Nanoarchaeota

Nanoarchaeota is a phylum within the Archaea characterized by extremely small cell size and highly reduced genomes. It was first described after the isolation of Nanoarchaeum equitans from hydrothermal vent samples in 2002. Members of this phylum have been detected in a variety of extreme environments, particularly hot springs and submarine hydrothermal vents, and many have been found in association with other archaea.

Taxonomy and phylogeny: Nanoarchaeota forms a distinct lineage within the Archaea and is commonly grouped within the DPANN superphylum, a collection of lineages with small genomes and often symbiotic or dependent lifestyles. The separately cultivated representative, N. equitans, serves as the best-studied member of the group.

Biology and ecology: The best-known member, Nanoarchaeum equitans, is an ultra-small archaeon that lives in close association with Ignicoccus hospitalis. The relationship is generally viewed as symbiotic or parasitic. N. equitans attaches to its host and relies on the host cell for many cellular metabolites and information transfer required for growth.

Genome and metabolism: N. equitans has an extremely reduced genome, on the order of about 490 kilobases, encoding a few hundred genes. The genome lacks many biosynthetic pathways, and the organism appears to depend on its host for nutrients and energy. The reduced genome and host dependence are characteristic features of Nanoarchaeota members described to date.

Research status: Because only one species has been cultivated, much of the biology of Nanoarchaeota remains inferred from metagenomic data and co-culture studies. The phylum provides insight into minimal genomes, extreme reduction, and host-dependent life strategies among Archaea, and continues to be a focus of studies on archaeal diversity and symbiosis.