Abstract
Although cryoconite holes, sediment-filled melt holes on glacier surfaces, appear small and homogenous, their microbial inhabitants may be spatially partitioned. This partitioning could be particularly important for maintaining biodiversity in holes that remain isolated for many years, such as in Antarctica. We hypothesized that cryoconite holes with greater species richness and biomass should exhibit greater partitioning between the sediments and water, promoting greater biodiversity through spatial niche partitioning. We tested this hypothesis by sampling frozen cryoconite holes along a gradient of biomass and biodiversity in the Taylor Valley, Antarctica, where ice-lidded cryoconite holes are a ubiquitous feature of glaciers. We extracted DNA and chlorophyll a from the sediments and water of these samples to describe biodiversity and quantify proxies for biomass. Contrary to our expectation, we found that cryoconite holes with greater richness and biomass showed less partitioning of phylotypes by the sediments versus the water, perhaps indicating that the probability of sediment microbes being mixed into the water is higher from richer sediments. Another explanation may be that organisms from the water were compressed by freezing down to the sediment layer, leaving primarily relic DNA of dead cells to be detected higher in the frozen water. Further evidence of this explanation is that the dominant sequences unique to water closely matched organisms that do not live in cryoconite holes or the Dry Valleys (e.g., vertebrates); so this cryptic biodiversity could represent unknown microbial animals or DNA from atmospheric deposition of dead biomass in the otherwise low-biomass water. Although we cannot rule out spatial niche partitioning occurring at finer scales or in melted cryoconite holes, we found no evidence of partitioning between the sediments and water in frozen holes. Future work should include more sampling of cryoconite holes at a finer spatial scale, and characterizing the communities of the sediments and water when cryoconite holes are melted and active.
Highlights
Cryoconite holes are a unique aquatic environment found on glacier surfaces that support active microbial communities capable of playing a significant role in regional nutrient cycles
Dispersion of community distances differed significantly among comparisons both for bacteria (F = 19, P < 0.001) and eukaryotes (F = 26, P = 0.043), PERMANOVA is robust to heterogeneous dispersions for balanced designs (Anderson and Walsh, 2013)
OTU richness and the concentrations of DNA and chlorophyll a were much greater in the sediments of cryoconite holes than in the water, and the sediments and water contained distinct assemblages of bacteria and microbial eukaryotes
Summary
Cryoconite holes are a unique aquatic environment found on glacier surfaces that support active microbial communities capable of playing a significant role in regional nutrient cycles. They start as sediment patches that accumulate on icy surfaces (McIntyre, 1984). The warmer sediment melts the underlying ice and migrates downward, forming holes of meltwater that deepen quickly to an equilibrium depth, where the rate of deepening is equal to the ablation rate of the ice surface (Gribbon, 1979; Wharton et al, 1985; Fountain et al, 2008). In warm years, they may flush with their accumulated carbon and nitrogen into ice-covered lakes and contribute to increased productivity (Bagshaw et al, 2013)
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