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Nitrososphaeria (previously phylum Nitrososphaerota or Thaumarchaeota) is a class of Archaea under the phylum Thermoproteota. The first species, Cenarchaeum symbiosum, was discovered in 1996 and was found to have a genome distinct from other known archaea at the time; hence, it was classified as a separate phylum. A decade later, three ammonia-oxidizing archaea were described, Nitrosopumilus maritimus, Nitrososphaera viennensis, and Nitrososphaera gargensis. Genome analysis in 2010 revealed that C. symbiosum and the three archaea are genetically of the same group.
Taxonomic reassessment in 2021 merged the archaeal group to the phylum Thermoproteota. Most species of Nitrososphaeria are chemolithoautotrophic ammonia-oxidizers and may play important roles in biogeochemical cycles, such as the nitrogen cycle and the carbon cycle. Metagenomics sequencing indicates that they constitute ~1% of the sea surface metagenome across many sites. The lipid crenarchaeol has been found only in Nitrososphaeria, making it a potential biomarker for the class.
Nitrososphaeria-derived membrane-spanning tetraether lipids (glycerol dialkyl glycerol tetraethers; GDGTs) from marine sediments can be used to reconstruct past temperatures via the TEX86 paleotemperature proxy, as these lipids vary in structure according to temperature. Because most Nitrososphaeria seem to be that fix CO2, their GDGTs can act as a record for past Carbon-13 ratios in the dissolved inorganic carbon pool, and thus have the potential to be used for reconstructions of the carbon cycle in the past.
In 2005, a team of German and American biologists at the University of Washington discovered Nitrification archaea from various water sources around Seattle and gave the name Nitrosopumilus maritimus. It was classified under the phylum Crenarchaeota. Another related ammonia-oxidizing archaea, Nitrososphaera gargensis, was discovered in 2008 from Siberian Garga hot spring . By then, C. symbiosum was established as capable of oxidizing ammonia. Genome sequence showed that the group differ significantly from other members of the Hyperthermophile Crenarchaeota . Two phyla of archaea were recognized: Crenarchaeota and Euryarchaeota. Since the genetic difference of the ammonia-oxidizing archaea was huge from member of the two existing phyla, a third phylum Thaumarchaeota was introduced in 2008. The classification was based on phylogenetic data, such as the sequences of these organisms' ribosomal RNA genes, and the presence of a form of type I topoisomerase that was previously thought to be unique to the .
In 2014, Nitrososphaera viennensis was discovered from a garden soil in Vienna, Austria, for which Michaela Stieglmeier and her colleagues created the taxonomic hierarchy, family Nitrososphaeraceae, order Nitrososphaerales and class Nitrososphaeria. International Code of Nomenclature of Prokaryotes (ICNP, Prokaryotic Code), Aharon Oren and George M. Garrity fomralized in 2021 the phylum as Nitrososphaerota for the ammonia-oxidizing archaea, since Stieglmeier's classification was the first valid publication. At the same time, a team of Australian scientists led by Christian Rinke and Philip Hugenholtz published a new classification on archaea, in which they merged Crenarchaeota and Nitrososphaerota (in fact the entire TACK superphylum) into the phylum Thermoproteota, thereby demoting the phylum to the class level.
Many members of the phylum assimilate carbon by Carbon fixation Bicarbonate−. This is done using a hydroxypropionate/hydroxybutyrate cycle similar to the Thermoproteota but which appears to have evolved independently. All Nitrososphaeria that have been identified by metagenomics thus far encode this pathway. Notably, the Nitrososphaeria CO2-fixation pathway is more efficient than any known aerobic autotrophic pathway. This efficiency helps explain their ability to thrive in low-nutrient environments. Some Nitrososphaeria such as Nitrosopumilus maritimus are able to incorporate organic carbon as well as inorganic, indicating a capacity for . At least two isolated strains have been identified as obligate mixotrophs, meaning they require a source of organic carbon in order to grow.
A study has revealed that Nitrososphaeria are most likely the dominant producers of the critical vitamin B12. This finding has important implications for eukaryotic phytoplankton, many of which are Auxotrophy and must acquire vitamin B12 from the environment; thus the Nitrososphaeria could play a role in and by extension global levels of atmospheric carbon dioxide. Because of the importance of vitamin B12 in biological processes such as the citric acid cycle and DNA synthesis, production of it by the Nitrososphaeria may be important for a large number of aquatic organisms.
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