Photosynthetic picoplankton
Encyclopedia
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
performing photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
composed by cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
between 0.2 and 2 µm (picoplankton
Picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either :* photosynthetic * heterotrophic Some species can also be mixotrophic....
). It is especially important in the central oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic regions of the world oceans that have very low concentration of nutrients.
History
- 1952: Description of the first truly picoplanktonic species, Chromulina pusilla, by Butcher. This species was renamed in 1960 to MicromonasMicromonasIn taxonomy, Micromonas is a genus of algae, specifically of the Mamiellaceae. The genus contains a single species, Micromonas pusilla, which is the dominant photosynthetic picoeukaryote in some marine ecosystems. Unlike many marine algae, it is distributed widely in both warm and cold waters. It...
pusilla and a few studies have found it to be abundant in temperate oceanic waters, although very little such quantification data exists for eukaryotic picophytoplankton. - 1979 : Discovery of marine SynechococcusSynechococcusSynechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
by Waterbury and confirmation with electron microscopy by Johnson and Sieburth. - 1982 : The same Johnson and Sieburth demonstrate the importance of small eukaryotes by electron microscopy.
- 1983 : W.K. Li and Platt show that a large fraction of marine primary production is due to organisms smaller than 2 µm.
- 1986 : Discovery of "prochlorophytes" by Chisholm and Olson in the Sargasso Sea, named in 1992 as ProchlorococcusProchlorococcusProchlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
marinus. - 1994 : Discovery in the Thau lagoon in France of the smallest photosynthetic eukaryote known to date, OstreococcusOstreococcusOstreococcus is a genus of unicellular coccoid or spherically shaped green alga belonging to the class Prasinophyceae. It includes prominent members of the global picoplankton community, which plays a central role in the oceanic carbon cycle.-History:...
tauri, by Courties. - 2001 : Through sequencing of the ribosomal RNARibosomal RNARibosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
gene extracted from marine samples, several European teams discover that eukaryotic picoplankton are highly diverseBiodiversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
. This finding followed on the first discovery of such eukaryotic diversity in 1998 by Rappe and colleagues at Oregon State University, who were the first to apply rRNA sequencing to eukaryotic plankton in the open-ocean, where they discovered sequences that seemed distant from known phytoplankton The cells containing DNA matching one of these novel sequences were recently visualized and further analyzed using specific probes and found to be broadly distributed.
Methods of study
- Epifluorescence microscopyFluorescence microscopeA fluorescence microscope is an optical microscope used to study properties of organic or inorganic substances using the phenomena of fluorescence and phosphorescence instead of, or in addition to, reflection and absorption...
allows to detect certain groups of cells possessing fluorescentFluorescenceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
pigments such as SynechococcusSynechococcusSynechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
which possess phycoerythrinPhycoerythrinPhycoerythrin is a red protein from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptomonads.Like all phycobiliproteins, phycoerythrin is composed of a protein part, organised in a hexameric structure of alpha and beta chains, covalently binding...
. - Flow cytometryFlow cytometryFlow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...
measures the size (" forward scatter ") and fluorescence of 1,000 in 10,000 cells per second. It allows one to determine very easily the concentration of the various picoplankton populations on marine samples. Three groups of cells (ProchlorococcusProchlorococcusProchlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
,SynechococcusSynechococcusSynechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
and picoeukaryotes) can be distinguished. For example Synechococcus is characterized by the double fluorescence of its pigments: orange for phycoerythrin and red for chlorophyllChlorophyllChlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
. Flow cytometry also allows to sort out specific populations (for example Synechococcus) in order put them in culture, or to make more detailed analyses. - Analysis of photosynthetic pigmentsBiological pigmentBiological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...
such as chlorophyll or carotenoids by high precision chromatography (HPLCHigh-performance liquid chromatographyHigh-performance liquid chromatography , HPLC, is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.HPLC typically utilizes different types of stationary...
) allows to determine the various groups of algae present in a sample. - Molecular biology techniques:
- CloningCloningCloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
and sequencingSequencingIn genetics and biochemistry, sequencing means to determine the primary structure of an unbranched biopolymer...
of genesGênesGênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
such as that of ribosomal RNARibosomal RNARibosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
, which allows to determine total diversity within a sample - DGGE (Denaturing Gel Electrophoresis), that is faster than the previous approach allows to have an idea of the global diversity within a sample
- In situ hybridization (FISHFluorescent in situ hybridizationFISH is a cytogenetic technique developed by biomedical researchers in the early 1980s that is used to detect and localize the presence or absence of specific DNA sequences on chromosomes. FISH uses fluorescent probes that bind to only those parts of the chromosome with which they show a high...
