Asgard or Asgardarchaeota is a proposed superphylum consisting of a group of

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

that includes

Lokiarchaeota Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group (DSAG), also known as Marine Benthic Group B (MBG-B). Lokiarchaeota is part of the superphylum Asgard contai ...

Thorarchaeota "''Candidatus'' Thorarchaeota", or simply Thorarchaeota, is a phylum within the superphylum Asgard archaea. The Asgard superphylum represents the closest prokaryotic relatives of eukaryotes. Since there is such a close relation between the two ...

Odinarchaeota Asgard or Asgardarchaeota is a proposed superphylum consisting of a group of archaea that includes Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. It appears the eukaryotes emerged within the Asgard, in a branch containing th ...

, and Heimdallarchaeota. It appears the

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s emerged within the Asgard, in a branch containing the Heimdallarchaeota. This supports the

two-domain system The two-domain system is a biological classification by which all organisms in the tree of life are classified into two big domains, Bacteria and Archaea. It emerged from development in the knowledge of archaea diversity and challenge over the wi ...

of classification over the three-domain system.

Discovery and nomenclature

In the summer of 2010, sediments were analysed from a gravity core taken in the rift valley on the Knipovich ridge in the Arctic Ocean, near the

Loki's Castle Loki's Castle is a field of five active hydrothermal vents in the mid-Atlantic Ocean, located at 73 degrees north on the Mid-Atlantic Ridge between Greenland and Norway at a depth of . The vents were discovered in mid-July 2008 and are the most ...

hydrothermal vent A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...

site. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages were subjected to metagenomic analysis. In 2015, an

Uppsala University Uppsala University ( sv, Uppsala universitet) is a public university, public research university in Uppsala, Sweden. Founded in 1477, it is the List of universities in Sweden, oldest university in Sweden and the Nordic countries still in opera ...

-led team proposed the

phylum based on

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

analyses using a set of

highly conserved In evolutionary biology, conserved sequences are identical or similar sequences in nucleic acids ( DNA and RNA) or proteins across species ( orthologous sequences), or within a genome ( paralogous sequences), or between donor and receptor taxa ( ...

protein-coding genes. Through a reference to the hydrothermal vent complex from which the first genome sample originated, the name refers to

Loki Loki is a god in Norse mythology. According to some sources, Loki is the son of Fárbauti (a jötunn) and Laufey (mentioned as a goddess), and the brother of Helblindi and Býleistr. Loki is married to Sigyn and they have two sons, Narfi or Na ...

, the Norse shape-shifting god. 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", analogous to the role of Lokiarchaeota in the debates about the origin of eukaryotes. In 2016, a

University of Texas The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...

-led team discovered

from samples taken from the

White Oak River The White Oak River is a blackwater river, close to 48 mi (77 km) long, on the coastal plain of southeast North Carolina in the United States. It empties in the Atlantic Ocean. Course It rises in the White Oak Pocosin in northern Onslo ...

in North Carolina, named in reference to

Thor Thor (; from non, Þórr ) is a prominent god in Germanic paganism. In Norse mythology, he is a hammer-wielding æsir, god associated with lightning, thunder, storms, sacred trees and groves in Germanic paganism and mythology, sacred groves ...

, another Norse god. Additional samples from Loki's Castle,

Yellowstone National Park Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellowston ...

Aarhus Bay The Bay of Aarhus, or Aarhus Bay, is a Danish waterway by Aarhus in eastern Jutland. The Bay of Aarhus is bounded by Kalø Vig in the north, Sletterhage and Helgenæs in the east, Samsø and Tunø to the south and the east Jutland coast to the wes ...

, an aquifer near the

Colorado River The Colorado River ( es, Río Colorado) is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and northern Mexico. The river drains an expansive, arid drainage basin, watershed that encompasses parts of ...

, New Zealand's Radiata Pool, hydrothermal vents near Taketomi Island, Japan, and the

estuary in the United States led researchers to discover Odinarchaeota and Heimdallarchaeota, and following the naming convention having been established to use Norse deities, the archaea were named for

Odin Odin (; from non, Óðinn, ) is a widely revered Æsir, god in Germanic paganism. Norse mythology, the source of most surviving information about him, associates him with wisdom, healing, death, royalty, the gallows, knowledge, war, battle, v ...

and

Heimdallr In Norse mythology, Heimdall (from Old Norse Heimdallr) is a god who keeps watch for invaders and the onset of Ragnarök from his dwelling Himinbjörg, where the burning rainbow bridge Bifröst meets the sky. He is attested as possessing forekno ...

, respectively. Researchers therefore named the superphylum containing these microbes “ Asgard”, after the realm of the deities in Norse mythology.

