Asgard or Asgardarchaeota is a proposed superphylumconsisting of a group of archaea that includes Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. A representative of the group was cultivated. The Asgard superphylum represents the closest prokaryotic relatives of eukaryotes, which possibly emerged from an ancestral lineage of Asgardarchaeota after assimilating bacteria through the process of symbiogenesis.
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 Castlehydrothermal vent site, were analysed. Specific sediment horizons previously shown to contain high abundances of novel archaeal lineages, were subjected to metagenomic analysis. In 2015, an Uppsala University-led team proposed the Lokiarchaeota phylum based on phylogenetic analyses using a set of highly conservedprotein-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-led team discovered Thorarchaeota from samples taken from the White Oak River in North Carolina, named in reference to Thor, another Norse god. Additional samples from Loki's Castle, Yellowstone National Park, Aarhus Bay, an aquifer near the Colorado River, New Zealand'sRadiata Pool, hydrothermal vents near Taketomi Island, 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”, after the realm of the deities in Norse mythology. 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.
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. Asgard archaeons have a regulated actin cytoskeleton, and the profilins and gelsolins they use can interact with eukaryotic actins. They also seem to form vesicles under cryoEM. Some may have a PKD domainS-layer.
Metabolism
Asgard archaea are obligate anaerobes. They have a Wood–Ljungdahl pathway and perform glycolysis. Members can be autotrophs, heterotrophs, or phototrophs using heliorhodopsin. One member, Candidatus Prometheoarchaeum syntrophicum, performs syntrophy with a sulfur-reducing proteobacteria and a methanogenic archaea. The RuBisCO they have are not carbon-fixing, but likely used for nucleoside salvaging.
Classification
The phylogenetic relationship of this group is still under discussion. The relationship of the members is approximately as follows: The Heimdallarchaeota are considered the deepest branching Asgard archaea. 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.
