After including the kingdom category intoICNP, the onlyvalidly published name of this group is kingdomPromethearchaeati, containing only onephylumPromethearchaeota. All formerly proposed "phyla" would be de-ranked toclasses in this framework.[7]
In the summer of 2010, sediments were analysed from a gravitycore taken in the rift valley on the Knipovich ridge in the Arctic Ocean, near theLoki's Castlehydrothermal vent site. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages were subjected tometagenomic analysis.[8][9] In 2015, anUppsala University-led team proposed theLokiarchaeota phylum based onphylogenetic analyses using a set ofhighly conserved protein-coding genes.[10] The group was named for the shape-shifting Norse godLoki, in an allusion to the hydrothermal vent complex from which the first genome sample originated.[11] The Loki of mythology has been described as "a staggeringly complex, confusing, and ambivalent figure who has been the catalyst of countless unresolved scholarly controversies",[12] analogous to the role of Lokiarchaeota in the debates about the origin of eukaryotes.[10][13]
In 2016, aUniversity of Texas-led team discoveredThorarchaeota from samples taken from theWhite Oak River in North Carolina, named in reference toThor, another Norse god.[14] Samples from Loki's Castle,Yellowstone National Park,Aarhus Bay, an aquifer near theColorado River, New Zealand'sRadiata Pool, hydrothermal vents nearTaketomi Island, Japan, and theWhite Oak River estuary in the United States contained Odinarchaeota and Heimdallarchaeota;[2] following the Norse deity naming convention, these groups were named forOdin andHeimdall respectively. Researchers therefore named the group containing these microbes "Asgard", after the home of the gods in Norse mythology.[2] Two Lokiarchaeota specimens have been cultured, enabling a detailed insight into their morphology.[15]
Promethearchaeati members encode many eukaryotic signature proteins, including novelGTPases, membrane-remodelling proteins likeESCRT andSNF7, aubiquitin modifier system, andN-glycosylation pathway homologs.[2]
Promethearchaeati archaeons have a regulatedactincytoskeleton, and theprofilins andgelsolins they use can interact with eukaryotic actins.[16][17] In addition, Promethearchaeati archaeatubulin from hydrothermal-living Odinarchaeota (OdinTubulin) was identified as a genuine tubulin. OdinTubulin forms protomers and protofilaments most similar to eukaryotic microtubules, yet assembles into ring systems more similar toFtsZ, indicating that OdinTubulin may represent an evolution intermediate between FtsZ andmicrotubule-forming tubulins.[18] They also seem to form vesicles undercryogenic electron microscopy. Some may have aPKD domainS-layer.[19] They also share the three-way ES39 expansion inLSU rRNA with eukaryotes.[20] Gene clusters or operons encoding ribosomal proteins are often less conserved in their organization in the Promethearchaeati kingdom than in other archaea, suggesting that the order of ribosomal protein coding genes may follow thephylogeny.[21]
Promethearchaeati are widely distributed around the world, both geographically and by habitat. Many of the known clades are restricted to sediments, whereas Lokiarchaeota, Thorarchaeota and another clade occupy many different habitats. Salinity and depth are important ecological drivers for most Promethearchaeati archaea. Other habitats include the bodies of animals, the rhizosphere of plants, non-saline sediments and soils, the sea surface, and freshwater. In addition, Promethearchaeati are associated with several other microorganisms.[24]
The phylum Heimdallarchaeota was found in 2017 to have N-terminal corehistone tails, a feature previously thought to be exclusively eukaryotic. Two other archaeal phyla, both outside of Promethearchaeati, were found to also have tails in 2018.[25]
In January 2020, scientists foundCandidatusPrometheoarchaeum syntrophicum, a member of the Lokiarcheota, engaging incross-feeding with two bacterial species. Drawing an analogy tosymbiogenesis, they consider this relationship a possible link between the simpleprokaryotic microorganisms and the complexeukaryotic microorganisms occurring approximately two billion years ago.[26][19]
The phylogenetic relationships of the Promethearchaeati archaea have been studied by several teams in the 21st century.[4][3][27][23] Varying results have been obtained, for instance using 53 marker proteins from theGenome Taxonomy Database.[28][29][30] In 2023, Eme, Tamarit, Caceres and colleagues reported that the Eukaryota are deep within Promethearchaeati, as sister of Hodarchaeales within the Heimdallarchaeota.[31]
In the depicted scenario, the Eukaryota are deep in the tree of Promethearchaeati. A favored scenario is syntrophy, where one organism depends on the feeding of the other. Anα-proteobacterium was incorporated to become themitochondrion.[33] In culture, extant Promethearchaeati archaea form various syntrophic dependencies.[34] Gregory Fournier and Anthony Poole have proposed that Promethearchaeati is part of "the Eukaryote tree", forming a superphylum they call "Eukaryomorpha" defined by "shared derived characters" (eukaryote signature proteins).[35]
Severalfamily-level groups ofviruses associated with Promethearchaeati archaea have been discovered using metagenomics.[39][40][41] The viruses were assigned to Lokiarchaeia, Thorarchaeia, Odinarchaeia and Helarchaeia hosts using CRISPR spacer matching to the corresponding protospacers within the viral genomes. Two groups of viruses (called 'verdandiviruses') are related to archaeal and bacterial viruses of the classCaudoviricetes, i.e., viruses with icosahedral capsids and helical tails;[39][41] two other distinct groups (called 'skuldviruses') are distantly related to tailless archaeal and bacterial viruses with icosahedral capsids of the realmVaridnaviria;[39][40] and the third group of viruses (calledwyrdviruses) is related to archaea-specific viruses with lemon-shaped virus particles (familyHalspiviridae).[39][40] The viruses have been identified in deep-sea sediments[39][41] and a terrestrial hot spring of the Yellowstone National Park.[40] All these viruses display very low sequence similarity to other known viruses but are generally related to the previously described prokaryotic viruses,[42] with no meaningful affinity to viruses of eukaryotes.[43][39]
In addition to viruses, several groups of crypticmobile genetic elements have been discovered throughCRISPR spacer matching to be associated with Promethearchaeati archaea of the Lokiarchaeia, Thorarchaeia and Heimdallarchaeota lineages.[39][44] These mobile elements do not encode recognizable viral hallmark proteins and could represent either novel types of viruses or plasmids.
^abEme, Laura; Spang, Anja; Lombard, Jonathan; Stairs, Courtney W.; Ettema, Thijs J. G. (November 2017). "Archaea and the origin of eukaryotes".Nature Reviews. Microbiology.15 (12):711–723.doi:10.1038/nrmicro.2017.133.PMID29123225.S2CID8666687.
^Nobs, Stephanie-Jane; MacLeod, Fraser I.; Wong, Hon Lun; Burns, Brendan P. (May 2022). "Eukarya the chimera: eukaryotes, a secondary innovation of the two domains of life?".Trends in Microbiology.30 (5):421–431.doi:10.1016/j.tim.2021.11.003.PMID34863611.S2CID244823103.
![Metabolic pathways of Promethearchaeati archaea, varying by phyla[22]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAsgard_archaea_Phyla_(cropped).png)
![Metabolic pathways of Promethearchaeati archaea, varying by environment[22]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAsgard_archaea_in_various_environments_(cropped).png)
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