# Korarchaeota

The Korarchaeota is a proposed phylum within the Archaea. The name is derived from the Greek noun koros or kore, meaning young man or young woman, and the Greek adjective archaios which means ancient. They are also known as Xenarchaeota. The name is equivalent to Candidatus Korarchaeota, and they go by the name Xenarchaeota or Xenarchaea as well.

## Taxonomy

The Korarchaeota are a proposed phylum in the domain, Archaea. They are thought to have diverged relatively early in the genesis of Archaea and are among the deep-branching lineages. Korarchaeota are also known as Xenarchaeota. Korarchaeaota, along with Thaumarchaeota, Aigarchaeota, Crenarchaeota, belong to the superphylum called TACK. The evolutionary link between Asgard archaea and Korarchaeota of TACK (Thaumarchaeota, Aigarchaeota, Crenarchaeota, Korarchaeota) is yet unknown.
The first member of Korarchaeota to have its genome reconstructed was Korarchaeum crypotfilum, which was found in a hot spring in Yellowstone National Park and described in 2008. Since then only a few Korarchaeal genomes have been described. To check for Korarchaeota, 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 Korarchaeote was identified as a “deep archaeal lineage” with a possible relationship to the Crenarchaeota. Furthermore, given the known genetic makeup of archaea, the Korarchaeota 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 korarchaeote 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.

### Species

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).
Phylogeny of "Korarchaeia" :
"Korarchaeia"
"Panguiarchaeales"
"Panguiarchaeaceae"
"Panguiarchaeum symbiosum"
"Korarchaeales"
"Korarchaeaceae"
"Ca. Korarchaeum cryptofilum"
"Ca. Methanodesulfokores washburnensis" :
Listed below are the known species of "Korarchaeota"
* Order "Panguiarchaeales" Qu et al. 2023
** Family "Panguiarchaeaceae" Qu et al. 2023
*** Genus "Panguiarchaeum" Qu et al. 2023
**** Species "Panguiarchaeum symbiosum" Qu et al. 2023
* Order "Korarchaeales" Petitjean et al. 2015
** Family "Korarchaeaceae" Rinke et al. 2020
*** Genus "Candidatus Korarchaeum" Elkins et al. 2008
**** Species "Ca. Korarchaeum cryptofilum" Elkins et al. 2008
***** "Ca. Korarchaeum cryptofilum" OPF8
*** Genus "Candidatus Methanodesulfokores" corrig. McKay et al. 2019
**** Species "Ca. Methanodesulfokores washburnensis" corrig. McKay et al. 2019
*** Genus Candidatus Korarchaeota archaeon
*** Genus Candidatus Korarchaeota archaeon NZ13-K
*** Genus Korarchaeote SRI-306
*** Genus environmental samples
**** uncultured korarchaeote pBA5
**** uncultured korarchaeote pJP27
**** uncultured korarchaeote pJP78

## Reference species

A strain of Korarchaeum cryptofilum was cultivated from an enrichment culture from a hot spring in Yellowstone National Park, USA and described in 2008. The cells are long and needle-shaped, which gave the species its name, alluding to its "cryptical filaments". This organism lacks the genes for purine nucleotide biosynthesis and thus relies on environmental sources to meet its purine requirements.

## Characteristics

Korarchaeota are a proposed phylum within the domain, Archaea, and therefore exhibit characteristics such as having a cell wall without peptidoglycan, as well as lipid membranes that are ether-linked. They have a surface layer of paracrystalline protein. This surface layer, known as the S-layer, is densely packed and consists of 1-2 proteins form various lattice structures and are most likely what maintains the cells’ structural integrity. They are typically rod-shaped, however, it has been found that this morphology can change to be thicker-shaped in the presence of higher sodium dodecyl sulfate (SDS) concentrations. Korarchaeota cells have an ultrathin filamentous morphology that may vary in length. They typically average 15 μm in length and 0.16 μm in diameter but can be seen up to 100 μm long. Some Archaea can fix carbon dioxide through the 3-hydroxypropionate/4-hydroxybutyrate pathway into organic compounds

## Ecology

Korarcheota have only been found in hydrothermal environments ranging from terrestrial, including hot springs to marine, including shallow hydrothermal vents and deep-sea hydrothermal vents. Previous research has shown greater diversity of Korarchaea found in terrestrial hot springs compared to marine environments. Korarchaeota have been found in nature in only low abundances. Korarcheota likely originated in marine environments and then adapted to terrestrial ones.
Geographically, Korarcheota have been found in a variety of locations around the world including Japan, Yellowstone National Park, the Gulf of California, Iceland and Russia.
Korarchaeota are thermophiles, having been found living in conditions of up to 128 degrees Celsius. The lowest temperature they have been found in is 52 degrees Celsius. While they have frequently been observed living in acidic conditions, they have also been found living in conditions up to a pH of 10.
Researchers have identified a virus that can potentially infect Korarcheota.