) uses fluorescent probes recognizing specific taxonTaxon|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, for example a speciesSpeciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, a genusGenusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
or a classClass (biology)In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
. This original description as a species is now thought to be composed of a number of different cryptic species, a finding that has been confirmed by a genome sequencing project of two strains led by researchers at the Monterey Bay Aquarium Research InstituteMonterey Bay Aquarium Research InstituteThe Monterey Bay Aquarium Research Institute is a not-for-profit oceanographic research center in Moss Landing, California affiliated with the Monterey Bay Aquarium. It was founded in 1987 by David Packard of Hewlett-Packard fame... - Real-time PCR can be used, as FISH, to determine, the abundance of specific groups. It has the main advantage to allow the rapid analysis of a large number of samples simultaneously, but requires more sophisticated controls and calibrations.
Composition
Three major groups of organisms constitute photosynthetic picoplankton.- Cyanobacteria belonging to the genus SynechococcusSynechococcusSynechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
of a size of 1 µm (micrometer) were first discovered in 1979 by J. Waterbury (Woods Hole Oceanographic InstitutionWoods Hole Oceanographic InstitutionThe Woods Hole Oceanographic Institution is a private, nonprofit research and higher education facility dedicated to the study of all aspects of marine science and engineering and to the education of marine researchers. Established in 1930, it is the largest independent oceanographic research...
). They are quite ubiquitous, but most abundant in relatively mesotrophic waters. - Cyanobacteria belonging to the genus ProchlorococcusProchlorococcusProchlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
are particularly remarkable. With a typical size of 0.6 µm, Prochlorococcus was discovered only in 1988 by two American researchers, Sallie W. (Penny) Chisholm (Massachusetts Institute of TechnologyMassachusetts Institute of TechnologyThe Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
) and R.J. Olson (Woods Hole Oceanographic InstitutionWoods Hole Oceanographic InstitutionThe Woods Hole Oceanographic Institution is a private, nonprofit research and higher education facility dedicated to the study of all aspects of marine science and engineering and to the education of marine researchers. Established in 1930, it is the largest independent oceanographic research...
). In spite of its small size, this photosynthetic organism is undoubtedly the most abundant of the planet: indeed its density can reach up to 100 million cells per liter and it can be found down to a depth of 150 m in all the intertropical belt. - Picoplanktonic eukaryotes are the least well known, as demonstrated by the recent discovery of major groups. Andersen created in 1993 a new classClass (biology)In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of brown algae, the Pelagophyceae. More surprising still, the discovery in 1994 of a eukaryote of very small size, OstreococcusOstreococcusOstreococcus is a genus of unicellular coccoid or spherically shaped green alga belonging to the class Prasinophyceae. It includes prominent members of the global picoplankton community, which plays a central role in the oceanic carbon cycle.-History:...
tauri, dominating the phytoplanktonic biomass of a French brackish lagoon (étang de Thau), shows that these organisms can also play a major ecological role in coastal environments. In 1999, yet a new class of alga was discovered, the Bolidophyceae, very close genetically of diatoms, but quite different morphologically. At the present time, about 50 species are known belonging to several classes.
-
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- {|class="wikitable" align="center" bgcolor="#F6FFB2"
-
|+Algal classes containing picoplankton species
!Classes
!Picoplanktonic genera
|----
|Chlorophyceae
Chlorophyceae
The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. For example the chlorophycean CW clade, and chlorophycean DO clade, are defined by the arrangement of their flagella. Members of the CW clade have flagella that are displaced...
|Nannochloris
|----
|Prasinophyceae
Prasinophyceae
In taxonomy, Prasinophytes are a class of the Division Chlorophyta. These are primitive eukaryotic, marine green algae. Its best known genus is Ostreococcus , which is considered to be the smallest free-living eukaryote and which has been detected in marine samples around the world...
|Micromonas , Ostreococcus, Pycnococcus
|----
|Prymnesiophyceae
Prymnesiophyceae
Prymnesiophyceae is a haptophyte class. Although it was originally described by Casper in 1972, it did not receive a Latin diagnosis until Hibberd provided one in 1976....
|Imantonia
|----
|Pelagophyceae
|Pelagomonas
|----
|Bolidophyceae
|Bolidomonas
|----
|Dictyochophyceae
|Florenciella
|}
The use of molecular approaches implemented since the 1990s for bacteria, were applied to the photosynthetic picoeukaryotes only 10 years later around 2000. They revealed a very wide diversity and brought to light the importance of the following groups in the picoplankton :
- PrasinophyceaePrasinophyceaeIn taxonomy, Prasinophytes are a class of the Division Chlorophyta. These are primitive eukaryotic, marine green algae. Its best known genus is Ostreococcus , which is considered to be the smallest free-living eukaryote and which has been detected in marine samples around the world...
- Haptophyta
- Cryptophyta
In temperate coastal environment, the genus Micromonas
Micromonas
In taxonomy, Micromonas is a genus of algae, specifically of the Mamiellaceae. The genus contains a single species, Micromonas pusilla, which is the dominant photosynthetic picoeukaryote in some marine ecosystems. Unlike many marine algae, it is distributed widely in both warm and cold waters. It...
(Prasinophyceae) seems dominant. However, in numerous oceanic environments, the dominant species of eukaryotic picoplankton remain still unknown.
Ecology
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
in the oceanic environment.
- The SynechococcusSynechococcusSynechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
cyanobacterium is generally abundant in mesotrophic environments, for example in the vicinity of the equatorial upwellingUpwellingUpwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
or in coastal regions. - The ProchlorococcusProchlorococcusProchlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
cyanobacterium replaces it when the waters becomes impoverished in nutrients (i.e. oligotrophOligotrophAn oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic). On the other hand in temperate region (for example in the North Atlantic Ocean),Prochlorococcus is absent because the cold waters prevent its development. - The diversity of eukaryotes, corresponds undoubtedly to a big variety of environments. In oceanic regions, they are often observed at depth at the base of the well-lit layer (the "euphotic" layer). In coastal regions, certain sorts of picoeukaryotes such as "Micromonas" dominate. Their abundance follows a seasonal cycle, as the plankton of bigger size, with a maximum in summer.
Thirty years ago, it was hypothesized that the speed of division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
for micro-organisms in central oceanic ecosystems was very slow, of the order of one week or one month. This hypothesis was consolidated by the fact that the biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
(estimated for example by the contents of chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
) was very stable over time. However with the discovery of the picoplankton, it was found that the system was much more dynamic than previously thought. In particular, small predators of a size of a few micrometres which ingest picoplanktonic algae as quickly as they were produced, were found to be ubiquitous. This extremely sophisticated predator-prey system is practically always at equilibrium and results in a quasi-constant picoplankton biomass. This perfect equivalence between production and consumption makes it however extremely difficult to measure precisely the speed at which the system turns over.
In 1988, two American researchers, Carpenter and Chang, had suggested estimating the speed of cell division of phytoplankton by following the course of DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
by microscopy. By replacing the microscope by a flow cytometer, it is possible to follow the DNA content of picoplankton cells over time. This allowed to establish that picoplankton cells are extremely synchronous: they replicate their DNA and then divide all at the same time at the end of the day. This synchronization could be due to the presence of an internal biological clock
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
.
Genomics
In the 2000s, genomicsGenomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
allowed to cross a supplementary stage. Genomics consists in determining the complete sequence of genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
of an organism and to list every gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
present. It is then possible to get an idea of the metabolic capacities of the targeted organisms and understand how it adapts to its environment. To date, the genomes of several types of Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
and Synechococcus
Synechococcus
Synechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
, and of a strain of Ostreococcus
Ostreococcus
Ostreococcus is a genus of unicellular coccoid or spherically shaped green alga belonging to the class Prasinophyceae. It includes prominent members of the global picoplankton community, which plays a central role in the oceanic carbon cycle.-History:...
have been determined. The complete genomes of two different Micromonas strains revealed that they were quite different (different species) - and had similarities with land plants . Several other cyanobacteria and of small eukaryotes (Bathycoccus, Pelagomonas) are under sequencing. In parallel, genome analyses begin to be done directly from oceanic samples (ecogenomics or métagenomics), allowing us to access to large sets of gene for uncultivated organisms.
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- {| align=center class="wikitable"
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|+Genomes of photosynthetic picoplankton strains
that have been sequenced to date
!Genus
!Strain
!Sequencing center
!Remark
|----
|Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
|MED4
|JGI
Joint Genome Institute
The U.S. Department of Energy Joint Genome Institute was created in 1997 to unite the expertise and resources in genome mapping, DNA sequencing, technology development, and information sciences pioneered at the DOE genome centers at Lawrence Berkeley National Laboratory , Lawrence Livermore...
|
|----
|
|SS120
|Genoscope
Genoscope
The French National Sequencing Center was created in 1996 in Evry, France. It has been involved in the sequencing of the Human genome....
|
|----
|
|MIT9312
|JGI
|
|----
|
|MIT9313
|JGI
|
|----
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|NATL2A
|JGI
|
|----
|
|CC9605
|JGI
|
|----
|
|CC9901
|JGI
|
|----
|Synechococcus
Synechococcus
Synechococcus is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 µm to 1.5 µm...
|WH8102
|JGI
|
|----
|
|WH7803
|Genoscope
|
|----
|
|RCC307
|Génoscope
|
|----
|
|CC9311
|TIGR
The Institute for Genomic Research
The Institute for Genomic Research was a non-profit genomics research institute founded in 1992 by Craig Venter in Rockville, Maryland, United States. It is now a part of the J. Craig Venter Institute.-History:...
|
|----
|Ostreococcus
|OTTH95
|Genoscope
|
|----
|Micromonas
|RCC299 and CCMP1545
|JGI
|
|}
See also
- PhytoplanktonPhytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
- BacterioplanktonBacterioplanktonBacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word , meaning "wanderer" or "drifter" , and , a Latin neologism coined in the 19th century by Christian Gottfried Ehrenberg...
- List of eukaryotic picoplankton species
- PicoeukaryotePicoeukaryotePicoeukaryotes are picoplanktonic eukaryotic organisms that range in size from 0.2 – 2.0 µm. They are distributed throughout the world’s marine and freshwater ecosystems and constitute a significant contribution to autotrophic communities...