The High Arctic is dominated by uncharacterized, genetically highly diverse bacteriophages

While the Earth's virosphere is estimated to be in the range of 10^31 viral particles, the vast majority of its diversity has yet to be discovered. In recent years, metagenomics has rapidly allowed the identification of viruses, from microenvironments to extreme environments like the High Arctic. However, the High Arctic virome is largely composed of viral sequences that have few, if any, matches to classified viruses in existing databases. Here, to bypass limitations posed by similarity-based strategies, we resorted to a metagenomics approach that placed viral genes found in Lake Hazen, a High Arctic lake, in a phylogenetic context with known viruses. We show that while High Arctic viruses clustered with known bacteriophages, they have undergone unique evolutionary processes characterized by high evolutionary rates, making them distinct from and more diverse than known viruses. A better understanding of how viruses from extreme polar conditions adapt and evolve could help us gain insights on the viral response to climate change and other environmental stressors.