Prochlorococcus is a genus of very small (0.6 µm) marine cyanobacteria with an unusual pigmentation (chlorophyll a2 and b2). These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth. Prochlorococcus microbes are among the major primary producers in the ocean, responsible for a large percentage of the photosynthetic production of oxygen. Analysis of the genome sequences of 12 Prochlorococcus strains show that 1,100 genes are common to all strains, and the average genome size is about 2,000 genes. In contrast, eukaryotic algae have over 10,000 genes.
Chisholm et al., 1992
Although there had been several earlier records of very small chlorophyll-b-containing cyanobacteria in the ocean, Prochlorococcus was discovered in 1986 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 first culture of Prochlorococcus was isolated in the Sargasso Sea in 1988 (strain SS120) and shortly another strain was obtained from the Mediterranean Sea (strain MED). The name Prochlorococcus 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 of the genus that has been described is Prochlorococcus marinus.
Marine cyanobacteria are to date the smallest known photosynthetic organisms; Prochlorococcus is the smallest at just 0.5 to 0.7 micrometres in diameter.. The coccoid shaped cells are non-motile and free-living. Their small size and large surface-area-to-volume ratio, gives them an advantage in nutrient poor water. Still, it is assumed that Prochlorococcus have a very small nutrient requirement. Moreover, Prochlorococcus have adapted to use sulfolipids instead of phospholipids in their membranes to survive in phosphate deprived environments. This adaptation allows them to avoid competition with heterotrophs that are dependent on phosphate for survival. Typically, Prochlorococcus divide once a day in the subsurface layer or oligotrophic waters.
Prochlorococcus is abundant in the euphotic zone of the world's tropical oceans. It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, the average yearly abundance is between 2.8 and 3.0 octillion (~1027) individuals (for comparison, that is approximately the number of atoms in a ton of gold). Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans. Prochlorococcus is mostly found in a temperature range of 10-33 °C and some strains can grow at depths with low light (<1% surface light). These strains are known as LL (Low Light) ecotypes, with strains that occupy shallower depths in the water column known as HL (High Light) ecotypes. LL type Prochlorococcus have a higher ratio of chlorophyll b to chlorophyll a, which aids in their ability to absorb blue light. Blue light is able to penetrate ocean waters deeper than the rest of the visible spectrum, and can reach depths of >200m, depending on the turbidity of the water. This penetration depth of blue light, combined with the ability of LL type Prochlorococcus to utilise it for photosynthesis, allows populations of LL Prochlorococcus to survive at depths of up to 200m. Furthermore, Prochlorococcus are more plentiful in the presence of heterotrophs that have catalase abilities. Prochlorococcus do not have mechanisms to degrade reactive oxygen species and rely on heterotrophs to protect them.  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.
Prochlorococcus is closely related to Synechococcus, another abundant photosynthetic cyanobacteria, which contains the light-harvesting antennae phycobilisomes. However, Prochlorochoccus has evolved to use a unique light-harvesting complex, consisting predominantly of divinyl derivatives of chlorophyll a (Chl a2) and b (Chl b2) and lacking monovinyl chlorophylls and phycobilisomes. 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.
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 has a high ratio of chlorophyll b2:a2 and HL has a low ratio of b2:a2), light requirements, nitrogen and phosphorus utilization, copper, and virus sensitivity. These "ecotypes" can be differentiated on the basis of the sequence of their ribosomal RNA gene. High-light adapted strains inhabit depths between 25 and 100 m, while low-light adapted strains inhabit waters between 80 and 200 m. Recently the genomes of several strains of Prochlorococcus have been sequenced. 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.
Despite Prochlorococcus being one of the smallest types of marine phytoplankton/bacteria in the worlds oceans , its substantial number makes it responsible for a major part of the oceans’ and world’s photosynthesis and oxygen production. The size of Prochlorococcus (i.e. around 0.5 to 0.7 micrometers in diameter) and the adaptations of the various ecotypes allow the organism to grow abundantly in low nutrient waters such as the waters of the tropics and the subtropics (~40N to 40S), however they can be found in higher latitudes as high up as 60° degrees north but at fairly minimal concentrations and the bacteria's distribution across the oceans suggest that the colder waters could be fatal to it. This wide range of latitude along with the bacteria’s ability to survive up to depths of 100 to 150 meters i.e. the average depth of the mixing layer of the surface ocean, allows it to grow to enormous numbers, up to 3 octillion individuals worldwide. This enormous number makes the Prochlorococcus play an important role in the global carbon cycle and oxygen production. Along with Synechococcus(another species of cyanobacteria that coexist with prochlorococcus) these cyanobacteria is responsible for approximately 50% of marine carbon fixation, making it an important carbon sink via the biological carbon pump(i.e. the transfer of organic carbon from the surface ocean to the deep via several biological, physical and chemical processes). 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.
- Munn, C. Marine Microbiology: ecology and applications Second Ed. Garland Science, 2011.
- "Life at the Edge of Sight — Scott Chimileski, Roberto Kolter | Harvard University Press". www.hup.harvard.edu. Retrieved 2018-01-26.
