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.[1][2][3][4][5]
Nomenclature
These archaea were initially called South-African Gold Mine Miscellaneous Euryarchaeal Group (SAGMEG), after their initial site of discovery.[6][7] The name Hadesarchaea was proposed by Baker et al. in 2016, a reference to the
Greek god of the underworld.[1]
Phylogeny
Previously, Hadesarchaea (or SAGMEG) were only known to exist through their distinctive phylogenetic position in the
tree of life. In 2016, scientists using
metagenomicshotgun sequencing were able to assemble several near-full genomes of these archaea.[1] It was shown that the genome of Hadesarchaea is approximately 1.5 Megabase pairs in size,[1] which is about 0.5 Mbp smaller than most archaea.[8] These archaea have not been successfully cultivated in the laboratory, but their metabolic properties have been inferred from the genomic reconstructions.[1] Hadesarchaea may have evolved from a methanogenic ancestor based on the genetic similarity with other methanogenic organisms.[9]
Taxonomy
"Hadarchaeota" Chuvochina et al. 2019 ["Hadesarchaeota" McGonigle et al. 2019; "Stygia" Adam et al. 2017[10][11][12][13]
"Persephonarchaeia" corrig. Mwirichia et al. 2016 (MSBL-1)
"Hadarchaeia" Chuvochina et al. 2019 ["Hadesarchaea" Baker et al. 2016] (SAGMEG)
These microbes were first discovered in a gold mine in South Africa at a depth of approximately 3 km (2 mi),[6] where they are able to live without oxygen or light.[8][14][15] They were later also found in the
White Oak River estuary in North Carolina and in
Yellowstone National Park's Lower Culex Basin.[16] These areas are approximately 70 °C (158 °F) and highly
alkaline.[16] Based on
phylogenetic marker gene survey, Hadesarchaeota might be present in soils in ancient mining areas in East Harz region, Germany.[17]
The microbes have been found in other marine environments as well. Some of these areas include cold seep systems in the
South China Sea. Hadesarchaea has been found to be a dominant member of the archaeal community in the area. These cold seeps contain gas hydrate bearing sediments in which microbes play a major role in
biogeochemical cycling. It is believed that Hadesarchaea is involved the oxidation of
carbon dioxide with water in this environment.[18] Hadesarchaea have also been found in subseafloor habitats located in the
Guaymas Basin and Sonora Margin around the
Gulf of California.[19]
In addition to being present in marine sediments, mines, and hot springs, Hadesarchaea has been identified in the
gut microbiome of certain fish species. The freshwater pufferfish (Tetraodon cutcutia), native to India, Assam, Bihar, and
West Bengal, was found to have Hadesarchaea present in their gut microbiome. Hadesarchaea was found to be in the second most abundant in the archaeal community of the freshwater pufferfish. This was found to be similar to community abundance found in the gut of carnivorous
Salmon and herbivorous
grass carp. While Hadesarchaea are found to be in such high abundance for these environments, it is not completely known how they influence the health and trophic level of these fish.[20]
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.[21][22] 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.[1][3] Initial research suggests that these organisms are also involved in significant
geochemical processes.[1]
Because of their relatively small genome, it is assumed that the genomes of Hadesarchaea have been subjected to
genome streamlining, possibly as a result of nutrient limitation.[1]