Metagenome-assembled genome of the glacier alga Ancylonema yields insights into the evolution of streptophyte life on ice and land

Contemporary glaciers and ice sheets are home to communities of streptophyte algae that balance their requirements for photosynthesis and growth with tolerance of low temperature, desiccation and UV radiation. These same environmental challenges have been hypothesized as the driving force behind the evolution of land plants from streptophyte algal ancestors in the Cryogenian (720–635 million years ago).

We sequenced, assembled and analyzed the metagenome-assembled genome of the glacier alga Ancylonema nordenskiöldii to investigate its adaptations to life in ice, and whether this represents a vestige of Cryogenian exaptations.

Phylogenetic analysis confirms the placement of glacier algae within the sister lineage to land plants, Zygnematophyceae. The metagenome-assembled genome is characterized by an expansion of genes involved in tolerance of high irradiance and UV light, while lineage-specific diversification is linked to the novel screening pigmentation of glacier algae. We found no support for the hypothesis of a common genomic basis for adaptations to ice and to land in streptophytes. Comparative genomics revealed that the reductive morphological evolution in the ancestor of Zygnematophyceae was accompanied by reductive genome evolution.

This first genome-scale data for glacier algae suggests an Ancylonema-specific adaptation to the cryosphere, and sheds light on the genome evolution of land plants and Zygnematophyceae.