Greenland's Dark Zone is the largest contiguous region of bare terrestrial ice in the Northern Hemisphere and microbial processes play an important role in driving its darkening and thereby amplifying melt and runoff from the ice sheet. However, the dynamics of these microbiota have not been fully identified. Here, we present joint 16S rRNA gene and 16S rRNA (cDNA) comparison of input (snow), storage (cryoconite) and output (supraglacial stream water) habitats across the Dark Zone over the melt season. We reveal that all three Dark Zone communities have a preponderance of rare taxa exhibiting high protein synthesis potential (PSP). Furthermore, taxa with high PSP represent highly connected 'bottlenecks' within community structure, consistent with their roles as metabolic hubs. Finally, low abundance-high PSP taxa affiliated with Methylobacterium within snow and stream water suggest a novel role for Methylobacterium in the carbon cycle of Greenlandic snowpacks, and importantly, the export of potentially active methylotrophs to the bed of the Greenland Ice Sheet. By comparing the dynamics of bulk and potentially active microbiota in the Dark Zone of the Greenland Ice Sheet, we provide novel insights into the mechanisms and impacts of the microbial colonization of this critical region of our melting planet.

Center for Permafrost University of Copenhagen Oester Voldgade 10 1350 Copenhagen Denmark

Centre for Glaciology Department of Geography and Earth Sciences Llandinam Building Aberystwyth University Ceredigion SY23 3DD UK

Department of Ecology Faculty of Sciences Charles University Prague Prague 128 44 Czechia

Department of Geochemistry Geological Survey of Denmark and Greenland Oester Voldgade 10 1350 Copenhagen Denmark

Institute of Biological Environmental and Rural Sciences Cledwyn Building Aberystwyth University Ceredigion SY23 3DD UK

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