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 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. 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. 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.
Distribution 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 (2.8 to 3.0)×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. 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. 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 a2 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. 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. 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).
DNA recombination, repair and replication Marine Prochlorococcus cyanobacteria have several genes that function in DNA recombination, repair and replication. These include the recBCD gene complex whose product, exonuclease V, functions in recombinational repair of DNA, and the umuCD gene complex whose product, DNA polymerase V, functions in error-prone DNA replication. 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.
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