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
Brown algae colonies of cells, and the brown algae are the only major group of seaweeds that does not include such forms. However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms. Whatever their form, the body of all brown algae is termed a "thallus", indicating that it lacks the complex xylem and phloem of vascular plants. This does not mean that brown algae completely lack specialized structures. But, because some botanists define "true" stems, leaves, and roots by the