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. Prochlorococcus strains, called ecotypes, have physiological differences enabling them to exploit different ecological niches. Analysis of the genome sequences of Prochlorococcus strains show that 1,273
The genus and the type species were made validly published names under the ICNP in 2001 with Validation list no. 79. They became valid under the ICNafp in 2020 with the description of Komárek et al.
Discovery
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. Chisholm was awarded the Crafoord Prize in 2019 for the discovery. 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 within the genus described is Prochlorococcus marinus, although two subspecies have been named for low-light and high-light adapted niche variations.
Morphology
Marine cyanobacteria are to date the smallest known photosynthetic organisms; Prochlorococcus is the smallest at just 0.5 to 0.7 micrometres in diameter. 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. 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 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. 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. According to a study published in 2025, the abundance of Prochlorococcus in tropical oceans could decline dramatically in the 21st century, with up to 51 percent of the population projected to disappear by 2100 under moderate and high warming scenarios, which could trigger a chain reaction in marine food webs.
Pigments
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 chlorophyll 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.
Genome
The genomes of several strains of Prochlorococcus have been sequenced. Research has shown that a massive genome reduction occurred during the Neoproterozoic Snowball Earth, which was followed by population bottlenecks.
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). These cyanobacteria also have the gene lexA that regulates an SOS response system, probably a system like the well-studied E. coli SOS system that is employed in the response to DNA damage. Despite Prochlorococcus 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. 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 to individuals worldwide. 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.
Ecotypes
Prochlorococcus has different "ecotypes" occupying different niches and can vary by pigments, light requirements, nitrogen and phosphorus utilization, copper, and virus sensitivity. LV species are found in highly iron scarce locations around the equator, and as a result, have lost several ferric proteins. The low-light adapted subspecies is otherwise known to have a higher ratio of chlorophyll b2 to chlorophyll a2, 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. Their genomes can range from 1,650,000 to 2,600,000 basepairs in size. In this process, 2-oxoglutarate decarboxylase (2OGDC) and succinic semialdehyde dehydrogenase (SSADH), replace the enzyme 2-oxoglutarate dehydrogenase (2-OGDH).
Strains
{| class="wikitable"
|+
!Strain
!Subtype
!Source
|-
|MIT9515
|HLI
|
|-
|MED4
|HLI
|
|-
|MIT0604
|HLII
|
|-
|XMU1408
|LLI
|
|-
|SS2
|LLII/III
|
|-
|MIT1306
|LLIV
|