The McMurdo Dry Valleys (MDV), Antarctica experience extreme environmental conditions including low temperatures, minimal available water, and intense ultraviolet radiation inputs. As a consequence, this is a microbially dominated ecosystem; higher plants and animals are absent. In contrast to the limited eukaryotic diversity, recent lines of evidence support the existence of an indigenous and diverse microbial community in MDV streams, lakes, soils, and glaciers. In MCMIV, we are using a molecular biology approach to identify the biodiversity and function of MDV communities in diverse habitats. In addition, we are determining how manipulations of water, nutrients, and temperature affect biodiversity and function in situ.