the hyper-arid zone displayed the lowest levels of diversity. We found bacterial taxa similar to those found in other arid soil communities with an abundance of Rubrobacterales, Actinomycetales, Acidimicrobiales, and a number of families from the Thermoleophilia. The extremely low abundance of Firmicutes, the phylum that comprises most sporulating bacteria, indicated that most bacteria in the soil were in the form of vegetative cells. No archaea were found in an of the soil samples. Integrating molecular data with climate and soil geochemistry, we found that air relative humidity (RH) and soil conductivity significantly correlated with microbial communities diversity metrics (least square linear regression for observed OTU0.03 and air RH and soil conductivity, p<0.001; UniFrac PCoA spearman’s correlation for air RH and soil conductivity, p<0.0001), indicating that water availability and salt content are key factors in shaping the Atacama soil microbiome. Mineralization studies showed communities actively metabolizing in all soil samples with increased rates in soils from the southern locations. These findings suggest that over geological time, and due to rare rain events, physicochemical factors potentially played a major role in selecting microorganisms that are most adapted to extreme desiccating conditions.