cryoconite hole metabarcoding Targeted loci environmental

Although cryoconite holes, sediment-filled melt holes on glacier surfaces, may appear small and homogenous, they are large relative to their microbial inhabitants and offer opportunities for spatial partitioning of their environment. Vertical partitioning by microbial communities at the water-sediment interface is a common characteristic of all bodies of water, including freezing lakes. Yet the relatively large size and inherent ecological variability among lakes makes the study of vertical partitioning challenging. But the small size and spatial concentration of cryoconite holes provide a compelling model system in which to study microbial communities in relatively simplified aquatic ecosystems. To determine the degree to which vertical partitioning occurs in cryoconite holes, we compared microbial communities from sediment and water columns of frozen cryoconite holes in Taylor Valley, Antarctica. Specifically, 30 frozen cryoconite holes were sampled on each of three glaciers along a gradient of biomass and richness in the valley: Taylor, Canada, and Commonwealth Glaciers. DNA was extracted separately from the bottom sediment (cryoconite) and frozen overlaying water column (ice) of each sample. Both bacterial and eukaryotic OTU richness was substantially higher in the sediment than the water column. Although we expected water column microbial communities to be dilute versions of those found in the sediments, only the most abundant sediment species were detected in the water column. Furthermore, water column communities contained unique sequences, which were primarily from organisms that do not live in cryoconite holes or the Dry Valleys (e.g. fish), and which likely represent DNA from atmospheric deposition in the low biomass water columns. Additional indices of microbial abundance and biomass, specifically dissolved organic matter, chlorophyll a, and total DNA, were at least an order of magnitude higher within the sediments than the water columns. These observations from frozen cryoconite holes, combined with previous observations of melted holes, suggest that there may be a seasonal component to vertical partitioning that could be driven by both biological (e.g. seasonal migration) or physical (e.g. gravitational settling or freeze concentration). Confirming the vertical partitioning of microbial communities within cryoconite holes provides an opportunity to further investigate aspects of niche partitioning, such as impacts to biogeochemical cycling, within an easily replicable system.