Photosynthetic picoplankton

Photosynthetic picoplankton is the fraction of the plankton performing photosynthesis composed by cells between 0.2 and 2 µm (picoplankton). It is especially important in the central oligotrophic 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 Micromonas pusilla and is now known as one of the most abundant in temperate oceanic waters.
 * 1979 : Discovery of marine Synechococcus 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 Prochlorococcus marinus.
 * 1994 : Discovery in the Thau lagoon in France of the smallest photosynthetic eukaryote known to date, Ostreococcus tauri, by Courties.
 * 2001 : Through sequencing of the ribosomal RNA gene extracted from marine samples, several European teams discover that eukaryotic picoplankton are highly diverse.

Methods of study
Because of its very small size, picoplankton is difficult to study by classic methods such as optical microscopy. More sophisiticated methods are needed.
 * Epifluorescence microscopy allows to detect certain groups of cells possessing fluorescent pigments such as Synechococcus which possess phycoerythrin.
 * Flow cytometry measures the size (" side scatter ") and fluorescence on 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 (Prochlorococcus,Synechococcus and picoeucaryotes) can be distinguished. For example Synechococcus is characterized by the double fluorescence of its pigments: orange for phycoerythrin and red for chlorophyll. 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 pigments such as chlorophyll or carotenoids by high precision chromatography (HPLC) allows to determine the various groups of algae present in a sample.
 * Molecular biology techniques:
 * Cloning and sequencing of genes such as that of ribosomal RNA, 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 (FISH) uses fluorescent probes recognizing specific taxon, for example a species, a genus or a class.
 * 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 Synechococcus of a size of 1 µm (micrometer) were first discovered in 1979 by J. Waterbury (Woods Hole Oceanographic Institution). They are quite ubiquist, but most abundant in relatively mesotrophic waters.
 * Cyanobacteria belonging to the genus Prochlorococcus 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 Technology) and R.J. Olson (Woods Hole Oceanographic Institution). 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 class of brown algae, the Pelagophyceae . More surprising still, the discovery in 1994 of an eukaryote of very small size, Ostreococcus 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.


 * {|class="wikitable" align="center" bgcolor="#F6FFB2"

!Classes !Picoplanktonic genera
 * +Algal classes containing picoplankton species
 * Chlorophyceae
 * Nannochloris
 * Prasinophyceae
 * Micromonas, Ostreococcus, Pycnococcus
 * Prymnesiophyceae
 * Imantonia
 * Pelagophyceae
 * Pelagomonas
 * Bolidophyceae
 * Bolidomonas
 * Dictyochophyceae
 * Florenciella
 * }
 * Bolidophyceae
 * Bolidomonas
 * Dictyochophyceae
 * Florenciella
 * }
 * }

In situ diversity
The introduction of the molecular biology in oceanography revolutionized the knowledge of the oceanic ecosystems. For the first time, we were able to determine the composition of the picoplanktonic compartment without having neither to observe it, nor to cultivate him. In practice, it is possible to determine the sequence of a gene present in all living organisms, the one coding the small sub-unit of the ribosomal RNA (rRNA). Every species has a sequence which is specific and two closely related species (for example, the man and the chimpanzee) have very similar sequences. The analysis of the sequence allows to place an organism in the phylogenetic tree of the life. Furthermore we can determine on this gene small regions characteristics of a group of organisms (for example a specific genus of the picoplankton such as "Ostreococcus") and synthetize a "probe" recognizing this region. If this probe is marked with a fluorescent compound and put in contact with cells, only cells belonging to the targeted group will be visible under a fluorescence microscope (FISH technique ) (see Methods of study).

These approaches implemented since the 1990s for bacteria, were applied to the photosynthetic picoeukaryotes only 10 years later. They revealed a very wide diversity and put in light the importance of the following groups in the picoplankton :


 * Prasinophyceae
 * Haptophyta
 * Cryptophyta

In temperate coastal environment, the genus Micromonas (Prasinophyceae) seems dominant. However, in numerous oceanic environments, the dominant species of eukaryotic picoplankton remain still unknown.

Ecology
Each picoplanktonic population occupies a specific ecological niche in the oceanic environment.
 * The Synechococcus cyanobacterium is generally abundant in mesotrophic environments, for example in the vicinity of the equatorial upwelling or in coastal regions.
 * The Prochlorococcus cyanobacterium replaces it when the waters becomes impoverished in nutrients (i.e. oligotrophic). 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 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 (estimated for example by the contents of chlorophyll) 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 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.

