Prochlorococcus microbes are among the major primary producers in the ocean, responsible for a large percentage of the photosynthetic production of oxygen.

[4] Although there had been several earlier records of very small chlorophyll-b-containing cyanobacteria in the ocean,[5][6] Prochlorococcus was discovered in 1986[7] by Sallie W. (Penny) Chisholm of the Massachusetts Institute of Technology, Robert J. Olson of the Woods Hole Oceanographic Institution, and other collaborators in the Sargasso Sea using flow cytometry.

The name Prochlorococcus[9] originated from the fact it was originally assumed that Prochlorococcus was related to Prochloron and other chlorophyll-b-containing bacteria, called prochlorophytes, but it is now known that prochlorophytes form several separate phylogenetic groups within the cyanobacteria subgroup of the bacteria domain.

The only species within the genus described is Prochlorococcus marinus, although two subspecies have been named for low-light and high-light adapted niche variations.

Worldwide, the average yearly abundance is (2.8 to 3.0)×1027 individuals[15] (for comparison, that is approximately the number of atoms in a ton of gold).

[17] The bacterium accounts for an estimated 13–48% of the global photosynthetic production of oxygen, and forms part of the base of the ocean food chain.

[18] Prochlorococcus is closely related to Synechococcus, another abundant photosynthetic cyanobacteria, which contains the light-harvesting antennae phycobilisomes.

[19] Prochlorococcus is the only known wild-type oxygenic phototroph that does not contain Chl a as a major photosynthetic pigment, and is the only known prokaryote with α-carotene.

[21][22] Twelve complete genomes have been sequenced which reveal physiologically and genetically distinct lineages of Prochlorococcus marinus that are 97% similar in the 16S rRNA gene.

[23] Research has shown that a massive genome reduction occurred during the Neoproterozoic Snowball Earth, which was followed by population bottlenecks.

[15] This enormous number makes the Prochlorococcus play an important role in the global carbon cycle and oxygen production.

[28] The abundance, distribution and all other characteristics of the Prochlorococcus make it a key organism in oligotrophic waters serving as an important primary producer to the open ocean food webs.

[11][30] LV species are found in highly iron scarce locations around the equator, and as a result, have lost several ferric proteins.