* By volume: Viruses are generally considered the smallest microorganisms, with some being as small as 20 nanometers in diameter. However, viruses are not technically living organisms.
* By diameter: Mycoplasma are a genus of bacteria that are among the smallest free-living organisms, with some species measuring only 0.1 micrometers in diameter.
* By genome size: Nanoarchaeum equitans is a type of archaeon with the smallest known genome, containing only 490,885 base pairs.
Ultimately, the "smallest" microorganism depends on your criteria.
