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.

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 controversy

In 2022, the proposed kingdom Methanobacteriati was introduced as a valid name for Euryarchaeota, which was claimed to be taxonomically invalid according to International Code of Nomenclature of Prokaryotes, which gives priority to the first description of euryarchaeal cultivated species/genus (using the systematic suffix -ati for kingdom). This proposal is preferred by LPSN, listing the Euryarchaeota as a not validly published phylum. The name Euryarchaeota is also currently considered as having no standing or validity according to the competitive SeqCode, which accepts descriptions of not cultivated taxa identified from sequence data. 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. Euryarchaeota/Methanobacteriati is not listed as a taxon in the Genome Taxonomy Database (GTDB) applying not the level kingdom, even if it could be identified as a clade (Euryarchaeota s.s.).

Phylogeny

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. A third phylogeny, 53 marker proteins based GTDB 08-RS214.