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'''Asgard''' or '''Asgardarchaeota'''<ref name="pmid28604769">{{cite journal |last=Da Cunha |first=Violette |last2=Gaia |first2=Morgan |last3=Gadelle |first3=Daniele |last4=Nasir |first4=Arshan |last5=Forterre |first5=Patrick |display-authors=3 |title=Lokiarchaea are close relatives of Euryarchaeota, not bridging the gap between prokaryotes and eukaryotes |journal=PLOS Genetics |volume=13 |issue=6 |pages=e1006810 |date=June 2017 |pmid=28604769 |pmc=5484517 |doi=10.1371/journal.pgen.1006810 }}</ref> is a proposed [[superphylum]] consisting of a group of [[archaea]] that contain eukaryotic signature proteins.<ref name="Zaremba">{{cite journal |last1=Zaremba-Niedzwiedzka |first1=Katarzyna |last2=Caceres |first2=Eva F. |last3=Saw |first3=Jimmy H. |last4=Bäckström |first4=Disa |last5=Juzokaite |first5=Lina |last6=Vancaester |first6=Emmelien |last7=Seitz |first7=Kiley W. |last8=Anantharaman |first8=Karthik |last9=Starnawski |first9=Piotr |display-authors=3 |title=Asgard archaea illuminate the origin of eukaryotic cellular complexity |journal=Nature |volume=541 |issue=7637 |pages=353–358 |date=January 2017 |pmid=28077874 |doi=10.1038/nature21031 |osti=1580084 |s2cid=4458094 |bibcode=2017Natur.541..353Z }}</ref> It appears that the [[eukaryote]]s, the [[Domain (biology)|domain]] that contains the [[animal]]s, [[plant]]s, and [[Fungus|fungi]], [[Eukaryogenesis|emerged within the Asgard]],<ref name="Eme">{{cite journal |last1=Eme |first1=Laura|last2=Spang |first2=Anja |last3=Lombard|first3=Jonathan |last4=Stairs |first4=Courtney W. |last5=Ettema |first5=Thijs J. G. |title=Archaea and the origin of eukaryotes |journal=Nature Reviews. Microbiology |volume=15 |issue=12 |pages=711–723 |date=November 2017 |pmid=29123225 |doi=10.1038/nrmicro.2017.133 |s2cid=8666687 }}</ref> in a branch containing the Heimdallarchaeota.<ref name="Williams 138–147">{{cite journal |last1=Williams |first1=Tom A. |last2=Cox |first2=Cymon J. |last3=Foster |first3=Peter G. |last4=Szöllősi |first4=Gergely J. |last5=Embley |first5=T. Martin |title=Phylogenomics provides robust support for a two-domains tree of life |journal=Nature Ecology & Evolution |volume=4 |issue=1 |pages=138–147 |date=January 2020 |pmid=31819234 |pmc=6942926 |doi=10.1038/s41559-019-1040-x }}</ref> This supports the [[two-domain system]] of classification over the [[three-domain system]].<ref name=" |
'''Asgard''' or '''Asgardarchaeota'''<ref name="pmid28604769">{{cite journal |last=Da Cunha |first=Violette |last2=Gaia |first2=Morgan |last3=Gadelle |first3=Daniele |last4=Nasir |first4=Arshan |last5=Forterre |first5=Patrick |display-authors=3 |title=Lokiarchaea are close relatives of Euryarchaeota, not bridging the gap between prokaryotes and eukaryotes |journal=PLOS Genetics |volume=13 |issue=6 |pages=e1006810 |date=June 2017 |pmid=28604769 |pmc=5484517 |doi=10.1371/journal.pgen.1006810 }}</ref> is a proposed [[superphylum]] consisting of a group of [[archaea]] that contain eukaryotic signature proteins.<ref name="Zaremba">{{cite journal |last1=Zaremba-Niedzwiedzka |first1=Katarzyna |last2=Caceres |first2=Eva F. |last3=Saw |first3=Jimmy H. |last4=Bäckström |first4=Disa |last5=Juzokaite |first5=Lina |last6=Vancaester |first6=Emmelien |last7=Seitz |first7=Kiley W. |last8=Anantharaman |first8=Karthik |last9=Starnawski |first9=Piotr |display-authors=3 |title=Asgard archaea illuminate the origin of eukaryotic cellular complexity |journal=Nature |volume=541 |issue=7637 |pages=353–358 |date=January 2017 |pmid=28077874 |doi=10.1038/nature21031 |osti=1580084 |s2cid=4458094 |bibcode=2017Natur.541..353Z }}</ref> It appears that the [[eukaryote]]s, the [[Domain (biology)|domain]] that contains the [[animal]]s, [[plant]]s, and [[Fungus|fungi]], [[Eukaryogenesis|emerged within the Asgard]],<ref name="Eme">{{cite journal |last1=Eme |first1=Laura|last2=Spang |first2=Anja |last3=Lombard|first3=Jonathan |last4=Stairs |first4=Courtney W. |last5=Ettema |first5=Thijs J. G. |title=Archaea and the origin of eukaryotes |journal=Nature Reviews. Microbiology |volume=15 |issue=12 |pages=711–723 |date=November 2017 |pmid=29123225 |doi=10.1038/nrmicro.2017.133 |s2cid=8666687 }}</ref> in a branch containing the Heimdallarchaeota.