Asgard (archaea)

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]

In January 2020, scientists reported that Candidatus Prometheoarchaeum syntrophicum, a member of Lokiarcheota, may be a possible link between the simple prokaryotic microorganisms and the complex eukaryotic microorganisms occurring approximately two billion years ago.[14][4]

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.[15] [16] They also seem to form vesicles under cryoEM. Some may have a PKD domain S-layer.[4]

The phylogenetic relationship of this group is still under discussion. The relationship of the members is approximately as follows:[5][18]

The Heimdallarchaeota are considered the deepest branching Asgard archaea.[4] The Eukaryotes may be sister to the Heimdallarchaeota or the Asgard archaea. 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.[19]

• Traci Watson: The trickster microbes that are shaking up the tree of life, in: Nature, 14 May 2019