- Kettler GC, Martiny AC, Huang K, et al. (December 2007). "Patterns and Implications of Gene Gain and Loss in the Evolution of Prochlorococcus". PLoS Genetics. 3 (12): e231. doi:10.1371/journal.pgen.0030231. PMC . PMID 18159947.
- P. W. Johnson & J. M. Sieburth (1979). "Chroococcoid cyanobacteria in the sea: a ubiquitous and diverse phototrophic biomass". Limnology and Oceanography. 24 (5): 928–935. doi:10.4319/lo.1979.24.5.0928.
- W. W. C. Gieskes & G. W. Kraay (1983). "Unknown chlorophyll a derivatives in the North Sea and the tropical Atlantic Ocean revealed by HPLC analysis". Limnology and Oceanography. 28 (4): 757–766. doi:10.4319/lo.1983.28.4.0757.
- S. W. Chisholm; R. J. Olson; E. R. Zettler; J. Waterbury; R. Goericke; N. Welschmeyer (1988). "A novel free-living prochlorophyte occurs at high cell concentrations in the oceanic euphotic zone". Nature. 334 (6180): 340–343. doi:10.1038/334340a0.
- Sallie W. Chisholm, S. L. Frankel, R. Goericke, R. J. Olson, B. Palenik, J. B. Waterbury, L. West-Johnsrud & E. R. Zettler (1992). "Prochlorococcus marinus nov. gen. nov. sp.: an oxyphototrophic marine prokaryote containing divinyl chlorophyll a and b". Archives of Microbiology. 157 (3): 297–300. doi:10.1007/BF00245165.
- Biller, Steven J.; Berube, Paul M.; Lindell, Debbie; Chisholm, Sallie W. (1 December 2014). "Prochlorococcus: the structure and function of collective diversity". Nature Reviews Microbiology. 13 (1): 13–27. doi:10.1038/nrmicro3378.
- Partensky F, Hess WR, Vaulot D (1999). "Prochlorococcus, a marine photosynthetic prokaryote of global significance". Microbiology and Molecular Biology Reviews. 63 (1): 106–127. PMC . PMID 10066832.
- Van Mooy, B. A. S.; Rocap, G.; Fredricks, H. F.; Evans, C. T.; Devol, A. H. (26 May 2006). "Sulfolipids dramatically decrease phosphorus demand by picocyanobacteria in oligotrophic marine environments". Proceedings of the National Academy of Sciences. 103 (23): 8607–8612. doi:10.1073/pnas.0600540103.
- Chisholm, S.W.; Frankel, S.; Goericke, R.; Olson, R.; Palenik, B.; Waterbury, J.; West-Johnsrud, L.; Zettler, E. (1992). "Prochlorococcus marinus nov. gen. nov. sp.: an oxyphototrophic marine prokaryote containing divinyl chlorophyll a and b". Archives of Microbiology. 157 (3): 297–300. doi:10.1007/bf00245165.
- Flombaum, P.; Gallegos, J. L.; Gordillo, R. A.; Rincon, J.; Zabala, L. L.; Jiao, N.; Karl, D. M.; Li, W. K. W.; Lomas, M. W.; Veneziano, D.; Vera, C. S.; Vrugt, J. A.; Martiny, A. C. (2013). "Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus". Proceedings of the National Academy of Sciences. 110 (24): 9824–9829. doi:10.1073/pnas.1307701110. PMC . PMID 23703908.
- F. Partensky, W. R. Hess & D. Vaulot (1999). "Prochlorococcus, a Marine Photosynthetic Prokaryote of Global Significance". Microbiology and Molecular Biology Reviews. 63 (1): 106–127. PMC . PMID 10066832.
- Coleman, M.; Sullivan, M.; Martiny, A.; Steglich, C.; Barry, K.; DeLong, E.; Chisholm, S. (2006). "Genomic islands and the ecology and evolution of Prochlorococcus". Science. 311 (5768): 1768–1770. doi:10.1126/science.1122050. PMID 16556843.
- Ralf, G.; Repeta, D. (1992). "The pigments of Prochlorococcus marinus: The presence of divinylchlorophyll a and b in a marine prokaryote". Limnology and Oceanography. 37 (2): 425–433. doi:10.4319/lo.1992.37.2.0425.
- Zinser, E.; Johnson, Z.; Coe, A.; Karaca, E.; Veneziano, D.; Chisholm, S. (2007). "Influence of light and temperature on Prochlorococcus ecotype distributions in the Atlantic Ocean". Limnology and Oceanography. 52 (5): 2205–2220. doi:10.4319/lo.2007.52.5.2205.
- Morris, J. J.; Kirkegaard, R.; Szul, M. J.; Johnson, Z. I.; Zinser, E. R. (23 May 2008). "Facilitation of Robust Growth of Prochlorococcus Colonies and Dilute Liquid Cultures by "Helper" Heterotrophic Bacteria". Applied and Environmental Microbiology. 74 (14): 4530–4534. doi:10.1128/AEM.02479-07.
