Prochlorococcus
Prochlorococcus is a genus of marine cyanobacteria that now includes some three dozen species, differentiated on the basis of their ribosomal DNA. Sallie W. Chisholm of the Massachusetts Institute of Technology, Robert J. Olson of the Woods Hole Oceanographic Institution, and "other collaborators" (according to the Scientific American article listed in the "References" section) gave the genus that name. They did so with the incorrect assumption Prochlorococcus is related to prochlorophytes.
The_Blue_Marble.jpg
Marine cyanobacteria are to date the smallest known photosynthetic organisms: Prochlorococcus is just 0.5 to 0.7 micrometres across. Possibly they are also the most plentiful species on Earth: a rich drop of surface seawater may contain 20,000 cells.
Historically, the cyanobacteria were the first organisms on Earth that produced oxygen. They were also antecedent to the chloroplasts in cells of algae and all higher plants.
References
- Steve Nadis, "The Cells That Rule the Seas: The ocean’s tiniest inhabitants, notes biological researcher Sallie W. Chisholm, hold the key to understanding the biosphere—and what happens when humans disrupt it", Scientific American. December 2003, pp 52f.