Description

Asgard members encode many eukaryotic signature proteins, including novel GTPases, membrane-remodelling proteins like ESCRT and SNF7, a ubiquitin modifier system, and

N-glycosylation ''N''-linked glycosylation, is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), ...

pathway homologs. Asgard archaeons have a regulated

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

, and the

profilin Profilin is an actin-binding protein involved in the dynamic turnover and reconstruction of the actin cytoskeleton. It is found in all eukaryotic organisms. Profilin is important for spatially and temporally controlled growth of actin microfilame ...

s and

gelsolin Gelsolin is an actin-binding protein that is a key regulator of actin filament assembly and disassembly. Gelsolin is one of the most potent members of the actin-severing gelsolin/villin superfamily, as it severs with nearly 100% efficiency. Cellu ...

s they use can interact with eukaryotic actins. In addition, Asgard archaea tubulin 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 to FtsZ, indicating that OdinTubulin may represent an evolution intermediate between FtsZ and

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...

-forming tubulins. They also seem to form vesicles under cryogenic electron microscopy. Some may have a PKD domain S-layer. They also share the three-way ES39 expansion in LSU rRNA with eukaryotes.

Metabolism

Asgard archaea Phyla.png, Metabolic pathways of Asgard archaea, variation by Phyla Asgard archaea in various environments.png, Metabolic pathways of Asgard archaea, variation by environment Asgard archaea are generally obligate anaerobes, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be facultative aerobes. They have a

Wood–Ljungdahl pathway The Wood–Ljungdahl pathway is a set of biochemical reactions used by some bacteria. It is also known as the reductive acetyl-coenzyme A (Acetyl-CoA) pathway. This pathway enables these organisms to use hydrogen as an electron donor, and carb ...

and perform

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

. Members can be autotrophs,

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

s, or

phototroph Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconcep ...

s using

heliorhodopsin Heliorhodopsin is a family of rhodopsins discovered in 2018 by Alina Pushkarev in the laboratory of Professor Oded Beja. The new family of heliorhodopsins has a distinct protein sequence from known Type 1 (microbial) and Type 2 (animal) rhodopsi ...

. One member, ''Candidatus'' ''Prometheoarchaeum syntrophicum'', is

syntrophic In biology, syntrophy, synthrophy, or cross-feeding (from Greek ''syn'' meaning together, ''trophe'' meaning nourishment) is the phenomenon of one species feeding on the metabolic products of another species to cope up with the energy limitations by ...

with a sulfur-reducing proteobacteria and a

methanogenic Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the Domain (biology), domai ...

archaea. The RuBisCO they have is not carbon-fixing, but likely used for nucleoside salvaging.

Eukaryotic-like features in subdivisions

The phylum "Heimdallarchaeota" was found in 2017 to have N-terminal core histone tails, a feature previously thought to be exclusively eukaryotic. Two other archaeal phyla, both outside of Asgard, were found to also have tails in 2018. In January 2020, scientists found ''Candidatus'' ''Prometheoarchaeum syntrophicum'', a member of the Lokiarcheota, engaging in

cross-feeding In biology, syntrophy, synthrophy, or cross-feeding (from Greek ''syn'' meaning together, ''trophe'' meaning nourishment) is the phenomenon of one species feeding on the metabolic products of another species to cope up with the energy limitations by ...

with two bacterial species. Drawing an analogy to symbiogenesis, they consider this relationship a possible link between the simple prokaryotic microorganisms and the complex

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

microorganisms occurring approximately two billion years ago.

Classification

The phylogenetic relationship of this group is still under discussion.

Taxonomy

In the depicted scenario, the Eukaryota are deep in the tree of Asgard, and any of the Eukaryota is significantly closer related to the Heimdalarchaeota than e.g. the Lokiarchaeota are. Some authors have suggested splitting Heimdallarchaeota into multiple groups (Hodarchaeota, Gerdarchaeota, Kariarchaeota and Heimdallarchaeota). Eukaryotes may be sister to the previous four groups and Wukongarchaeota, or to the entire Asgard archaea group. A favored scenario is syntrophy, where one organism depends on the feeding of the other. In this case, the syntrophy may have been due to the Asgard archaea having been incorporated in an unknown type of bacteria, developing into the nucleus. An α-proteobacterium was incorporated to become the

mitochondrion A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

. The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and

National Center for Biotechnology Information The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. The ...