- Johnson, Zachary I.; Zinser, Erik R.; Coe, Allison; McNulty, Nathan P.; Woodward, E. Malcolm S.; Chisholm, Sallie W. (2006). "Niche Partitioning among Prochlorococcus Ecotypes along Ocean-Scale Environmental Gradients". Science. 311 (5768): 1737–1740. doi:10.1126/science.1118052.
- Ting CS, Rocap G, King J, Chisholm S (2002). "Cyanobacterial photosynthesis in the oceans: the origins and significance of divergent light-harvesting strategies". Trends in Microbiology. 10 (3): 134–142. doi:10.1016/s0966-842x(02)02319-3.
- Goericke R & Repeta D (1992). "The pigments of Prochlorococcus marinus: the presence of divinyl chlorophyll a and b in a marine prokaryote". Limnology and Oceanography. 37 (2): 425–433. doi:10.4319/lo.1992.37.2.0425.
- N. J. West & D. J. Scanlan (1999). "Niche-partitioning of Prochlorococcus in a stratified water column in the eastern North Atlantic Ocean". Applied and Environmental Microbiology. 65: 2585–2591.
- Martiny AC, Tai A, Veneziano D, Primeau F, Chisholm S (2009). "Taxonomic resolution, ecotypes and biogeography of Prochlorococcus". Environmental Microbiology. 11 (4): 823–832. doi:10.1111/j.1462-2920.2008.01803.x.
- G. Rocap; F. W. Larimer; J. Lamerdin; S. Malfatti; P. Chain; N. A. Ahlgren; A. Arellano; M. Coleman; L. Hauser; W. R. Hess; Z. I. Johnson; M. Land; D. Lindell; A. F. Post; W. Regala; M. Shah; S. L. Shaw; C. Steglich; M. B. Sullivan; C. S. Ting; A. Tolonen; E. A. Webb; E. R. Zinser; S. W. Chisholm (2003). "Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation" (PDF). Nature. 424 (6952): 1042–1047. doi:10.1038/nature01947. PMID 12917642. Archived from the original (– Scholar search) on December 11, 2004.
- A. Dufresne; M. Salanoubat; F. Partensky; F. Artiguenave; I. M. Axmann; V. Barbe; S. Duprat; M. Y. Galperin; E. V. Koonin; F. Le Gall; K. S. Makarova; M. Ostrowski; S. Oztas; C. Robert; I. B. Rogozin; D. J. Scanlan; N. Tandeau de Marsac; J. Weissenbach; P. Wincker; Y. I. Wolf; W. R. Hess (2003). "Genome sequence of the cyanobacterium Prochlorococcus marinus SS120, a nearly minimal oxyphototrophic genome". Proceedings of the National Academy of Sciences. 100 (17): 10020–10025. doi:10.1073/pnas.1733211100. PMC . PMID 12917486.
- Partensky F, Hess WR, Vaulot (1999). "Prochlorococcus, a Marine Photosynthetic Prokaryote of Global Significance". Microbiology and Molecular Biology Reviews: 106–127.
- Partensky, Blanchot, Vaulot (1999). "DifferentiaI distribution and ecology of Prochlorococcus and Synechococcus in oceanic waters: a review". Bulletin de l'Institut océanographique. Monaco. n°. spécial 19 (spécial 19).
- Erik R. Zinser1,‡, Allison Coe1, Zackary I. Johnson1,§, Adam C. Martiny1, Nicholas J. Fuller2, David J. Scanlan2 and Sallie W. Chisholm1,* (January 2006). "Prochlorococcus Ecotype Abundances in the North Atlantic Ocean As Revealed by an Improved Quantitative PCR Method†". Applied and Environmental Microbiology. vol. 72.
- Fei-Xue Fu, Mark E. Warner, Yaohong Zhang, Yuanyuan Feng, David A. Hutchins (16 May 2007). "Effects of Increased Temperature and CO2 on Photosynthesis, Growth, and Elemental Ratios in Marine Synechococcus and Prochlorococcus (Cyanobacteria)". Journal of Phycology. 43 (3): 485–496. doi:10.1111/j.1529-8817.2007.00355.x.
- L. Campbell, H. A. Nolla & D. Vaulot (1994). "The importance of Prochlorococcus to community structure in the central North Pacific Ocean". Limnology and Oceanography. 39 (4): 954–961. doi:10.4319/lo.1994.39.4.0954.
- Jagroop Pandhal, Phillip C. Wright & Catherine A. Biggs (2007). "A quantitative proteomic analysis of light adaptation in a globally significant marine cyanobacterium Prochlorococcus marinus MED4". Journal of Proteome Research. 6 (3): 996–1005. doi:10.1021/pr060460c.
- Steve Nadis (2003). "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: 52f.
- Melissa Garren (2012). "The sea we've hardly seen". TEDx Monterey: 52f.
- M. D. Guiry. "Prochlorococcus S.W. Chisholm, S.L. Frankel, R. Goericke, R.J. Olson, B. Palenik, J.B. Waterbury, L. West-Johnsrud & E.R. Zettler 1992: 299". AlgaeBase.
- The Most Important Microbe You've Never Heard Of: NPR Story on Prochlorococcus
- The Habitable Planet Video
- Genome Information for Prochlorococcus marinus: National Center for Biotechnology Information.