Prochlorococcus is agenus of very small (0.6 μm)marinecyanobacteria with an unusual pigmentation (chlorophylla2 andb2). These bacteria belong to thephotosynthetic picoplankton and are probably the most abundantphotosynthetic organism on Earth.Prochlorococcus microbes are among the majorprimary producers in the ocean, responsible for a large percentage of the photosynthetic production ofoxygen.^[1][2]Prochlorococcus strains, called ecotypes, have physiological differences enabling them to exploit different ecological niches.^[3] Analysis of thegenome sequences ofProchlorococcus strains show that 1,273^[4] genes are common to all strains, and the average genome size is about 2,000genes.^[1] In contrast,eukaryoticalgae have over 10,000 genes.^[4]

Prochlorococcus
TEM image ofProchlorococcus marinus (pseudo-colored)
Scientific classification
Domain:	Bacteria
Kingdom:	Bacillati
Phylum:	Cyanobacteriota
Class:	Cyanophyceae
Order:	Synechococcales
Family:	Prochloraceae
Genus:	Prochlorococcus Chisholm et al., 1992
Species:	P. marinus
Binomial name
Prochlorococcus marinus Chisholm et al., 1992

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] bySallie W. (Penny) Chisholm of theMassachusetts Institute of Technology, Robert J. Olson of theWoods Hole Oceanographic Institution, and other collaborators in theSargasso Sea usingflow cytometry. Chisholm was awarded theCrafoord Prize in 2019 for the discovery.^[8] The first culture ofProchlorococcus was isolated in the Sargasso Sea in 1988 (strain SS120) and shortly another strain was obtained from theMediterranean Sea (strain MED). The nameProchlorococcus^[9] originated from the fact it was originally assumed thatProchlorococcus was related toProchloron and other chlorophyll-b-containing bacteria, called prochlorophytes, but it is now known that prochlorophytes form several separatephylogenetic groups within the cyanobacteria subgroup of thebacteria domain. The only species within the genus described isProchlorococcus marinus, although two subspecies have been named for low-light and high-light adapted niche variations.^[10]

Marine cyanobacteria are to date the smallest knownphotosynthetic organisms;Prochlorococcus is the smallest at just 0.5 to 0.7 micrometres in diameter.^[11][2] The coccoid shaped cells are non-motile and free-living. Their small size and largesurface-area-to-volume ratio, gives them an advantage in nutrient-poor water. Still, it is assumed thatProchlorococcus have a very small nutrient requirement.^[12] Moreover,Prochlorococcus have adapted to usesulfolipids instead of phospholipids in their membranes to survive in phosphate deprived environments.^[13] This adaptation allows them to avoid competition with heterotrophs that are dependent on phosphate for survival.^[13] Typically,Prochlorococcus divide once a day in the subsurface layer or oligotrophic waters.^[12]

Prochlorococcus is abundant in theeuphotic zone of the world's tropical oceans.^[14] 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(2.8 to 3.0)×10²⁷ individuals^[15] (for comparison, that is approximately the number of atoms in aton ofgold).Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in theoligotrophic (nutrient-poor) regions of the oceans.^[12]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.^[16] Furthermore,Prochlorococcus are more plentiful in the presence of heterotrophs that have catalase abilities.^[17]Prochlorococcus do not have mechanisms to degrade reactive oxygen species and rely on heterotrophs to protect them.^[17] The bacterium accounts for an estimated 13–48% of the global photosynthetic production ofoxygen, and forms part of the base of the oceanfood chain.^[18]

Prochlorococcus is closely related toSynechococcus, another abundant photosynthetic cyanobacteria, which contains the light-harvesting antennaephycobilisomes. However,Prochlorochoccus has evolved to use a unique light-harvesting complex, consisting predominantly of divinyl derivatives ofchlorophyll a (Chl a2) andchlorophyll b (Chl b2) and lacking monovinyl chlorophylls and 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.^[20]

The genomes of several strains ofProchlorococcus have been sequenced.^[21][22] Twelve complete genomes have been sequenced which reveal physiologically and genetically distinct lineages ofProchlorococcus marinus that are 97% similar in the 16S rRNA gene.^[23] Research has shown that a massive genome reduction occurred during the NeoproterozoicSnowball Earth, which was followed bypopulation bottlenecks.^[24]

The high-light ecotype has the smallest genome (1,657,990 basepairs, 1,716 genes) of any known oxygenic phototroph, but the genome of the low-light type is much larger (2,410,873 base pairs, 2,275 genes).^[21]

