The Watson River drains a portion of the SW Greenland ice sheet, transporting microbial communities from subglacial environments to a delta at the head of Søndre Strømfjord. This study investigates the potential activity and community shifts of glacial microbiota deposited and buried under layers of sediments within the river delta. A long-term (12-month) incubation experiment was established using Watson River delta sediment under anaerobic conditions, with and without CO2/H2enrichment. Within CO2/H2-amended incubations, sulphate depletion and a shift in the microbial community to a 52% predominance of Desulfosporosinus meridiei by day 371 provides evidence for sulphate reduction. We found evidence of methanogenesis in CO2/H2-amended incubations within the first 5 months, with production rates of ~4 pmol g-1 d-1, which was likely performed by methanogenic Methanomicrobiales- and Methanosarcinales-related organisms. Later, a reduction in methane was observed to be paired with the depletion of sulphate, and we hypothesise that sulphate reduction out competed hydrogenotrophic methanogenesis. The structure and diversity of the original CO2/H2-amended incubation communities changed dramatically with a major shift in predominant community members and a decline in diversity and cell abundance. These results highlight the need for further investigations into the fate of subglacial microbiota within downstream environments.

Center for Permafrost University of Copenhagen Øster Voldgade 10 DK 1350 Copenhagen Denmark

Department of Geochemistry Geological Survey of Denmark and Greenland Aberystwyth University Penglais Aberystwyth SY23 3FL UK

Department of Geochemistry Geological Survey of Denmark and Greenland University of Copenhagen Øster Voldgade 10 DK 1350 Copenhagen Denmark

Department of Geochemistry Geological Survey of Denmark and Greenland University of Copenhagen Øster Voldgade 10 DK 1350 Copenhagen Denmark Department of Ecology Faculty of Science Charles University Viničná 7 128 43 Prague Czech Republic

Department of Geochemistry Geological Survey of Denmark and Greenland University of Copenhagen Øster Voldgade 10 DK 1350 Copenhagen Denmark Department of Environmental Science Aarhus University Frederiksborgvej 399 DK 4000 Roskilde Denmark

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