Marine life from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. Microorganisms constitute more than 90% of the marine biomass. A microorganism (or microbe) is a microscopic living organism, which may be single-celled or multicellular. Microorganisms are very diverse and include all bacteria, archaea and most protozoa. This group also contains some species of fungi, algae, and certain microscopic animals, such as rotifers. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving. In
Bacteria sizes, called morphologies. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometres in length. However, a few species are visible to the unaided eye—for example, "Thiomargarita namibiensis" is up to half a millimetre long and "Epulopiscium fishelsoni" reaches 0.7 mm. Among the smallest bacteria are members of the genus "Mycoplasma", which measure only 0.3 micrometres, as small as the largest viruses. Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied. Most bacterial species are either spherical, called "cocci" ("sing". coccus, from Greek "kókkos", grain, seed), or rod-shaped, called "bacilli" ("sing". bacillus,