Nanoarchaeum equitans is a species of tiny microbe, discovered in 2002 in a hydrothermal vent off the coast of Iceland by Karl Stetter. Since it grows in temperatures approaching boiling, it is considered to be a thermophile. Nanoarchaeum appears to be an obligatory symbiont on the archaeon Ignicoccus; it must be in contact with the host organism to survive. Its cells are only 400 nm in diameter, making it the next smallest known living organism, excepting possibly nanobacteria and nanobes, whose status as living organisms are controversial. Its genome is only 490,885 nucleotides long; the smallest non-viral genome ever sequenced next to C. Ruddii's in 2006.

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