Antarctic cryoconite holes, or surface melt-holes in ice containing sediments, represent a system of isolated mesocosms in which to study community assembly in aquatic ecosystems. The cryoconite holes on the glaciers of the McMurdo Dry Valleys probably form from blown onto the ice from the adjacent valley floor, a polar desert among the driest of terrestrial environments. Microbial communities of the cryoconite holes differ from those of the nearby terrestrial sediments. To characterize their transition from a terrestrial to an aquatic community during cryoconite hole formation, we created cryoconite holes from sediments on the glacier surface and characterized their microbial communities before and after melt-in. During the 2016-2017 austral summer, we redistributed sediments from deposits on the Canada Glacier as smaller patties of sediments that melted into the ice, mimicking the formation of natural cryoconite holes. We sampled six of these experimental holes after one month and another eight after two months by drilling through the ice lid and pipetting or scooping sediments into sterile bags. We used high-throughput sequencing to characterize the communities of bacteria and microbial eukaryotes.
Results/Conclusions
The bacterial communities changed significantly in composition each month. In particular, the dominant cyanobacterium, Nostoc sp. further increased its dominance, while other cyanobacteria, such as Leptolyngbya sp., initially increased then decreased in relative abundance. The putative heterotrophs Hymenobacter sp. and Flavobacterium sp. greatly increased their relative abundance. By contrast, the eukaryotic communities showed slight but nonsignificant signs of a community shift. The community continued to be dominated by green and golden algae, tardigrades, and rotifers, but the relative abundances of some green algae increased and some rotifers decreased. These first stages of a functionally terrestrial soil community transitioning to an aquatic community are consistent with faster rates of bacterial turnover than larger eukaryotes and with communities previously characterized in cryoconite holes. The transition in environment and resulting effect on microbial communities has the potential to provide insight into other terrestrial to aquatic transitions, such as the expected changes as lake levels rise in the McMurdo Dry Valleys, and mechanisms differentiating terrestrial and aquatic microbial communities more generally.