<ref name="Williams 138–147">{{cite journal |last1=Williams |first1=Tom A. |last2=Cox |first2=Cymon J. |last3=Foster |first3=Peter G. |last4=Szöllősi |first4=Gergely J. |last5=Embley |first5=T. Martin |title=Phylogenomics provides robust support for a two-domains tree of life |journal=Nature Ecology & Evolution |volume=4 |issue=1 |pages=138–147 |date=January 2020 |pmid=31819234 |pmc=6942926 |doi=10.1038/s41559-019-1040-x }}</ref> This supports the [[two-domain system]] of classification over the [[three-domain system]].<ref name="Nobs-2022">{{cite journal |last1=Nobs |first1=Stephanie-Jane |last2=MacLeod |first2=Fraser I. |last3=Wong |first3=Hon Lun |last4=Burns |first4=Brendan P. |title=Eukarya the chimera: eukaryotes, a secondary innovation of the two domains of life? |journal=Trends in Microbiology |volume=30 |issue=5 |pages=421–431 |date=May 2022 |pmid=34863611 |doi=10.1016/j.tim.2021.11.003 |s2cid=244823103 }}</ref><ref name="Doolittle-2020">{{cite journal |last=Doolittle |first=W. Ford |title=Evolution: Two Domains of Life or Three? |journal=Current Biology |volume=30 |issue=4 |pages=R177–R179 |date=February 2020 |pmid=32097647 |doi=10.1016/j.cub.2020.01.010 |doi-access=free }}</ref> |
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==Discovery and nomenclature== |
==Discovery and nomenclature== |
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Asgard members encode many eukaryotic signature proteins, including novel [[Guanosine triphosphate|GTPases]], membrane-remodelling proteins like [[ESCRT]] and [[CHMP4B|SNF7]], a [[ubiquitin]] modifier system, and [[N-glycosylation]] pathway homologs.<ref name="Zaremba"/> |
Asgard members encode many eukaryotic signature proteins, including novel [[Guanosine triphosphate|GTPases]], membrane-remodelling proteins like [[ESCRT]] and [[CHMP4B|SNF7]], a [[ubiquitin]] modifier system, and [[N-glycosylation]] pathway homologs.<ref name="Zaremba"/> |
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Asgard archaeons have a regulated [[actin]] [[cytoskeleton]], and the [[profilin]]s and [[gelsolin]]s they use can interact with eukaryotic actins.<ref>{{cite journal |last=Akıl |first=Caner |last2=Robinson |first2=Robert C. |title=Genomes of Asgard archaea encode profilins that regulate actin |journal=Nature |volume=562 |issue=7727 |pages=439–443 |date=October 2018 |pmid=30283132 |doi=10.1038/s41586-018-0548-6 |s2cid=52917038 |bibcode=2018Natur.562..439A }}</ref><ref>{{cite journal |last=Akıl |first=Caner |last2=Tran |first2=Linh T. |last3=Orhant-Prioux |first3=Magali |last4=Baskaran |first4=Yohendran |last5=Manser |first5=Edward |last6=Blanchoin |first6=Laurent |last7=Robinson |first7=Robert C. |title=Insights into the evolution of regulated actin dynamics via characterization of primitive gelsolin/cofilin proteins from Asgard archaea |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=117 |issue=33 |pages=19904–19913 |date=August 2020 |pmid=32747565 |pmc=7444086 |doi=10.1073/pnas.2009167117 |doi-access=free |biorxiv=10.1101/768580 }}</ref> 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]]-forming tubulins.<ref>{{cite journal |last1=Akıl |first1=Caner |last2=Ali |first2=Samson |last3=Tran |first3=Linh T. |last4=Gaillard |first4=Jeremie |last5=Li |first5=Wenfei |last6=Hayashida |first6=Kenichi |last7=Mika |first7=Hirose |last8=Oshima |first8=Atsunori |last9=Fujishima |first9=Kosuke |last10=Blanchoin |first10=Laurent |last11=Narita |first11=Akihiro |last12=Robinson |first12=Robert C. |display-authors=3 |title=Structure and dynamics of Odinarchaeota tubulin and the implications for eukaryotic microtubule evolution |journal=Science Advances |volume=8 |issue=12 |pages=eabm2225 |date=March 2022 |pmid=35333570 |pmc=8956254 |doi=10.1126/sciadv.abm2225 |bibcode=2022SciA....8M2225A }}</ref> They also seem to form vesicles under [[cryogenic electron microscopy]]. Some may have a [[PKD domain]] [[S-layer]].<ref name=" |
