Nitrososphaeria

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. Metagenomic 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 autotrophs 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.