Korarchaeia

Taxonomy

"Candidatus Korarchaeia" had been designated as Korarchaeota in the domain, Archaea. They are thought to have diverged relatively early in the genesis of Archaea and are among the deep-branching lineages. They have been classified as phylum (and sometimes as kingdom) Korarchaeota, along with Thaumarchaeota, Aigarchaeota, Crenarchaeota, under the kingdom Thermoproteati. The evolutionary link between Promethearchaeati and Thermoproteati.

The first member of "Candidatus Korarchaeia" to have its genome reconstructed was Korarchaeum cryptofilum, which was found in a hot spring, Obsidian Pool, in Yellowstone National Park and described in 2008. Since then only a few Korarchaeal genomes have been described. To check for "Candidatus Korarchaeia", samples from a variety of hot springs in Iceland and Kamchatka were gathered. According to the samples and analysis, the Icelandic samples contained about 87 distinct 16S ribosomal nucleic acid sequences, whereas the Kamchatkan samples contained about 33.

Based on protein sequences and phylogenetic analysis of conserved single genes, the "Candidatus Korarchaeia" was identified as a "deep archaeal lineage" with a possible relationship to the Crenarchaeota. Furthermore, given the known genetic makeup of archaea, the "Candidatus Korarchaeia" may have preserved a set of biological traits that correspond to the earliest known archaeal form.

Analysis of their 16S rRNA gene sequences suggests that they are a deeply branching lineage that does not belong to the main archaeal groups, Thermoproteota and Euryarchaeota. Analysis of the genome of one "Candidatus Korarchaeia" that was enriched from a mixed culture revealed a number of both Crenarchaeota- and Euryarchaeota-like features and supports the hypothesis of a deep-branching ancestry.

Revision

In 2001, George M. Garrity and John G. Holt described the phylum Crenarchaeota to include a single class Thermoprotei. With the growing number of prokaryotic taxa and the inconsistent nomenclature and classification, there was a need to revise the overall classification of archaea. In 2014, French taxonomists led by David Moreira designated "TACK", generally considered as a superphylum, into a single kingdom named Proteoarchaeota. However, such revision was not valid under the rules of the International Committee on Systematics of Prokaryotes (ICSP). In 2021, a team of Australian scientists led by Christian Rinke and Philip Hugenholtz published a new classification on archaea in which their genetic evidence indicated Thermoproteati archaea belong to a unified phylum Thermoproteota, which could be divided into the classes "Candidatus Korarchaeia", Thermoprotei, Methanomethylicia (former phylum Ca. Verstraetearchaeota), Bathyarchaeia and Nitrososphaeria (former phylum Ca. Thaumachaeota). The List of Prokaryotic names with Standing in Nomenclature (LPSN), an authority on maintenance and cataloguing prokaryotic taxons, upheld this classification of "Candidatus Korarchaeia" as a "preferred name" as a class.

In 2022, the ICSP revised its International Code of Nomenclature of Prokaryotes (ICNP, Prokaryotic Code). Following the revised code, Markus Göker and Aharon Oren (leaders of the ICSP), revised the domain and kingdom classifications 2024 in which the traditionally named "TACK" group was renamed to kingdom Thermoproteati. In this way, previous designations such as phylum Korarchaeota or kingdom Proteoarchaeota are invalidated.