Smallest organisms

The smallest organisms found on Earth can be determined according to various aspects of organism size; including volume, mass, height, length, or genome size.

Given the incomplete nature of scientific knowledge, it is possible that the smallest organism is undiscovered. Furthermore, there is some debate over the definition of life, and what entities qualify as organisms; consequently the smallest known organism (microorganism) is debatable.

Many 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 and can employ the metabolism of their host, they can be considered organisms. The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kilo base pairs (kb) and particle diameters of 80 nanometres (nm). The smallest double stranded 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. It is important to consider other self replicating genetic elements, such as satelliviruses, viroids and ribozymes.

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