Asgard archaeons have a regulated [[actin]] [[cytoskeleton]], and the [[profilin]]s and [[gelsolin]]s they use can interact with eukaryotic actins.<ref>{{cite journal |last=Akıl |first=Caner |last2=Robinson |first2=Robert C. |title=Genomes of Asgard archaea encode profilins that regulate actin |journal=Nature |volume=562 |issue=7727 |pages=439–443 |date=October 2018 |pmid=30283132 |doi=10.1038/s41586-018-0548-6 |s2cid=52917038 |bibcode=2018Natur.562..439A }}</ref><ref>{{cite journal |last=Akıl |first=Caner |last2=Tran |first2=Linh T. |last3=Orhant-Prioux |first3=Magali |last4=Baskaran |first4=Yohendran |last5=Manser |first5=Edward |last6=Blanchoin |first6=Laurent |last7=Robinson |first7=Robert C. |title=Insights into the evolution of regulated actin dynamics via characterization of primitive gelsolin/cofilin proteins from Asgard archaea |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=117 |issue=33 |pages=19904–19913 |date=August 2020 |pmid=32747565 |pmc=7444086 |doi=10.1073/pnas.2009167117 |doi-access=free |biorxiv=10.1101/768580 }}</ref> 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]]-forming tubulins.<ref>{{cite journal |last1=Akıl |first1=Caner |last2=Ali |first2=Samson |last3=Tran |first3=Linh T. |last4=Gaillard |first4=Jeremie |last5=Li |first5=Wenfei |last6=Hayashida |first6=Kenichi |last7=Mika |first7=Hirose |last8=Oshima |first8=Atsunori |last9=Fujishima |first9=Kosuke |last10=Blanchoin |first10=Laurent |last11=Narita |first11=Akihiro |last12=Robinson |first12=Robert C. |display-authors=3 |title=Structure and dynamics of Odinarchaeota tubulin and the implications for eukaryotic microtubule evolution |journal=Science Advances |volume=8 |issue=12 |pages=eabm2225 |date=March 2022 |pmid=35333570 |pmc=8956254 |doi=10.1126/sciadv.abm2225 |bibcode=2022SciA....8M2225A }}</ref> They also seem to form vesicles under [[cryogenic electron microscopy]]. Some may have a [[PKD domain]] [[S-layer]].<ref name="Imachi-2020">{{cite journal |last1=Imachi |first1=Hiroyuki |last2=Nobu |first2=Masaru K. |last3=Nakahara |first3=Nozomi|last4=Morono |first4=Yuki |last5=Ogawara |first5=Miyuki |last6=Takaki |first6=Yoshihiro |last7=Takano |first7=Yoshinori |last8=Uematsu |first8=Katsuyuki |last9=Ikuta |first9=Tetsuro |last10=Ito |first10=Motoo |last11=Matsui |first11=Yohei |display-authors=3 |title=Isolation of an archaeon at the prokaryote-eukaryote interface |journal=Nature |volume=577 |issue=7791 |pages=519–525 |date=January 2020 |pmid=31942073 |pmc=7015854 |doi=10.1038/s41586-019-1916-6 |bibcode=2020Natur.577..519I }}</ref> They also share the three-way ES39 expansion in [[LSU rRNA]] with eukaryotes.<ref>{{cite journal |last1=Penev |first1=Petar I. |last2=Fakhretaha-Aval |first2=Sara |last3=Patel |first3=Vaishnavi J. |last4=Cannone |first4=Jamie J. |last5=Gutell |first5=Robin R. |last6=Petrov |first6=Anton S. |last7=Williams |first7=Loren Dean |last8=Glass |first8=Jennifer B. |display-authors=3 |title=Supersized Ribosomal RNA Expansion Segments in Asgard Archaea |journal=Genome Biology and Evolution |volume=12 |issue=10 |pages=1694–1710 |date=October 2020 |pmid=32785681 |pmc=7594248 |doi=10.1093/gbe/evaa170 |doi-access=free }}</ref> |
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===Metabolism=== |
===Metabolism=== |
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</gallery> |
</gallery> |