(NCBI). * Phylum ?" Freyrarchaeota" Caceres 2019 ex Xie et al. 2021 * Phylum ?" Friggarchaeota" Caceres 2019 * Phylum ?" Gefionarchaeota" Caceres 2019 * Phylum ?" Idunnarchaeota" Caceres 2019 * Phylum ?" Njordarchaeota" Xie et al. 2021 * Phylum ?" Sigynarchaeota" Xie et al. 2021 * Phylum ?" Tyrarchaeota" Xie et al. 2021 * Class "Sifarchaeia" Sun et al. 2021 ** Order "Borrarchaeales" Liu et al. 2020 *** Family "Borrarchaeaceae" Liu et al. 2020 **** "''Candidatus'' Borrarchaeum" Liu et al. 2020 ** Order "Sifarchaeales" Sun et al. 2021 *** Family "Sifarchaeaceae" Sun et al. 2021 **** "''Candidatus'' Sifarchaeum" corrig. Farag, Zhao & Biddle 2020 * Class "Wukongarchaeia" Liu et al. 2020 ** Order "Wukongarchaeales" Liu et al. 2020 *** Family "Wukongarchaeaceae" Liu et al. 2020 **** "''Candidatus'' Wukongarchaeum" Liu et al. 2020 * Class " Heimdallarchaeia" ** Order " Gerdarchaeales" (JABLTI01) ** Order " Heimdallarchaeales" ** Order "Hodarchaeales" Liu et al. 2020 *** Family "Hodarchaeaceae" Liu et al. 2020 **** "''Candidatus'' Hodarchaeum" Liu et al. 2020 ** Order "Kariarchaeales" Liu et al. 2020 *** Family "Kariarchaeaceae" Liu et al. 2020 **** "''Candidatus'' Kariarchaeum" Liu et al. 2020 * Class "Jordarchaeia" Sun et al. 2021 ** Order "Jordarchaeales" Sun et al. 2021 *** Family "Jordarchaeaceae" Sun et al. 2021 **** "''Candidatus'' Jordarchaeum" Sun et al. 2021 * Class "

Odinarchaeia Asgard or Asgardarchaeota is a proposed superphylum consisting of a group of archaea that includes Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. It appears the eukaryotes emerged within the Asgard, in a branch containing ...

" Tamarit et al. 2022 ** Order "Odinarchaeales" Tamarit et al. 2022 *** Family "Odinarchaeaceae" Tamarit et al. 2022 **** "Candidatus Odinarchaeum" Tamarit et al. 2022 * Class "Baldrarchaeia" Liu et al. 2020 ** Order "Baldrarchaeales" Liu et al. 2020 *** Family "Baldrarchaeaceae" Liu et al. 2020 **** "Candidatus Baldrarchaeum" Liu et al. 2020 * Class "Thorarchaeia" ** Order "Thorarchaeales" *** Family " Thorarchaeaceae" (MBG-B) * Class "Hermodarchaeia" Liu et al. 2020 ** Order "Hermodarchaeales" Liu et al. 2020 *** Family "Hermodarchaeaceae" Liu et al. 2020 **** "''Candidatus'' Hermodarchaeum'' Hermodarchaeum''" Liu et al. 2020 * Class "

Lokiarchaeia Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group (DSAG), also known as Marine Benthic Group B (MBG-B). Lokiarchaeota is part of the superphylum Asgard contai ...

" corrig. Spang et al. 2015 ** Order " Helarchaeales" ** Order "Lokiarchaeales" Spang et al. 2015 *** Family "Lokiarchaeaceae" Vanwonterghem et al. 2016 **** "''Candidatus''

Lokiarchaeum Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group (DSAG), also known as Marine Benthic Group B (MBG-B). Lokiarchaeota is part of the superphylum Asgard (archa ...

''" corrig. Spang et al. 2015 (MBGB, DSAG) *** Family "MK-D1" **** "''Candidatus'' Promethearchaeum'' Promethearchaeum''" corrig. Imachi, Nobu & Takai 2020

Asgard archaeal mobilome

Viruses

Several family-level groups of viruses associated with Asgard archaea have been discovered using metagenomics. 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 are related to archaeal and bacterial viruses of the class ''

Caudoviricetes ''Caudovirales'' is an order of viruses known as the tailed bacteriophages (''cauda'' is Latin for "tail"). Under the Baltimore classification scheme, the ''Caudovirales'' are group I viruses as they have double stranded DNA (dsDNA) genomes ...

'', i.e., viruses with icosahedral capsids and helical tails; two other distinct groups are distantly related to tailless archaeal and bacterial viruses with icosahedral capsids of the realm '' Varidnaviria''; and the third group of viruses is related to archaea-specific viruses with lemon-shaped virus particles. The viruses have been identified in deep-sea sediments and a terrestrial hot spring of the Yellowstone National Park. All these viruses display very low sequence similarity to other known viruses but are generally related to the previously described prokaryotic viruses, with no meaningful affinity to viruses of eukaryotes.

Mobile genetic elements

In addition to viruses, several groups of cryptic mobile genetic elements have been discovered through CRISPR spacer matching to be associated with Asgard archaea of the Lokiarchaeia, Thorarchaeia and Heimdallarchaeia lineages. These mobile elements do not encode recognizable viral hallmark proteins and could represent either novel types of viruses or plasmids.

References

External links

* Traci Watson
The trickster microbes that are shaking up the tree of life
in:

Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...

, 14 May 2019 {{Taxonbar, from=Q45003302 Archaea Superphyla