Genomics
In the 2000s, genomics allowed to cross a supplementary stage. Genomics consists in determining the complete sequence of genome of an organism and to list every gene 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 and Synechococcus, and of a strain of Ostreococcus have been determined, while those of several other cyanobacteria and of small eukaryotes (Bathycoccus, Micromonas) 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.
 * {| align=center class="wikitable"

!Genus !Strain !Sequencing center !Remark
 * +Genomes of photosynthetic picoplankton strains that have been sequenced to date
 * Prochlorococcus
 * MED4
 * JGI
 * SS120
 * Genoscope
 * MIT9312
 * JGI
 * MIT9313
 * JGI
 * NATL2A
 * JGI
 * CC9605
 * JGI
 * CC9901
 * JGI
 * Synechococcus
 * WH8102
 * JGI
 * WH7803
 * Genoscope
 * RCC307
 * Génoscope
 * CC9311
 * TIGR
 * Ostreococcus
 * OTTH95
 * Genoscope
 * }
 * JGI
 * CC9901
 * JGI
 * Synechococcus
 * WH8102
 * JGI
 * WH7803
 * Genoscope
 * RCC307
 * Génoscope
 * CC9311
 * TIGR
 * Ostreococcus
 * OTTH95
 * Genoscope
 * }
 * RCC307
 * Génoscope
 * CC9311
 * TIGR
 * Ostreococcus
 * OTTH95
 * Genoscope
 * }
 * CC9311
 * TIGR
 * Ostreococcus
 * OTTH95
 * Genoscope
 * }
 * Genoscope
 * }

Cyanobacteria

 * Zehr, J. P., Waterbury, J. B., Turner, P. J., Montoya, J. P., Omoregie, E., Steward, G. F., Hansen, A. & Karl, D. M. 2001. Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean. Nature 412:635-8

Eukaryotes

 * Butcher, R. 1952. Contributions to our knowledge of the smaller marine algae. J. Mar. Biol. Assoc. UK. 31:175-91.
 * Manton, I. & Parke, M. 1960. Further observations on small green flagellates with special reference to possible relatives of Chromulina pusilla Butcher. J. Mar. Biol. Assoc. UK. 39:275-98.
 * Eikrem, W., Throndsen, J. 1990. The ultrastructure of Bathycoccus gen. nov. and B. prasinos sp. nov., a non-motile picoplanktonic alga (Chlorophyta, Prasinophyceae) from the Mediterranean and Atlantic. Phycologia 29:344-350
 * Chrétiennot-Dinet, M. J., Courties, C., Vaquer, A., Neveux, J., Claustre, H., et al. 1995. A new marine picoeucaryote: Ostreococcus tauri gen et sp nov (Chlorophyta, Prasinophyceae). Phycologia 34:285-292
 * Sieburth, J. M., M. D. Keller, P. W. Johnson, and S. M. Myklestad. 1999. Widespread occurrence of the oceanic ultraplankter, Prasinococcus capsulatus (Prasinophyceae), the diagnostic "Golgi-decapore complex" and the newly described polysaccharide "capsulan". J. Phycol. 35: 1032-1043.

Ecology

 * Platt, T., Subba-Rao, D. V. & Irwin, B. 1983. Photosynthesis of picoplankton in the oligotrophic ocean. Nature 300:701-4.
 * Stomp M, Huisman J, de Jongh F, Veraart AJ, Gerla D, Rijkeboer M, Ibelings BW, Wollenzien UIA, Stal LJ. 2004. Adaptive divergence in pigment composition promotes phytoplankton biodiversity. Nature 432: 104-107.
 * Campbell, L., Nolla, H. A. & Vaulot, D. 1994. The importance of Prochlorococcus to community structure in the central North Pacific Ocean. Limnol. Oceanogr. 39:954-61.

Molecular Biology and Genomes

 * Rappé, M. S., P. F. Kemp, and S. J. Giovannoni. 1995. Chromophyte plastid 16S ribosomal RNA genes found in a clone library from Atlantic Ocean seawater. J. Phycol. 31: 979-988.

picoplancton photosynthétique