Meltwater runoff from the Greenland Ice Sheet reveals microbial consortia from contrasting subglacial drainage systems

Ice sheets overlay active and putatively widespread microbial ecosystems. An active subglacial biota has the potential to impact strongly on the (bio)geochemistry of local as well as downstream environments. Such impacts partly depend on the distribution of microbial populations, the types of habitats present beneath the ice, and their connectivity. In the ablation zone of the Greenland Ice Sheet (GrIS), supraglacial meltwaters are routed to the ice-sheet bed during the melt season, flushing out subglacial waters, sediments, and cells to proglacial environments via runoff. Here, we report on the diversity, composition, and niche differentiation of microbial assemblages exported in bulk runoff from a large (~600 km2) GrIS catchment. Proglacial river samples were collected over a period of subglacial drainage evolution in order to capture potential shifts in exported microbial community alongside hydrochemical transitions. We use high-resolution hydrochemical and hydrological information from the proglacial river to guide microbial (16S rRNA gene) interpretations. Core populations closely matched sequences previously isolated from other (pro)glacial environments, and phylogenetic characterisation of main OTUs alluded to a central role for subglacial iron, sulphur, and methane cycling. Whilst results indicate that bulk populations exported are likely true members of sub ice-sheet communities, we also find evidence of a supraglacial signature influencing composition of exported assemblages. Changes in assemblage structure accompanied those of major hydrological periods, with enhanced subglacial flushing coinciding with distinct shifts in microbial composition. Timing of sampling therefore matters when attempting to infer more nuanced changes in exported communities, or reveal the biogeochemical processes likely occurring in regions of the bed less influenced by surface melt. This is likely especially true when studying larger glacial systems, which experience complex hydrological changes throughout the melt-season, and that periods of extensive subglacial flushing offer opportunities to assess diversity from more isolated regions of the bed. Still, an apparent strong buffering signal from marginal zones appear to mask some of the diversity intrinsic to more remote, likely anoxic, subglacial niches, which may ultimately only be sampled via direct access to the subsurface.