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Scientific classification
Kingdom: Bacteria
Phylum: Cyanobacteria
Order: Synechococcales
Family: Prochloraceae
Genus: Prochlorococcus
Chisholm et al., 1992

P. marinus

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.[1] 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.[1] In contrast, eukaryotic algae have over 10,000 genes.[2]


Although there had been several earlier records of very small chlorophyll-b-containing cyanobacteria in the ocean,[3][4] Prochlorococcus was discovered in 1986[5] 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[6] 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 0.8 micrometres across. The coccoid shaped cells are non-motile and free-living. Their small size, thus 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.[7] Typically, Prochlorococcus divide once a day in the subsurface layer or oligotrophic waters.[7]


Prochlorococcus is abundant in the euphotic zone of the world's tropical oceans.[8] 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[9] (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.[10] 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).[1] 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.[11] LL type Prochlorococcus have a higher ratio of chlorophyll b to chlorophyll a, which aids in their ability to absorb blue light.[12] 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.[13] The bacterium accounts for an estimated 20% of the global photosynthetic production of oxygen, and forms part of the base of the ocean food chain.[14]


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.[15] 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.[16]

Prochlorococcus occupies two distinct niches, leading to the nomenclature of the low light (LL) and high light (HL) groups,[17] 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.[18] Recently the genomes of several strains of Prochlorococcus have been sequenced.[19][20] 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.[18]


P. marina is Earth's smallest plant. It is typically present at a density of on the order of a hundred thousand individuals per milliliter in the ocean. It sequesters ∼25% of the global CO
total. P. ubique is one of the ocean's most abundant microbes dominates the microbial loop.[21]

See also[edit]


  1. ^ a b c Munn, C. Marine Microbiology: ecology and applications Second Ed. Garland Science, 2011.
  2. ^ 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. PMC 2151091Freely accessible. PMID 18159947. doi:10.1371/journal.pgen.0030231. 
  3. ^ 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. 
  4. ^ 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. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ a b 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 98958Freely accessible. PMID 10066832. 
  8. ^ 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. 
  9. ^ 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. PMC 3683724Freely accessible. PMID 23703908. doi:10.1073/pnas.1307701110. 
  10. ^ 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 98958Freely accessible. PMID 10066832. 
  11. ^ 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. PMID 16556843. doi:10.1126/science.1122050. 
  12. ^ 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. 
  13. ^ 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. 
  14. ^ The Most Important Microbe You've Never Heard Of
  15. ^ 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. 
  16. ^ 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. 
  17. ^ 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. 
  18. ^ a b 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. 
  19. ^ 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. PMID 12917642. doi:10.1038/nature01947. Archived from the original (– Scholar search) on December 11, 2004. 
  20. ^ 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. PMC 187748Freely accessible. PMID 12917486. doi:10.1073/pnas.1733211100. 
  21. ^ Zehr, Jonathan P.; Weitz, Joshua S.; Joint, Ian (2017-08-18). "How microbes survive in the open ocean". Science. 357 (6352): 646–647. ISSN 0036-8075. doi:10.1126/science.aan5764. 

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