Nanoarchaeota: and Korachaeota

Nanoarchaeota

• The phylum Nanoarchaeota consists of one genus, Nanoarchaeum, and one species, Nanoarchaeum equitans
• Nanoarchaeum is a nano-sized, hyperthermophilic obligate symbiont and an early-branching member of Archaea
• It has the smallest genome of all known organisms, at 0.49 Mb
• It lacks genes for core molecular processes, depending on its host, Ignicoccus, for cellular needs
• It does contain genes that encode key enzymes involved in DNA replication, transcription, translation, and repair
• Nanoarchaeota are regarded as the least evolved of all Archaea
• They lack genes for most metabolic functions, relying on Ignicoccus
• They cannot grow in pure culture and replicate only when attached to the surface of their host
• It is proposed that Ignicoccus provides N. equitans with organic carbon, as it can't metabolize H₂ and So for energy
• Nanoarchaeum cells are coccoid, with a diameter of 300-500 nm
• They grow under strictly anaerobic conditions and at temperatures of 70 to 98°C
• They are found in hydrothermal vent systems and continental samples
• Mid-Atlantic Ridge (depth ≈106 m)
• East Pacific Rise (black smoker fragment; pH 6.5)
• Obsidian Pool (Temp 80°C, pH 6.0), Yellowstone National Park, USA
• Uzon Caldera (Temp 85°C, pH 5.5), Kamchatka, Russia
• They have a broad distribution in high-temperature biotopes, deep sea, shallow marine areas, and solfataric fields
• Habitats on different continents suggest a worldwide distribution
• The phylum is well defined by 16S rRNA gene sequence comparisons
• The phylum Nanoarchaeota represents an isolated deep lineage within the Archaea
• Forms a distinct cluster, separate from the three other phyla
• The placement of the Nanoarchaeota branch within Archaea is problematic due to indefinite branching
• Position varies depending on the analytical method and filter sets
• They possess highly conserved base pair substitutions among all other groups of Archaea
• Phylogenetic analyses indicate Nanoarchaeota branches off very deeply within the archaeal domain
• Detailed phylogenetic analyses suggest N. equitans divergence occurred when Euryarchaeota formed

Korarchaeota

• "Korarchaeota" is represented by a small number of small-subunit rRNA gene sequences; there are no cultivated representatives
• Genome sequence has been determined by metagenomics analyses of an enriched culture
• Analyses with small-subunit sequences indicate that the group branches more deeply than Crenarchaeota and Euryarchaeota
• Korarchaeota is represented by one named species, Candidatus Korarchaeum cryptofilum
• It remains uncultivated to date
• Ca. K. cryptofilum is an obligate anaerobic chemoorganotroph and a hyperthermophile growing at 85°C
• The cells are long, thin (0.2 µm diameter) filaments of variable length that have a tough paracrystalline S-layer
• Each of the six hot springs was found to contain Korarchaeota
• The genome sequence has provided insights into its lifestyle
• It cannot perform aerobic respiration
• It is a fermentative organism, growing by fermenting peptides or amino acids
• Lacks many core genes in biosynthesis including the ability to synthesize purines, coenzyme A and several essential cofactors
• Some uncertainty exists regarding its exact phylogenetic position
• The genome includes genes with affinity to both Euryarchaeota and Crenarchaeota
• Phylogenetic analysis indicates its ribosomal proteins, RNA polymerase subunits, and ribosomal RNA genes have an affinity to Crenarchaeota
• Its genes for cell division, tRNA maturation, and DNA replication and repair have an affinity to Euryarchaeota
• A unique genetic composition supports placement near the base of the archaeal radiation
• The species was first found in a survey of archaeal rRNA genes in Yellowstone's Obsidian Pool
• Sequences have been detected in other Yellowstone hot springs
• Has been observed in a range of geothermal habitats, both submarine and terrestrial