Nanoarchaeum equitans is a species of tiny microbe, discovered in 2002 in a hot vent off the coast of Iceland by Karl Stetter. It appears to be an obligatory symbiont on the archaeon Ignicoccus. Its cells are only 400 nm in diameter and its genome only half a million bases long, both considerably smaller than those of other known organisms. Genetically, Nanoarchaeum is sufficiently peculiar that its 16S rDNA sequence is undetectable by the most common methods. It has been given its own phylum, called Nanoarchaeota, which is somewhat tentatively placed among the Archaea.

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