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''Prochlorococcus'' is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of very small (0.6  μm) marine
cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
with an unusual pigmentation ( chlorophyll ''a2'' and ''b2''). These bacteria belong to the photosynthetic picoplankton and are probably the most abundant
photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
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 Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. ''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
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s. In contrast,
eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
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 The ''International Code of Nomenclature for algae, fungi, and plants'' (ICN or ICNafp) is the set of rules and recommendations dealing with the formal botanical names that are given to plants, fungi and a few other groups of organisms, all th ...
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 The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...
, Robert J. Olson of the
Woods Hole Oceanographic Institution The Woods Hole Oceanographic Institution (WHOI, acronym pronounced ) is a private, nonprofit research and higher education facility dedicated to the study of marine science and engineering. Established in 1930 in Woods Hole, Massachusetts, it i ...
, and other collaborators in the Sargasso Sea using flow cytometry. Chisholm was awarded the
Crafoord Prize The Crafoord Prize () is an annual science prize established in 1980 by Holger Crafoord, a Swedish industrialist, and his wife Anna-Greta Crafoord following a donation to the Royal Swedish Academy of Sciences. It is awarded jointly by the Acade ...
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 The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...
(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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
groups within the cyanobacteria subgroup of the
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
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 Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
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 The surface-area-to-volume ratio or surface-to-volume ratio (denoted as SA:V, SA/V, or sa/vol) is the ratio between surface area and volume of an object or collection of objects. SA:V is an important concept in science and engineering. It is use ...
, 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 individuals (for comparison, that is approximately the number of atoms in a ton of
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
). ''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 Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
, and forms part of the base of the ocean
food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
.


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. 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 The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...
, 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 Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
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.


Ecology

Ancestors of ''Prochlorococcus'' contributed to the production of early atmospheric oxygen. 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. The size of ''Prochlorococcus'' (0.5 to 0.7 μm) 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); 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. This enormous number makes the ''Prochlorococcus'' play an important role in the global
carbon cycle The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycl ...
and oxygen production. Along with '' Synechococcus'' (another genus of cyanobacteria that co-occurs with ''Prochlorococcus'') these cyanobacteria are responsible for approximately 50% of marine carbon fixation, making it an important
carbon sink A carbon sink is a natural or artificial carbon sequestration process that "removes a  greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere". These sinks form an important part of the natural carbon cycle. An overar ...
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.


Ecotypes

''Prochlorococcus'' has different "ecotypes" occupying different niches and can vary by pigments, light requirements, nitrogen and phosphorus utilization, copper, and virus sensitivity. It is thought that ''Prochlorococcus'' may occupy potentially 35 different ecotypes and sub-ecotypes within the worlds' oceans. They can be differentiated on the basis of the sequence of the
ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...
gene. It has been broken down by NCBI Taxonomy into two different subspecies, Low-light Adapted (LL) or High-light Adapted (HL). There are six clades within each subspecies.


Low-light adapted

''Prochlorococcus marinus'' subsp. ''marinus'' is associated with low-light adapted types. It is also further classified by sub-ecotypes LLI-LLVII, where LLII/III has not been yet phylogenetically uncoupled. 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, which aids in its 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 >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.


High-light adapted

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


Metabolism

Most cyanobacterium are known to have an incomplete tricarboxylic acid cycle (TCA). In this process, 2-oxoglutarate decarboxylase (2OGDC) and succinic semialdehyde dehydrogenase (SSADH), replace the enzyme 2-oxoglutarate dehydrogenase (2-OGDH). Normally, when this enzyme complex joins with NADP+, it can be converted to succinate from 2-oxoglutarate (2-OG). This pathway is non-functional in ''Prochlorococcus'', as succinate dehydrogenase has been lost evolutionarily to conserve energy that may have otherwise been lost to phosphate metabolism.


Strains

Table modified from


See also

* Photosynthetic picoplankton * '' Pelagibacter'' * '' Synechococcus''


References


Further reading

* * * *


External links


The Most Important Microbe You've Never Heard Of
NPR Story on ''Prochlorococcus'' {{Taxonbar, from=Q18645284, from2=Q310210 Synechococcales Environmental microbiology Monotypic bacteria genera Cyanobacteria genera Marine microorganisms