Glacial meltwater microbes: are there seasonal trends in exported assemblages over different catchment sizes?

Glaciers and ice sheets host diverse microbial life within the hydrologically connected supraglacial, englacial, and subglacial habitats. Microbial cells are collected from the entire glacial ecosystem by seasonally-generated meltwater and exported by proglacial streams. Over the course of the melt season, a subglacial drainage system develops beneath outlet glaciers from the Greenland Ice Sheet (GrIS). This system evolves from an inefficient distributed network to a more efficient channelized pathway. The extent and interconnectivity of the subglacial drainage system with the surface and sediment bed is hypothesized to differ with catchment size.

In this study, we ask whether microbial export from GrIS outlet glacier systems depend on catchment size and whether they evolve with subglacial hydrology over time. We hypothesize that larger catchments will have proportionally greater subglacial drainage, which may be reflected in a greater proportion of subglacial microbes compared to smaller catchments, where the supraglacial inputs might have a higher influence on the exported meltwater. We also expect that changes in assemblage structure are likely to coincide with the evolution of the subglacial drainage system of larger catchments as the season progresses, with supraglacial inputs increasing in importance as the channelized efficient system fully develops. To test these hypotheses, we sampled three outlet glaciers of the GrIS with different catchment sizes (from biggest to smallest: Isunnguata Sermia, Leverett and Russell glaciers) over the 2018 summer. Meltwater samples were taken at the same time each day over a period of three weeks to catch temporal patterns of microbial assemblages. DNA was extracted from samples, and 16S rRNA gene amplicons sequenced to characterize assemblage structure.

This study will help us better understand the meltwater hydrology of the GrIS by describing patterns in its microbial export and the degree of influence from supra- and subglacial systems. In this current age of glacier recession, it is furthermore important to make these characterizations as we might not have opportunity in near future to investigate them in the same unchanged environment.