The colonization of the underside of rocks normally requires that the material is sufficiently translucent to allow the penetration of photosynthetically active radiation. We examined the underside of 950 opaque rocks in sixteen locations in the Arctic for hypolithic colonization by photosynthetic microorganisms. Greater than 90% of rocks were colonized. The mean width of the bands of colonization was 3.1 ± 1.9 cm on Devon Island, and 3.0 ± 1.6 cm on Cornwallis Island. The width of the bands of colonization was less in the interior of frost-sorted polygons compared to their edges (in the arctic location, 0.7 ± 0.8 and 3.6 ± 1.4 cm in the interior and at the edges, respectively), suggesting the importance of frost sorting in enhancing the penetration of light around the edges of rocks to their undersides, and thus allowing colonization by photosynthetic organisms. We observed a similar pattern of colonization in antarctic polygons. The hypolithic habitat provides protection from environmental extremes. We show that within the hypolithic habitat organisms are protected from UV radiation. From radiolabeled carbon uptake measurements we estimate the productivity of the arctic communities to be approximately 0.8 g m−2 a−1, potentially as high as above-ground productivity. We discuss the potential implications of climate change for these communities.
