Since glaciers and ice sheets hold about 70% of Earth's freshwater, there is an argument that the microbial processes associated with glacial ice masses comprise Earth's largest freshwater biome. Cryoconite can be described as a microbial "ice-cold hot-spot" within this glacial biome. It occurs as microbial biofilms that trap debris on ice surfaces, forming a darkened microbe-mineral aggregate. A cryoconite-induced reduction in glacier albedo promotes the evolution of depressions called cryoconite holes, and contributes to the release of surface meltwater via the "biological darkening" of glaciers.

Cryoconite is found on glaciers in the polar regions and on mountain glaciers. It has been the subject of renewed interest in the last decade, revealing surprisingly high rates of cryoconite biogeochemical cycling and the biodiversity of the cryoconite ecosystem through microscopy, culturing and the analysis of marker genes.

The last decade has also seen explosive progress in our ability to sequence genomes rapidly, in some cases using genomes directly extracted from environmental samples, a strategy called metagenomics. In September 2010, we sampled every cryoconite hole we could find on Rotmoosferner, a glacier in the Austrian Alps. Ultimately we directly sequenced 27 billion base pairs of cryoconite DNA at Aberystwyth University's Translational Genomics Facility in the UK.

Our cryoconite metagenome revealed a microbial community dominated by bacteria expert in efficiently acquiring and recycling organic carbon and nutrients blown on to the ice surface from elsewhere. The cryoconite metagenome also showed the genes associated with surviving extremes – in particular stress from fluctuating temperatures and water availability. In summary, our analyses supported the notion of Alpine cryoconite ecosystems as recycling "dustbins" capable of enduring the extreme conditions at the ice surface.

While our metagenome presents a singular high-resolution "snapshot" of an Alpine cryoconite ecosystem, we advocate comparative analyses of cryoconite metagenomes to identify interactions with environmental change. This point is borne out by subsequent visits to Rotmoosferner, which reveal glacial wastage rapidly affecting its cryoconite-rich areas.

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