MarineProchlorococcuscyanobacteria have several genes that function in DNArecombination,repair andreplication. These include therecBCD gene complex whose product,exonuclease V, functions in recombinational repair of DNA, and theumuCD gene complex whose product,DNA polymerase V, functions in error-prone DNA replication.^[25] These cyanobacteria also have the genelexA that regulates anSOS response system, probably a system like the well-studiedE. coli SOS system that is employed in the response toDNA damage.^[25]

Ancestors ofProchlorococcus contributed to the production of early atmospheric oxygen.^[26] DespiteProchlorococcus being one of the smallest types of marine phytoplankton in the world's oceans, its substantial number make it responsible for a major part of the oceans', world's photosynthesis, and oxygen production.^[2] The size ofProchlorococcus (0.5 to 0.7 μm)^[12] 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 (c. 40°N to 40°S);^[27] however, they can be found in higher latitudes as high up as 60° north but at fairly minimal concentrations and the bacteria's distribution across the oceans suggest that the colder waters could be fatal. This wide range of latitude along with the bacteria's ability to survive up to depths of 100 to 150 metres, i.e. the average depth of the mixing layer of the surface ocean, allows it to grow to enormous numbers, up to3×10²⁷ individuals worldwide.^[15] This enormous number makes theProchlorococcus play an important role in the globalcarbon cycle and oxygen production. Along withSynechococcus (another genus of cyanobacteria that co-occurs withProchlorococcus) these cyanobacteria are responsible for approximately 50% of marine carbon fixation, making it an importantcarbon 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).^[28] The abundance, distribution and all other characteristics of theProchlorococcus make it a key organism in oligotrophic waters serving as an important primary producer to the open ocean food webs.

Prochlorococcus has different"ecotypes" occupying different niches and can vary by pigments, light requirements, nitrogen and phosphorus utilization, copper, and virus sensitivity.^[29][11][21] It is thought thatProchlorococcus may occupy potentially 35 different ecotypes and sub-ecotypes within the worlds' oceans. They can be differentiated on the basis of the sequence of theribosomal RNA gene.^[11][29] It has been broken down byNCBI Taxonomy into two different subspecies, Low-light Adapted (LL) or High-light Adapted (HL).^[10] There are six clades within each subspecies.^[11]

Prochlorococcus marinus subsp.marinus is associated with low-light adapted types.^[10] It is also further classified by sub-ecotypes LLI-LLVII, where LLII/III has not been yet phylogenetically uncoupled.^[11][30] LV species are found in highly iron scarce locations around the equator, and as a result, have lost several ferric proteins.^[31] The low-light adapted subspecies is otherwise known to have a higher ratio of chlorophyll b2 to chlorophyll a2,^[29] which aids in its ability to absorb blue light.^[32] Blue light is able to penetrate ocean waters deeper than the rest of the visible spectrum, and can reach depths of >200 m, depending on the turbidity of the water. Their ability to photosynthesize at a depth where blue light penetrates allows them to inhabit depths between 80 and 200 m.^[23][33] Their genomes can range from 1,650,000 to 2,600,000 basepairs in size.^[30]

Prochlorococcus marinus subsp.pastoris is associated with high-light adapted types.^[10] It can be further classified by sub-ecotypes HLI-HLVI.^[30][11] HLIII, like LV, is also located in an iron-limited environment near the equator, with similar ferric adaptations.^[31] The high-light adapted subspecies is otherwise known to have a low ratio of chlorophyll b2 to chlorophyll a2.^[29] High-light adapted strains inhabit depths between 25 and 100 m.^[23] Their genomes can range from 1,640,000 to 1,800,000 basepairs in size.^[30]

Mostcyanobacterium are known to have an incompletetricarboxylic acid cycle (TCA).^[34][35] In this process,2-oxoglutarate decarboxylase (2OGDC) andsuccinic semialdehyde dehydrogenase (SSADH), replace the enzyme2-oxoglutarate dehydrogenase (2-OGDH).^[35] Normally, when this enzyme complex joins withNADP+, it can be converted to succinate from2-oxoglutarate (2-OG).^[35] This pathway is non-functional inProchlorococcus,^[35] assuccinate dehydrogenase has been lost evolutionarily to conserve energy that may have otherwise been lost to phosphate metabolism.^[36]

Table modified from^[30]

• Photosynthetic picoplankton
• Pelagibacter
• Synechococcus

• The Most Important Microbe You've Never Heard Of:NPR Story onProchlorococcus