Nanoarchaeota is a phylum of very small parasitic Archaea that branches closest to the root of the archaeal phylogentic tree. Cells of Nanoarchaeum, the only genus in this phylum, are small coccoids that live as parasites, or possibly as symbionts, of the crenarchaeota Ignicoccus. Cells of Nanoarchaeum are about 0.4 μm in diameter and replicate only when attached to the surface of Ignicoccus. Nanoarchaeum is hyperthermophilic, with an optimal growth of about 90°C. The metabolism of Nanoarchaeum is unknown, but its host is an autotroph, growing with H2 as electron donor and elemental sulfur as electron acceptor. Isolates from Nanoarchaeum have been obtained from submarine hydrothermal vents as well as terrestrial hot springs. The genome of Nanoarchaeum is only 0.49 Mbp, the smallest genome known. It lacks identifiable genes for most known metabolic functions, including the synthesis of monomers, such as aminoacids, nucleotides, and coenzymes.