e- Report on Hypoliths
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High-latitude polar deserts are among the most extreme environments on Earth. Here we describe a large and previously unappreciated habitat for photosynthetic life under opaque rocks in the Arctic and Antarctic polar deserts. This habitat is created by the periglacial movement of the rocks, which allows some light to reach their underside. The productivity of this ecosystem is at least as great as that of above-ground biomass and potentially doubles previous productivity estimates for the polar desert ecozone.
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The underside of rocks in climatically extreme deserts acts as a refugium for photosynthetic microorganisms (defined as ‘hypoliths’) and their community (the ‘hypolithon’)1. Here, the organisms are protected from harsh ultraviolet radiation2 and wind scouring, and trapped moisture can provide them with a source of liquid water3. Colonization also usually requires the rocks to be sufficiently translucent to allow for the penetration of light — all hypoliths reported so far have been found under quartz, which is one of the most common translucent rocks4,5.
We examined 850 randomly selected opaque dolomitic rocks, without regard to the local patterns of periglacial rock sorting, on Cornwallis Island and Devon Island in the Canadian high Arctic. These are regions typical of extreme polar desert, where vegetation cover was measured to be 1.2% or less6. On Devon Island, 95% of rocks were found to be colonized, and on Cornwallis Island, 94% were colonized. The photosynthetic organisms form a well defined green band on the underside of the rocks. The mean (±s.d.) band width of these communities was 3.1 ± 1.9 cm and 3.0 ± 1.6 cm on Devon Island and Cornwallis, respectively.
The Arctic hypoliths are dominated by cyanobacteria. Species found include Gloeocapsa cf. atrata Kützing, Gloeocapsa cf. punctata Nägeli, Gloeocapsa cf. kuetzingiana Nägeli and Chroococcidiopsis-like cells; unicellular algal chlorophytes were also present7.
We investigated the colonization of well developed polygons, which are just one manifestation of a diversity of linear and circular features caused by the long-term action of periglacial processes8. In the Arctic, we found colonization on 68% of rocks within polygons, with a mean (±s.d.) band width of 0.7 ± 0.8 cm, where the rocks are surrounded by fine soil sorted into their centres.