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Asgard archaea are generally [[obligate anaerobe]]s, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be [[facultative aerobes]].<ref name="Liu_2020">{{cite journal |last=Liu |first=Yang |last2=Makarova |first2=Kira S. |last3=Huang |first3=Wen-Cong |last4=Wolf |first4=Yuri I. |last5=Nikolskaya |first5=Anastasia |last6=Zhang |first6=Xinxu |last7=Cai |first7=Mingwei |last8=Zhang |first8=Cui-Jing |last9=Xu |first9=Wei |last10=Luo |first10=Zhuhua |last11=Cheng |first11=Lei |last12=Koonin |first12=Eugene V. |author12-link=Eugene Koonin |last13=Li |first13=Meng |display-authors=3 |title=Expanding diversity of Asgard archaea and the elusive ancestry of eukaryotes |journal=bioRxiv |year=2020 |doi=10.1101/2020.10.19.343400 |s2cid=225056970 |url=https://www.biorxiv.org/content/10.1101/2020.10.19.343400v3.full}}</ref> They have a [[Wood–Ljungdahl pathway]] and perform [[glycolysis]]. Members can be [[autotroph]]s, [[heterotroph]]s, or [[phototroph]]s using [[heliorhodopsin]].<ref name="kindler2019">{{cite journal |last1=MacLeod |first1=Fraser |last2=Kindler |first2=Gareth S. |last3=Wong |first3=Hon Lun |last4=Chen |first4=Ray |last5=Burns |first5=Brendan P. |title=Asgard archaea: Diversity, function, and evolutionary implications in a range of microbiomes |journal=AIMS Microbiology |volume=5 |issue=1 |pages=48–61 |date=2019 |pmid=31384702 |pmc=6646929 |doi=10.3934/microbiol.2019.1.48 }}</ref> One member, [[Candidatus Prometheoarchaeum syntrophicum|''Candidatus'' ''Prometheoarchaeum syntrophicum'']], is [[syntrophy|syntrophic]] with a sulfur-reducing proteobacteria and a [[methanogenic]] archaea.<ref name=" |
Asgard archaea are generally [[obligate anaerobe]]s, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be [[facultative aerobes]].<ref name="Liu_2020">{{cite journal |last=Liu |first=Yang |last2=Makarova |first2=Kira S. |last3=Huang |first3=Wen-Cong |last4=Wolf |first4=Yuri I. |last5=Nikolskaya |first5=Anastasia |last6=Zhang |first6=Xinxu |last7=Cai |first7=Mingwei |last8=Zhang |first8=Cui-Jing |last9=Xu |first9=Wei |last10=Luo |first10=Zhuhua |last11=Cheng |first11=Lei |last12=Koonin |first12=Eugene V. |author12-link=Eugene Koonin |last13=Li |first13=Meng |display-authors=3 |title=Expanding diversity of Asgard archaea and the elusive ancestry of eukaryotes |journal=bioRxiv |year=2020 |doi=10.1101/2020.10.19.343400 |s2cid=225056970 |url=https://www.biorxiv.org/content/10.1101/2020.10.19.343400v3.full}}</ref> They have a [[Wood–Ljungdahl pathway]] and perform [[glycolysis]]. Members can be [[autotroph]]s, [[heterotroph]]s, or [[phototroph]]s using [[heliorhodopsin]].<ref name="kindler2019">{{cite journal |last1=MacLeod |first1=Fraser |last2=Kindler |first2=Gareth S. |last3=Wong |first3=Hon Lun |last4=Chen |first4=Ray |last5=Burns |first5=Brendan P. |title=Asgard archaea: Diversity, function, and evolutionary implications in a range of microbiomes |journal=AIMS Microbiology |volume=5 |issue=1 |pages=48–61 |date=2019 |pmid=31384702 |pmc=6646929 |doi=10.3934/microbiol.2019.1.48 }}</ref> One member, [[Candidatus Prometheoarchaeum syntrophicum|''Candidatus'' ''Prometheoarchaeum syntrophicum'']], is [[syntrophy|syntrophic]] with a sulfur-reducing proteobacteria and a [[methanogenic]] archaea.<ref name="Imachi-2020" /> |
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The [[RuBisCO]] they have is not carbon-fixing, but likely used for nucleoside salvaging.<ref name="kindler2019"/> |
The [[RuBisCO]] they have is not carbon-fixing, but likely used for nucleoside salvaging.<ref name="kindler2019"/> |
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The phylum Heimdallarchaeota was found in 2017 to have N-terminal core [[Histone code|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.<ref name=pmid30212449>{{cite journal |last=Henneman |first=Bram |last2=van Emmerik |first2=Clara |last3=van Ingen |first3=Hugo |last4=Dame |first4=Remus T. |title=Structure and function of archaeal histones |journal=PLOS Genetics |volume=14 |issue=9 |pages=e1007582 |date=September 2018 |pmid=30212449 |pmc=6136690 |doi=10.1371/journal.pgen.1007582 |bibcode=2018BpJ...114..446H }}</ref> |
The phylum Heimdallarchaeota was found in 2017 to have N-terminal core [[Histone code|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.<ref name=pmid30212449>{{cite journal |last=Henneman |first=Bram |last2=van Emmerik |first2=Clara |last3=van Ingen |first3=Hugo |last4=Dame |first4=Remus T. |title=Structure and function of archaeal histones |journal=PLOS Genetics |volume=14 |issue=9 |pages=e1007582 |date=September 2018 |pmid=30212449 |pmc=6136690 |doi=10.1371/journal.pgen.1007582 |bibcode=2018BpJ...114..446H }}</ref> |
