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The smallest organism found on Earth can be measured using a variety of different methods, and can be defined as the smallest by volume, mass, height, or length. Because there is some dispute over what the definition of life is and what entities qualify as organisms, the actual "smallest organism" (microorganisms) may be unclear.

Microorganisms

Viruses

The majority of biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves, requiring a host cell to replicate and synthesise new products. A minority of scientists hold that because viruses do have genetic material they can be considered as organisms. The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kb and particle diameters of 80 nm. The smallest DNA viruses are the hepadnaviruses such as Hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18-26 nm, but larger genomes, at 5 kb. The smallest DNA bacteriophage is the Phi-X174 phage, thought to be larger than Hepatitis B, at about 4 kb.

Nanobes

Nanobes are thought by some to be the smallest known organism, about ten times smaller than the smallest known bacteria. Nobes, tiny filamental structures first found in some rocks and sediments, are ten times smaller than the smallest known bacteria and were first described in 1996 by Philipa Uwins of the University of Queensland. The smallest are 20 nm long. However, some researchers believe them to be merely crystal growths, but a purported find of DNA in nanobe samples may prove otherwise. They are similar to the life-like structures found in ALH84001, the famous Mars meteorite from the Antarctic.

Nanoarchaeum

Nanoarchaeum equitans is a species of tiny microbe discovered in 2002 in a hydrothermal vent off the coast of Iceland by Karl Stetter. A thermophile that grows in near-boiling temperatures, 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 smallest known living organism, excepting possibly nanobes, if the latter is considered to be living. Its genome is only 490,885 nucleotides long; as of 2005 it remains the smallest non-viral genome ever sequenced.

Mycoplasma genitalium

Mycoplasma genitalium, a parasitic bacterium which lives in the primate genital and respiratory tracts, is thought to be the smallest known organism capable of independent growth and reproduction. With a size of 0.2 to 0.3 µm (micrometers), M. genitalium is an ultramicrobacteria smaller than other small bacteria, including rickettsia and chlamydia. However, the vast majority of bacterial strains have not been studied, and the marine ultramicrobacteria Sphingomonas sp strain RB2256 is reported to have passed through 0.22 µm ultrafilter. A compliating factor is nutrient-downsized bacteria, bacteria that become much smaller due to lack of available nutrients.