Riding the fire sphere: Nanoarchaeum equitans

Nanoarchaeum equitans is one of the smallest living organisms known so far: a nano-sized (about 400 nm in diameter) microbe of the third domain,  named archaea [1-4]. The epithet equitans relates to the Latin nouns equus and equitatus, meaning “horse” and “horse riding,” respectively. N. equitans is too small to ride a horse: it is riding as a symbiont on other archaea in the genus Ignicoccus. Ignicocci (such as I.  islandicus and I. hospitalis) are sphere-shaped hyperthermophiles, (extremophiles, typically growing at 80 °C (176 F), but also at higher temperatures); hence the association that N. equitans is riding the fire sphere.

The symbiotic relationship between these two types of archaea has been described as N. equitans parasites attached to the surface of their Ignicoccus host. The parasites are living off the metabolism of its host. N. equitans lacks genes for its own metabolism, but possesses genes for DNA repair and reproduction. It has a highly compact genome—the smallest microbial genome sequenced to date [3,4].

N. equitans was discovered in 2002 by Karl Otto Stetter of the University of Regensburg (Bavaria, Germany), while exploring hydrothermal vents of the Kolbeinsey Ridge [1,2],  a stretch of the Mid-Atlantic Ridge named after a submarine volcano north of Iceland [5]. Stetter is responsible for the name Nanoarchaeum equitans.
 
Keywords: microbiology, nanobiology, archaeal kingdom Nanoarchaeota, hyperthermophile, crenarchaeon, nomenclature.

References and more to explore
[1] Tim Friend: The Third Domain. The Untold Story of Archaea and the Future of Biotechnology. Joseph Henry Press, Washington, D.C., 2007.
[2]  Microbe Wiki: Nanoarchaeum equitans [biowiki.kenyon.edu/index.php/Nanoarchaeum_equitans].
[3] E. Waters et al.: The genome of Nanoarchaeum rquitans: Insights into early archaeal evolution and derived parasitism. Proc. Natl. Acad. Sci. USA October 2003, 100 (22), pp. 12984-12988 [www.ncbi.nlm.nih.gov/pmc/articles/PMC240731].
[4] K. S. Makarova and E. V. Koonin: Evolutionary and functional genomics of the Archaea. Curr. Opin. Microbiol. October 2005, 8 (5), pp. 586-594 [www.ncbi.nlm.nih.gov/pubmed/16111915].
[5] Global Volcanism Program > Kolbeinsey Ridge: www.volcano.si.edu/world/volcano.cfm?vnum=1705-01=.