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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]], they consider this relationship a possible link between the simple [[prokaryotic]] microorganisms and the complex [[eukaryotic]] microorganisms occurring approximately two billion years ago.<ref name="NYT-2020115">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=This Strange Microbe May Mark One of Life's Great Leaps - A organism living in ocean muck offers clues to the origins of the complex cells of all animals and plants. |url=https://www.nytimes.com/2020/01/15/science/cells-eukaryotes-archaea.html |date=15 January 2020 |work=[[The New York Times]] |access-date=16 January 2020 }}</ref><ref name=" |
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]], they consider this relationship a possible link between the simple [[prokaryotic]] microorganisms and the complex [[eukaryotic]] microorganisms occurring approximately two billion years ago.<ref name="NYT-2020115">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=This Strange Microbe May Mark One of Life's Great Leaps - A organism living in ocean muck offers clues to the origins of the complex cells of all animals and plants. |url=https://www.nytimes.com/2020/01/15/science/cells-eukaryotes-archaea.html |date=15 January 2020 |work=[[The New York Times]] |access-date=16 January 2020 }}</ref><ref name="Imachi-2020"/> |
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== Classification == |
== Classification == |
Revision as of 21:30, 6 June 2023
Asgard | |
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Scientific classification | |
Domain: | Archaea |
Kingdom: | Proteoarchaeota |
Superphylum: | Asgard Katarzyna Zaremba-Niedzwiedzka , et al. 2017 |
Phyla | |
see text | |
Synonyms | |
|
Asgard or Asgardarchaeota[2] is a proposed superphylum consisting of a group of archaea that contain eukaryotic signature proteins.[3] It appears that the eukaryotes, the domain that contains the animals, plants, and fungi, emerged within the Asgard,[4] in a branch containing the Heimdallarchaeota.[5] This supports the two-domain system of classification over the three-domain system.[6][7]
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 hydrothermal vent site. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages were subjected to metagenomic analysis.[8][9] In 2015, an Uppsala University-led team proposed the Lokiarchaeota phylum based on phylogenetic analyses using a set of highly conserved protein-coding genes.[10] The group was named for the shape-shifting Norse god Loki, 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, 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.[14] 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 contained Odinarchaeota and Heimdallarchaeota;[3] following the Norse deity naming convention, these groups were named for Odin and Heimdallr respectively. Researchers therefore named the superphylum containing these microbes "Asgard", after the home of the gods in Norse mythology.[3] Two Lokiarchaeota specimens have been cultured, enabling a detailed insight into their morphology.[15]
Description
Proteins
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.[16][17] 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-forming tubulins.[18] They also seem to form vesicles under cryogenic electron microscopy. Some may have a PKD domain S-layer.[19] They also share the three-way ES39 expansion in LSU rRNA with eukaryotes.[20]
Metabolism
-
Metabolic pathways of Asgard archaea, varying by phyla[21]
-
Metabolic pathways of Asgard archaea, varying by environment[21]
