Prochlorococcus is a genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of very small (0.6 µm
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

) marine
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 cyanobacteria with an unusual pigmentation (chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 b). These bacteria belong to the photosynthetic picoplankton
Photosynthetic picoplankton
Photosynthetic picoplankton is the fraction of the plankton performing photosynthesis composed by cells between 0.2 and 2 µm . It is especially important in the central oligotrophic regions of the world oceans that have very low concentration of nutrients.- History :* 1952: Description of the...

 and are probably the most abundant photosynthetic
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 organism on Earth.
Although there had been several earlier records of very small chlorophyll-b-containing cyanobacteria in the ocean, Prochlorococcus was actually discovered in 1986 by Sallie W. (Penny) Chisholm
Sallie W. Chisholm
Sallie W. Chisholm is a U.S. biological oceanographer. She attended Skidmore College and received her Ph.D. from the SUNY Albany in 1974. Since 1976, she has been a member of the faculty at the Massachusetts Institute of Technology, where she is the McAfee Professor of Engineering.Chisholm's...

 of the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

, Robert J.