Euryarchaeota

The euryarchaea (Euryarcheota) or euriotes are a phylum or recently a superphylum of the Archaea domain. It is one of the two main groups of archaea and was originally separated from the other group (Crenarchaeota) based on rRNA sequences. They are simple prokaryotes and encompass a large and diverse set of microorganisms. It has recently been proposed that Euryarchaeota should be considered a superphylum and that some of its classes would comprise separate phyla in their own right, along with other recently discovered Archaean phyla.

Classification

We can distinguish the following groups of euryarchaeotes:

Extreme halophiles

These archaea belong to the class Haloarchaea. They live in extremely saline environments, salt lakes, seawater evaporation pools, saline underground deposits, but now also on the surfaces of highly salted foods such as salted fish and meat. Other halophyte organisms (mushrooms, shrimps, etc.) have normal cytoplasmic salt concentrations, permanently expending energy in carrying salt out of the cell and water inside. These archaea can live at higher salt concentrations and have the advantage of having the same salt concentration inside the cells as outside. Therefore, to survive they need solutions with a minimum of 9% salt, but most species prefer 12-23% and can survive with 32% salt concentrations.

Haloarchaea have the unique characteristic of using light as a source of energy without having chlorophyll. To do this, they have a pigment called bacteriorhodopsin (functioning similar to rhodopsin in the retina of vertebrates) that gives the membrane a purple color and that reacts with light, producing a proton gradient along the membrane that allows them to the synthesis of ATP. It comprises about twenty genres.

Methanogens

This is a large group of archaea that produce methane as part of their metabolism. It includes the classes Methanobacteria, Methanococci, Methanomicrobia and Methanopyri. They are widely distributed in sediments, anoxic swamps, lakes, hydrothermal vents, sewage, and also in the rumen of artiodactyl mammals, intestines of dogs and humans, and in the hindgut (or proctodeum) of insects such as termites and cockroaches. In contrast to their great phylogenetic diversity, they can only use a few simple substrates, most of them being C₁ compounds like H₂/CO₂ >, methanoate, methanol or methylamine, although any specific species is only capable of using two or three of them. Some of them are also hyperthermophilic or acidophilic. It comprises about thirty genres.

Extreme acidophiles

It is a small group of organisms (class Thermoplasmata) that contains the most extreme hyperacidophiles known, Picrophilus, with an optimum pH of 0.7, being able to grow at -0.06 and die at pH values greater than 4, 0. Most are thermophiles, living in sulphurous springs and burning coal piles. Includes the genera Ferroplasma, Picrophilus and Thermoplasma.

Hyperthermophiles

A small group of organisms that live in extremely hot environments such as hydrothermal vents, hot deep-sea sediments, and hot oil reservoirs. It includes the classes Archaeoglobi and Thermococci, plus some methanogens and acidophiles. The optimum growth temperature is above 80 °C, with Pyrococcus supporting a maximum temperature of 105 °C. Most prefer a neutral pH, although Thermococcus alcaliphilus has an optimum pH of 9.0 and a maximum of 10.5. Includes the genera Archaeoglobus, Ferroglobus, Geoglobus (class Archaeoglobi), Palaeococcus, Pyrococcus and Thermococcus (class Thermococci).

Planktonic

Analyzes of rRNA sequences collected in seawater suggest that archaea constitute an important part of oceanic plankton. These genetic sequences have been classified into four groups, the first two being the most widely distributed. Group I is included in the Crenarchaeota, while the remaining three are included in the Euryarchaeota. Group II is more abundant in surface waters and is peripherally related to Thermoplasmata. The other two groups appear to be less abundant. Group III has been found in waters below the photic zone and is also related to Thermoplasmata. Group IV was discovered by Professor Francisco Rodríguez-Valera and his collaborators from the Miguel Hernández University (Spain). It seems that they inhabit the deep waters of the ocean, since they have never been detected on the surface, but rather in the deepest parts of the water column.

Cladograms

The protein phylogenetic analysis also suggests that methanogenesis is very old, since it would be almost 3,500 million years old, obtaining the following cladogram:

A somewhat agreed phylogeny in the GTDB database and the Annotree is the following:

Other phylogenetic analyzes have suggested that the archaea of the DPANN clade also belong to the Euryarchaeota, with the Nanohaloarchaeota phylum being totally separate from the rest. The DPANN clade without Nanohaloarchaeota has been named "Micrarchaea". The following phylogeny summarizes this proposal: