# Thermoproteati

Thermoproteati is a kingdom of archaea. Its synonym, "TACK", is an acronym for Thaumarchaeota (now Nitrososphaerota), Aigarchaeota, Crenarchaeota (now Thermoproteota), and Korarchaeota, the first groups discovered. They are found in different environments ranging from acidophilic thermophiles to mesophiles and psychrophiles and with different types of metabolism, predominantly anaerobic and chemosynthetic. Thermoproteati is a kingdom that is sister to the Asgard branch that gave rise to the eukaryotes. It has been proposed that the Thermoproteati kingdom be classified as "Crenarchaeota" and that the traditional "Crenarchaeota" (Thermoproteota) be classified as a class called "Sulfolobia", along with the other phyla with class rank or order. After including the kingdom category into ICNP, the only validly published name of this group is kingdom Thermoproteati (Guy and Ettema 2024).

## Classification

* Thermoproteota (formerly Crenarchaeota). It is the best known edge and the most abundant archaea in the marine ecosystem. They were previously called sulfobacteria because of their dependence on sulfur and are important as carbon fixers. There are hyperthermophiles in hydrothermal vents and other groups are the most abundant at depths of less than 100 m.
* "Aigarchaeota". It is a phylum proposed from the genome of the candidate species Caldiarchaeum subterraneum found deep within a gold mine in Japan. Genomic sequences of this group have also been found in geothermal environments, both terrestrial and marine.
* "Geoarchaeota". It includes thermophilic organisms that live in acidic environments reducing ferric iron. Alternatively it has been proposed that this and earlier group actually belong to the phylum Nitrososphaerota.
* "Nitrososphaerota" (formerly Thaumarchaeota). It includes mesophilic or psychrophilic organisms (medium and low temperatures), of ammonia-oxidant chemolytoautotrophic metabolism (nitrifying) and that can play an important role in biochemical cycles, such as the nitrogen and carbon cycles.
* "Bathyarchaeota". It is abundant in the sediments of the seabed with a shortage of nutrients. At least some lineages develop through homoacetogenesis, a type of metabolism hitherto thought unique to bacteria.
* "Korarchaeota". They have only been found in hydrothermal environments and in low abundance. They seem diversified at different phylogenetic levels according to temperature, salinity (fresh or marine water) and geography.

## Phylogeny

The relationships are roughly as follows:
| McKay et al. 2019 | 16S rRNA based LTP_06_2022 | 53 marker proteins based GTDB 09-RS220 (24 April 2024) |
| Thermoproteati "Korarchaeota" "Bathyarchaeota" "Aigarchaeota" Nitrososphaerota "Verstraetearchaeota" Thermoproteota "Geoarchaeota" "Marsarchaeota" | Thermoproteati Nitrososphaerota Conexivisphaeria Conexivisphaerales Nitrososphaeria Nitrososphaerales Nitrosopumilales Thermoproteota Thermoproteia Thermoproteales Fervidicoccales Desulfurococcales 2 Desulfurococcales Sulfolobales | Thermoproteati: "JANJXX01" "Panguiarchaeales" "Korarchaeia" "Korarchaeales" "BAT" "Bathyarchaeia" "Bifangarchaeales" [B24] "Hecatellales" [B25] "Xuanwuarculales" [RBG-16-48-13] "Houtuarculales" [40CM-2-53-6] "Wuzhiqiibiales" [TCS64] "Zhuquarculales" [EX4484-135] "Bathyarchaeales" [B26-1] (MCG) Nitrososphaeria_A "Caldarchaeales" Nitrososphaeria "Geothermarchaeales" Conexivisphaerales Nitrososphaerales "Sulfobacteria" "Methanomethylicia" "Nezhaarchaeales" "Culexarchaeles" "Methanomethylicales" ("Verstraetearchaeota") "Thermoproteia" "Gearchaeales" Thermofilales Thermoproteales "Sulfolobia" "Marsarchaeales" Sulfolobales Thermoproteota |

## Eocyte hypothesis

The eocyte hypothesis proposed in the 1980s by James Lake suggests that eukaryotes emerged within the prokaryotic eocytes.
One piece of evidence supporting a close relationship between Thermoproteati and eukaryotes is the presence of a homolog of the RNA polymerase subunit Rbp-8 in Thermoproteota but not in Euryarchaea.