|Ca. Hadesararchaeum tengchongensis
Ca. Methanourarchaum thermotelluricum|
Hadesarchaea, formerly called the South-African Gold Mine Miscellaneous Euryarchaeal Group, are a class of thermophile microorganisms that have been found in deep mines, hot springs, marine sediments and other subterranean environments.
These archaea were initially called South-African Gold Mine Miscellaneous Euryarchaeal Group (SAGMEG) after their initial site of discovery. The name Hadesarchaea was proposed by Baker et al. in 2016, a reference to the Greek god of the underworld.
Previously, Hadesarchaea (or SAGMEG) were only known to exist through their distinctive phylogenetic position in the tree of life. In 2016, scientists using metagenomic shotgun sequencing were able to assemble several near-full genomes of these archaea. It was shown that the genome of Hadesarchaea is approximately 1.5 Megabase pairs in size, which is about 0.5 Mbp smaller than most archaea. These archaea have not been successfully cultivated in the laboratory, but their metabolic properties have been inferred from the genomic reconstructions. Hadesarchaea may have evolved from a methanogenic ancestor based on the genetic similarity with other methanogenic organisms.
Habitat and metabolism
These microbes were first discovered in a gold mine in South Africa at a depth of approximately 3 km (2 mi), where they are able to live without oxygen or light. They were later also found in the White Oak River estuary in North Carolina and in Yellowstone National Park's Lower Culex Basin. These areas are approximately 70 °C (158 °F) and highly alkaline. Based on phylogenetic marker gene survey, Hadesarchaeota might be present in soils in ancient mining areas in East Harz region, Germany.
Hadesarchaea are unique among known archaea in that they can convert carbon monoxide and water to carbon dioxide and oxygen, producing hydrogen as a by-product. From metagenome-assembled genome (MAG) data, Hadesarchaea possess genes associated with Wood-Ljungdahl carbon fixation pathway, methanogenesis and alkane metabolism.  Hadesarchaeal genomes have also been reported to contain genes that enable them to metabolize sugars and amino acids in a heterotrophic lifestyle, and perform dissimilatory nitrite reduction to ammonium. Initial research suggests that these organisms are also involved in significant geochemical processes.
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