Asgard (archaea)

Asgard
Scientific classification
Domain: Archaea
Kingdom: Proteoarchaeota
Superphylum: Asgard
Katarzyna Zaremba-Niedzwiedzka, et al. 2017
Phyla
Synonyms
  • "Asgardarchaeota" Violette Da Cunha et al. 2017
  • "Asgardaeota" Whitman 2018
  • "Eukaryomorpha" Fournier & Poole 2018[1]

Asgard or Asgardarchaeota[2] is a proposed superphylum consisting of a group of archaea that includes Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota.[3] A representative of the group was cultivated.[4] It appears the eukaryotes have emerged within the Asgard, in a branch containing the Heimdallarchaeota.[5][6]

Discovery

In the summer of 2010, sediments from a gravity core taken in the rift valley on the Knipovich ridge in the Arctic Ocean, near the so-called Loki's Castle hydrothermal vent site, were analysed. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages, were subjected to metagenomic analysis.[7][8]

In 2015, an Uppsala University-led team proposed the Lokiarchaeota phylum based on phylogenetic analyses using a set of highly conserved protein-coding genes.[9] 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.[10] 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",[11] analogous to the role of Lokiarchaeota in the debates about the origin of eukaryotes.[9][12]

In 2016, a University of Texas-led team discovered Thorarchaeota from samples taken from the White Oak River in North Carolina, named in reference to Thor, another Norse god.[13]

Additional samples from Loki's Castle, Yellowstone National Park, Aarhus Bay, an aquifer near the Colorado River, New Zealand's Radiata Pool, hydrothermal vents near Taketomi Island, Japan, and the White Oak River estuary in the United States led researchers to discover Odinarchaeota and Heimdallarchaeota,[3] 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”, after the realm of the deities in Norse mythology.[3]

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 pathway homologs.[3]

Asgard archaeons have a regulated actin cytoskeleton, and the profilins and gelsolins they use can interact with eukaryotic actins.[14][15] They also seem to form vesicles under Cryogenic electron microscopy. Some may have a PKD domain S-layer.[4] They also share the three-way ES39 expansion in LSU rRNA with eukaryotes.[16]

Metabolism

Asgard archaea are generally obligate anaerobes, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be facultative aerobes.[18] They have a Wood–Ljungdahl pathway and perform glycolysis. Members can be autotrophs, heterotrophs, or phototrophs using heliorhodopsin.[17] One member, Candidatus Prometheoarchaeum syntrophicum, performs syntrophy with a sulfur-reducing proteobacteria and a methanogenic archaea.[4]

The RuBisCO they have are not carbon-fixing, but likely used for nucleoside salvaging.[17]

Eukaryotic-like features in subdivisions

The phylum "Heimdallarchaeota" was found to have N-terminal core histone tails, a feature previously thought to be exclusively eukaryotic, in 2017. Two other archaeal phyla, both outside of Asgard, were found to also have tails in 2018.[19]

In January 2020, scientists found Candidatus Prometheoarchaeum syntrophicum, a member of Lokiarcheota, engaging in cross-feeding 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 microorganisms occurring approximately two billion years ago.[20][4]

Classification

The phylogenetic relationship of this group is still under discussion. The relationship of the members is approximately as follows:[5][6][21]

Proteoarchaeota

TACK

Asgard

"Borrarchaeota"

"Helarchaeota"

"Lokiarchaeota"

"Baldrarchaeota"

"Odinarchaeota"

"Hermodarchaeota"

"Thorarchaeota"

"Wukongarchaeota"

"Hodarchaeota"

"Gerdarchaeota"

"Kariarchaeota"

"Heimdallarchaeota"

(+α─Proteobacteria)

Eukaryota

Some authors have suggested to split 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.[22]

References

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  2. Violette Da Cunha, Morgan Gaia, Daniele Gadelle, Arshan Nasir, Patrick Forterre: Lokiarchaea are close relatives of Euryarchaeota, not bridging the gap between prokaryotes and eukaryotes, in: PLoS Genet. 2017 Jun; 13(6): e1006810. 2017 Jun 12, doi: 10.1371/journal.pgen.1006810
  3. 1 2 3 4 Zaremba-Niedzwiedzka, Katarzyna; Caceres, Eva F.; Saw, Jimmy H.; Bäckström, Disa; Juzokaite, Lina; Vancaester, Emmelien; Seitz, Kiley W.; Anantharaman, Karthik; Starnawski, Piotr (11 January 2017). "Asgard archaea illuminate the origin of eukaryotic cellular complexity" (PDF). Nature. 541 (7637): 353–358. Bibcode:2017Natur.541..353Z. doi:10.1038/nature21031. ISSN 1476-4687. OSTI 1580084. PMID 28077874. S2CID 4458094.
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