Illuminating the functional rare biosphere of the Greenland Ice Sheet’s Dark Zone

The Dark Zone of the western Greenland Ice Sheet is the most expansive region of contiguous bare terrestrial ice in the Northern Hemisphere. Microbial processes within the Dark Zone play an important role in driving extensive albedo reduction and amplified melting, yet the composition and function of those consortia have not been fully identified. Here we present the first results from joint 16S rRNA gene and 16S rRNA (cDNA) analysis for the comparison of input (snow), storage (cryoconite), and output (supraglacial stream water) habitats across the Dark Zone over the melt season. Our analysis reveals that all three Dark Zone communities are characterized by 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 roles as metabolic hubs within their communities. Finally, the detection of low abundance-high PSP taxa affiliated with Methylobacterium within snow and stream water indicates a potential 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 microbial communities in the Dark Zone of the Greenland Ice Sheet our study provides insight into the mechanisms and impacts of the microbial colonization of this critical region of our melting planet.