Two Worlds on a Stone: Arctic Desert Hypoliths and Epiliths Show Spatial Niche Differentiation.

In Arctic polar deserts, rocks can be extensively colonized by phototrophic hypolithic communities that exploit periglacial sorting processes to grow beneath opaque rocks. These communities are distinguished by green bands that are distinctly and abruptly separated from the black-pigmented communities on the rock surface (epiliths). We used 16S and 18S rDNA culture-independent methods to address the hypothesis that the two communities are different. Although both communities were dominated by cyanobacterial species (Chroococcidiopsis and Nostoc spp.), we found that the hypolithic and epilithic habitats host distinct microbial communities. We found that eukaryotic hypolithic and epilithic communities were statistically similar but that the hypolithic habitats contained tardigrade DNA, showing that the more clement subsurface habitat supports animal life in contrast to the surface of the rocks. These results reveal the distinctive communities and sharp demarcations that can develop across small spatial scales in the Earth's rocky extreme environments.