A korarchaeal genome reveals new insights into the evolution of the Archaea

The candidate division Korarchaeota comprises a group of uncultivated microorganisms which, by their small subunit rRNA phylogeny, may have diverged early from the major archaeal phyla Crenarchaeota and Euryarchaeota. Here we report the initial characterization of a member of the Korarchaeota with the proposed name, “Candidatus Korarchaeum cryptofilum” which exhibits an ultra-thin, filamentous morphology. To investigate possible ancestral relationships between deep-branching Korarchaeota and other phyla, we used whole-genome shotgun (WGS) sequencing to construct a complete, composite korarchaeal genome from enriched cells. The genome was assembled into a single contig 1.59 Mb in length with a G+C content of 49%. Of the 1602 predicted protein-coding genes, 1382 (86%) could be assigned to a revised set of archaeal COGs. The predicted gene functions suggest that the organism relies on a simple mode of peptide fermentation for carbon and energy and lacks the ability to synthesize de novo purines, coenzyme A, and several other cofactors. Phylogenetic analyses based on conserved single genes and concatenated protein sequences positioned the korarchaeote as a deep archaeal lineage with an apparent affinity to the Crenarchaeota. However, the predicted gene content revealed that several conserved cellular systems such as cell division, DNA replication, and tRNA maturation resemble the counterparts in the Euryarchaeota. In light of the known composition of archaeal genomes, the Korarchaeota might have retained a set of cellular features that represents the ancestral archaeal form.