# Euryarchaeota

Euryarchaeota (from Ancient Greek εὐρύς eurús, "broad, wide") is a kingdom of archaea. Euryarchaeota are highly diverse and include methanogens, which produce methane and are often found in intestines; halobacteria, which survive extreme concentrations of salt; and some extremely thermophilic aerobes and anaerobes, which generally live at temperatures between 41 and 122 °C. They are separated from the other archaeans based mainly on rRNA sequences and their unique DNA polymerase. The only validly published name for this group under the Prokaryotic Code is Methanobacteriati.

## Description

The Euryarchaeota are diverse in appearance and metabolic properties. The phylum contains organisms of a variety of shapes, including both rods and cocci. Euryarchaeota may appear either gram-positive or gram-negative depending on whether pseudomurein is present in the cell wall. Euryarchaeota also demonstrate diverse lifestyles, including methanogens, halophiles, sulfate-reducers, and extreme thermophiles in each. Others live in the ocean, suspended with plankton and bacteria. Although these marine euryarchaeota are difficult to culture and study in a lab, genomic sequencing suggests that they are motile heterotrophs.
Though it was previously thought that euryarchaeota only lived in extreme environments (in terms of temperature, salt content and/or pH), a paper by Korzhenkov et al. published in January 2019 showed that euryarchaeota also live in moderate environments, such as low-temperature acidic environments. In some cases, euryarchaeota outnumbered the bacteria present. Euryarchaeota have also been found in other moderate environments such as water springs, marshlands, soil and rhizospheres. Some euryarchaeota are highly adaptable; an order called Halobacteriales are usually found in extremely salty and sulfur-rich environments but can also grow in salt concentrations as low as that of seawater 2.5%. In rhizospheres, the presence of euryarchaeota seems to be dependent on that of mycorrhizal fungi; a higher fungal population was correlated with higher euryarchaeotal frequency and diversity, while absence of mycorrihizal fungi was correlated with absence of euryarchaeota.

## Nomenclatural status

Under the International Code of Nomenclature of Prokaryotes, there has been no (and cannot be any) valid name for this group, as it exceeds the level of a phylum. In 2024, the Code was emeded to included the levels of kingdom and domain. In the same year, the name Methanobacteriati was validly published for this group, making it the first and only "valid name" under the Code. The LPSN, which aligns itself with the Code, adopts this view and lists the earlier Euryarchaeota as a invalidly published phylum.
The competing SeqCode for uncultivated taxa also has no level above phylum, hence "Euryarchaeota" also has no standing there. The Methanobacteriati name has no standing either: SeqCode only automatically accepts a name made under the prokaryotic Code if it's older than 2023.
Euryarchaeota was listed in National Center for Biotechnology Information (NCBI) taxonomy browser as a current name for phylum (Euryarchaeota Garrity and Holt 2002) till September 2024, considering Methanobacteriota as heterotypic synonym. From October 2024 the names Methanobacteriati for kingdom and Halobacteriota, Methanobacteriota and Thermoplasmatota for included phyla are listed.
The taxon Euryarchaeota is also listed in the Bergey's Manual of Systematics of Archaea and Bacteria, but this is because the latest version of the chapter was published in 2017.
Euryarchaeota/Methanobacteriati is not listed as a taxon in the Genome Taxonomy Database (GTDB), as the algorithm does not generate kingdoms.

## Phylogeny

16S rRNA based LTP_12_2021. :
Archaea
Methanopyri
TACK
Nitrososphaerota
Thermoproteota
Euryarchaeota
Methanococci
Thermoplasmata
Archaeoglobi
Thermococci
Methanobacteria
Methanonatronarchaeia
Methanomicrobia
Halobacteria :
Other phylogenetic analyzes have suggested that the archaea of the clade DPANN may also belong to Euryarchaeota and that they may even be a polyphyletic group occupying different phylogenetic positions within Euryarchaeota. It is also debated whether the phylum Altiarchaeota should be classified in DPANN or Euryarchaeota. A cladogram summarizing this proposal is graphed below. The groups marked in quotes are lineages assigned to DPANN, but phylogenetically separated from the rest.
| Dombrowski et al. 2019, Jordan et al. 2017 and Cavalier-Smith 2020. | 53 marker proteins based GTDB 09-RS220. |
| Archaea Euryarchaeota Thermococci Hadesarchaea Methanobacteria Methanopyri Methanococci Thermoplasmata Archaeoglobi Methanomicrobia "Nanohaloarchaeota" Haloarchaea "Altiarchaeota" DPANN Diapherotrites Micrarchaeota Undinarchaeota Aenigmarchaeota Nanoarchaeota Parvarchaeota Mamarchaeota Pacearchaeota Woesearchaeota Proteoarchaeota TACK Asgard Lokiarchaeota Odinarchaeota Thorarchaeota Heimdallarchaeota (+α─Proteobacteria) Eukaryota | Archaea "Undinarchaeota" "Huberarchaeaota" "Aenigmarchaeota" "Nanohalarchaeota" Nanobdellota "Altarchaeota" "Iainarchaeota" "Micrarchaeota" Methanobacteriota_A "Methanococcia" "Hadarchaeota" "Hadarchaeia" Methanobacteriota_B "Thermococcia" "Methanomada" "Hydrothermarchaeota" "Hydrothermarchaeia" Methanobacteriota "Methanopyria" "Methanobacteriia" "Neoeuryarchaeota" Thermoplasmatota "Izemarchaea" (MBG-D, E2) "Poseidoniia" (MGII & MGIII) "Thermoplasmatia" Halobacteriota "Methanomicrobia" "Methanoliparia" "Archaeoglobia" "Syntropharchaeia" Methanocellia Methanosarcinia Methanonatronarchaeia Halobacteria "Proteoarchaeota" Promethearchaeati Promethearchaeota Thermoproteati Thermoproteota DPANN/ [Nanobdellati] "Euryarchaeota"/ [Methanobacteriati] |