Streams in the McMurdo Dry Valleys of Antarctica (MDV) are harsh habitats for life. Months of perpetual darkness and subzero temperatures are punctuated by months of perpetual sunlight, intense UV radiation, and temperatures that hover around freezing. The availability of liquid water regulates biological activity and limits the growing season to 6–10 weeks each year. These harsh conditions likely allow perennial microbial mats to persist in this freshwater environment (Fig. 1), as the rates of biomass accumulation exceed losses due to grazing metazoans and scouring (McKnight and Tate, 1997).

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Despite the extreme selective conditions, MDV microbial...

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