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. It was originally assumed that Prochlorococcus was related to Prochloron and other chlorophyll b containing bacteria, called prochlorophytes, but it is now known that they form several separate lines of cyanobacteria.

Marine cyanobacteria are to date the smallest known photosynthetic organisms: Prochlorococcus is the smallest at just 0.5 to 0.8 micrometres across. Possibly they are also the most plentiful species on Earth: a rich drop of surface seawater may contain 100,000 cells. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans. The light harvesting pigment complement of Prochlorococcus is unique, consisting predominantly of divinyl derivatives of chlorophyll a (Chl a2) and b (Chl b2) and lacking monovinyl chlorophylls. Prochlorococcus occupies two distinct niches, leading to the nomenclature of the low light (LL) and high light (HL) groups, which vary in pigment ratios (LL possess a high ration of chlorophyll b2: a2 and HL low b2: a2), light requirements, nitrogen and phosphorus utilization, copper and virus sensitivity.

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.

Cyanobacteria

Prochlorococcus