Asgard archaea are generally obligate anaerobes, though Kariarchaeota, Gerdarchaeota and Hodarchaeota may be facultative aerobes.[22] They have a Wood–Ljungdahl pathway and perform glycolysis. Members can be autotrophs, heterotrophs, or phototrophs using heliorhodopsin.[21] One member, Candidatus Prometheoarchaeum syntrophicum, is syntrophic with a sulfur-reducing proteobacteria and a methanogenic archaea.[19]
The RuBisCO they have is not carbon-fixing, but likely used for nucleoside salvaging.[21]
Ecology
Asgard 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 Asgard archaea. Other habitats include the bodies of animals, the rhizosphere of plants, non-saline sediments and soils, the sea surface, and freshwater. In addition, Asgard are associated with several other microorganisms.[23]
Eukaryote-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.[24]
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, they consider this relationship a possible link between the simple prokaryotic microorganisms and the complex eukaryotic microorganisms occurring approximately two billion years ago.[25][19]
Classification
The phylogenetic relationships of the Asgard archaea are still under discussion.
Williams et al. 2019,[5] Eme et al. 2017,[4] Liu et al. 2021[26] & Liu et al. 2020[22] | 53 marker proteins based GTDB 07-RS207[27][28][29] | |||||||||||||||||||||||||||||||||||||||||||||
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In 2023, Eme, Tamarit and colleagues reported that the Eukaryota are deep within Asgard, as sister of Hodarcheales within the Heimdallarchaeia.[30]
Asgard | |
Taxonomy
In the depicted scenario, the Eukaryota are deep in the tree of Asgard. A favored scenario is syntrophy, where one organism depends on the feeding of the other. An α-proteobacterium was incorporated to become the mitochondrion.[32] In culture, extant Asgard archaea form various syntrophic dependencies.[33] Gregory Fournier and Anthony Poole have proposed that Asgard is part of "the Eukaryote tree", forming a superphylum they call "Eukaryomorpha" defined by "shared derived characters" (eukaryote signature proteins).[34]
The taxonomy is uncertain and the phylum names are therefore somewhat speculative. The list of phyla is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)[35] and National Center for Biotechnology Information (NCBI).[36]
- Phylum Baldrarchaeota
- Phylum Borrarchaeota
- Phylum Freyrarchaeota
- Phylum Friggarchaeota
- Phylum Gefionarchaeota
- Phylum Gerdarchaeota
- Phylum Heimdallarchaeota
- Phylum Helarchaeota
- Phylum Hermodarchaeota
- Phylum Hodarchaeota
- Phylum Idunnarchaeota
- Phylum Kariarchaeota
- Phylum Lokiarchaeota
- Phylum Njordarchaeota
- Phylum Odinarchaeota
- Phylum Sifarchaeota
- Phylum Sigynarchaeota
- Phylum Thorarchaeota
- Phylum Tyrarchaeota
- Phylum Wukongarchaeota
Genomic elements
Viruses
Several family-level groups of viruses associated with Asgard archaea have been discovered using metagenomics.[37][38][39] 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 class Caudoviricetes, i.e., viruses with icosahedral capsids and helical tails;[37][39] two other distinct groups (called 'skuldviruses') are distantly related to tailless archaeal and bacterial viruses with icosahedral capsids of the realm Varidnaviria;[37][38] and the third group of viruses (called wyrdviruses) is related to archaea-specific viruses with lemon-shaped virus particles (family Halspiviridae).[37][38] The viruses have been identified in deep-sea sediments[37][39] and a terrestrial hot spring of the Yellowstone National Park.[38] All these viruses display very low sequence similarity to other known viruses but are generally related to the previously described prokaryotic viruses,[40] with no meaningful affinity to viruses of eukaryotes.[41][37]
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.[37][42] These mobile elements do not encode recognizable viral hallmark proteins and could represent either novel types of viruses or plasmids.
See also
References
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External links
- Traci Watson: The trickster microbes that are shaking up the tree of life, in: Nature, 14 May 2019