Asgard or Asgardarchaeota is a proposed superphylum consisting of a group of archaea 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, Odinarchaeota, and Heimdallarchaeota. It appears the eukaryotes 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 w ...

of classification over the

three-domain system The three-domain system is a biological classification introduced by Carl Woese, Otto Kandler, and Mark Wheelis in 1990 that divides cellular life forms into three domains, namely Archaea, Bacteria, and Eukaryota or Eukarya. The key difference ...

Discovery and nomenclature

In the summer of 2010, sediments were analysed from a gravity

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

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 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 research university in Uppsala, Sweden. Founded in 1477, it is the oldest university in Sweden and the Nordic countries still in operation. The university rose to significance during ...

-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 protein-coding genes. Through a reference to the hydrothermal vent complex from which the first genome sample originated, the name refers to Loki, 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 Thorarchaeota from samples taken from the White Oak River 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 god associated with lightning, thunder, storms, sacred groves and trees, strength, the protection of humankind, hallowing, an ...

, 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 Yellowst ...

, Aarhus Bay, 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 watershed that encompasses parts of seven U.S. s ...

, New Zealand's

Radiata Pool Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of animals that do not form a monophyletic ...

, hydrothermal vents near

Taketomi Island is an island in the town of Taketomi, within Yaeyama District of Okinawa Prefecture, Japan. Taketomi is one of the Yaeyama Islands. The population of Taketomi Island was 323 as of January 2012. Geography Taketomi Island is located south of I ...

, Japan, and the White Oak River 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 and Heimdallr, respectively. Researchers therefore named the superphylum containing these microbes “

Asgard In Nordic mythology, Asgard (Old Norse: ''Ásgarðr'' ; "enclosure of the Æsir") is a location associated with the gods. It appears in a multitude of Old Norse sagas and mythological texts. It is described as the fortified home of the Æsir ...

”, after the realm of the deities in Norse mythology.

Description

Asgard members encode many

eukaryotic signature protein 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. Bacter ...

s, including novel GTPases, membrane-remodelling proteins like

ESCRT The endosomal sorting complexes required for transport (ESCRT) machinery is made up of cytosolic protein complexes, known as ESCRT-0, ESCRT-I, ESCRT-II, and ESCRT-III. Together with a number of accessory proteins, these ESCRT complexes enable a uni ...

and SNF7, a

ubiquitin Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...

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 ov ...

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 com ...

, and the profilins 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. Cell ...

s they use can interact with eukaryotic actins. In addition, Asgard archaea

tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...

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 FtsZ is a protein encoded by the ''ftsZ'' gene that assembles into a ring at the future site of bacterial cell division (also called the Z ring). FtsZ is a prokaryotic homologue of the eukaryotic protein tubulin. The initials FtsZ mean "Filam ...

, indicating that OdinTubulin may represent an evolution intermediate between FtsZ and microtubule-forming tubulins. They also seem to form vesicles under

cryogenic electron microscopy Cryogenic electron microscopy (cryo-EM) is a cryomicroscopy technique applied on samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An aqueous sample so ...

. Some may have a PKD domain

S-layer An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) ...

. They also share the three-way ES39 expansion in

LSU rRNA Large subunit ribosomal ribonucleic acid (LSU rRNA) is the largest of the two major RNA components of the ribosome. Associated with a number of ribosomal proteins, the LSU rRNA forms the large subunit of the ribosome. The LSU rRNA acts as a rib ...

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 anaerobe Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environme ...

s, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be facultative aerobes. They have a Wood–Ljungdahl pathway and perform glycolysis. Members can be

autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Wo ...

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. One member, ''Candidatus'' ''Prometheoarchaeum syntrophicum'', is syntrophic with a sulfur-reducing proteobacteria and a methanogenic archaea. The

RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...

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 with two bacterial species. Drawing an analogy to

symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and pos ...

, they consider this relationship a possible link between the simple

prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

microorganisms and the complex

eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, 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 ...

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. The currently accepted taxonomy is based on the

List of Prokaryotic names with Standing in Nomenclature List of Prokaryotic names with Standing in Nomenclature (LPSN) is an online database that maintains information on the nomenclature, naming and Taxonomy (biology), taxonomy of prokaryotes, following the taxonomy requirements and rulings of the In ...

(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" 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" corrig. Spang et al. 2015 ** Order " Helarchaeales" ** Order "Lokiarchaeales" Spang et al. 2015 *** Family "Lokiarchaeaceae" Vanwonterghem et al. 2016 **** "''Candidatus'' Lokiarchaeum'' Lokiarchaeum''" 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'', 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 ''Varidnaviria'' is a realm of viruses that includes all DNA viruses that encode major capsid proteins that contain a vertical jelly roll fold. The major capsid proteins (MCP) form into pseudohexameric subunits of the viral capsid, which store ...

''; 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 physical world or universe. "Nature" can refer to the 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. Although humans are ...

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