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The McMurdo Dry Valley lakes, Antarctica, one of the Earth’s southernmost ecosystems containing liquid water, harbor some of the most environmentally extreme (cold, nutrient-deprived) conditions on the planet. Lake Bonney has a permanent ice cover that supports a unique microbial habitat, provided by soil particles blown onto the lake surface from the surrounding, ice-free valley floor. During continuous sunlight summers (Nov.-Feb.), the dark soil particles are heated by solar radiation and melt their way into the ice matrix. Layers and parches of aggregates and liquid water are formed. Aggregates contain a complex cyanobacterial-bacterial community, concurrently conducting photosynthesis (CO2 fixation), nitrogen (N-2) fixation, decomposition, and biogeochemical zonation needed to complete essential nutrient cycles. Aggregate-associated CO2- and N-2-fixation rates were low and confined to liquid water (i.e., no detectable activities in the ice phase). CO2 fixation was mediated by cyanobacteria; both cyanobacteria and eubacteria appeared responsible for N-2 fixation. CO2 fixation was stimulated primarily by nitrogen (NO3-), but also by phosphorus (PO43-). PO43- and iron (FeCl3 + EDTA) enrichment stimulated of N-2 fixation. Microautoradiographic and physiological studies indicate a morphologically and metabolically diverse microbial community, exhibiting different cell-specific photosynthetic and heterotrophic activities. The microbial community is involved in physical (particle aggregation) and chemical (establishing redox gradients) modification of a nutrient- and organic matter-enriched microbial "oasis," embedded in the desertlike (i.e., nutrient depleted) lake ice cover. Aggregate-associated production and nutrient cycling represent microbial self-sustenance in a microenvironment supporting "life at the edge," as it is known on Earth.