A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont

According to small subunit ribosomal RNA (ss rRNA) sequence comparisons all known Archaea belong to the phyla Crenarchaeota, Euryarchaeota, and-indicated only by environmental DNA sequences-to the `Korarchaeota'. Here we report the cultivation of a new nanosized hyperthermophilic archaeon from a submarine hot vent. This archaeon cannot be attached to one of these groups and therefore must represent an unknown phylum which we name `Nanoarchaeota' and species, which we name `Nanoarchaeum equitans'. Cells of `N. equitans' are spherical, and only about 400nm in diameter. They grow attached to the surface of a specific archaeal host, a new member of the genus Ignicoccus. The distribution of the `Nanoarchaeota' is so far unknown. Owing to their unusual ss rRNA sequence, members remained undetectable by commonly used ecological studies based on the polymerase chain reaction. `N. equitans' harbours the smallest archaeal genome; it is only 0.5 megabases in size. This organism will provide insight into the evolution of thermophily, of tiny genomes and of interspecies communication.