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Photosynthetic picoplankton

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Photosynthetic picoplanktonorpicophytoplanktonis the fraction of thephotosyntheticphytoplanktonofcellsizes between 0.2 and 2 μm (i.e.picoplankton). It is especially important in the centraloligotrophicregions of the world oceans that have very low concentration ofnutrients.

Picoplankton observed by epifluorescence

History

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  • 1952: Description of the first truly picoplanktonic species,Chromulina pusilla,by Butcher.[1]This species was renamed in 1960 toMicromonaspusilla[2]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 marineSynechococcusby Waterbury[3]and confirmation with electron microscopy by Johnson and Sieburth.[4]
  • 1982: The same Johnson and Sieburth demonstrate the importance of small eukaryotes by electron microscopy.[5]
  • 1983:W.K.W Liand colleagues, includingTrevor Plattshow that a large fraction of marine primary production is due to organisms smaller than 2 μm.[6]
  • 1986: Discovery of "prochlorophytes" by Chisholm and Olson in the Sargasso Sea,[7]named in 1992 asProchlorococcusmarinus.[8]
  • 1994: Discovery in the Thau lagoon in France of the smallest photosynthetic eukaryote known to date,Ostreococcustauri,by Courties.[9]
  • 2001: Through sequencing of theribosomal RNAgene extracted from marine samples, several European teams discover that eukaryotic picoplankton are highlydiverse.[10][11]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[12]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.[13]

Methods of study

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Analysis of picoplankton by flow cytometry

Because of its very small size, picoplankton is difficult to study by classic methods such as optical microscopy. More sophisticated methods are needed.

  • Epifluorescence microscopyallows researchers to detect certain groups of cells possessingfluorescentpigments such asSynechococcuswhich possessphycoerythrin.
  • Flow cytometrymeasures 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 (Prochlorococcus,Synechococcusand picoeukaryotes) can be distinguished. For exampleSynechococcusis characterized by the double fluorescence of its pigments: orange for phycoerythrin and red forchlorophyll.Flow cytometry also allows researchers to sort out specific populations (for exampleSynechococcus) in order put them in culture, or to make more detailed analyses.
  • Analysis of photosyntheticpigmentssuch as chlorophyll orcarotenoidsby high precision chromatography (HPLC) allows researchers to determine the various groups of algae present in a sample.
  • Molecular biology techniques:
  • Cloningandsequencingofgenessuch as that ofribosomal RNA,which allows researchers to determine total diversity within a sample
  • DGGE(denaturing gel electrophoresis) that is faster than the previous approach allows researchers to have an idea of the global diversity within a sample
  • In situ hybridization (FISH) uses fluorescent probes recognizing specifictaxon,for example aspecies,agenusor aclass.[14]This original description as a species is now thought to be composed of a number of differentcryptic species,a finding that has been confirmed by a genome sequencing project of two strains led by researchers at theMonterey Bay Aquarium Research Institute.[15]
  • Quantitative PCRcan 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,[16]but requires more sophisticated controls and calibrations.

Composition

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Three major groups of organisms constitute photosynthetic picoplankton:

  • Cyanobacteriabelonging to the genusSynechococcusof a size of 1 μm (micrometer) were first discovered in 1979 by J. Waterbury[3](Woods Hole Oceanographic Institution). They are quite ubiquitous, but most abundant in relativelymesotrophic waters.
  • Cyanobacteria belonging to the genusProchlorococcusare particularly remarkable. With a typical size of 0.6 μm,Prochlorococcuswas discovered only in 1988[7]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.[17]
  • Picoplanktoniceukaryotesare the least well known, as demonstrated by the recent discovery of major groups. Andersen created in 1993 a newclassof brown algae, thePelagophyceae.[18]More surprising still, the discovery in 1994[9]of a eukaryote of very small size,Ostreococcustauri,dominating the phytoplanktonic biomass of a French brackish lagoon (étang deThau), shows that these organisms can also play a major ecological role in coastal environments. In 1999, yet a new class of alga was discovered,[19]theBolidophyceae,very close genetically ofdiatoms,but quite different morphologically. At the present time, about 50 species are known belonging to several classes.
Main article:Picoeukaryotes
Algal classes containing picoplankton species
Classes Picoplanktonic genera
Chlorophyceae Nannochloris
Prasinophyceae Micromonas,Ostreococcus,Pycnococcus
Prymnesiophyceae 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[10][11]and brought to light the importance of the following groups in the picoplankton:

In temperate coastal environment, the genusMicromonas(Prasinophyceae) seems dominant.[14]However, in numerous oceanic environments, the dominant species of eukaryotic picoplankton remain still unknown.[20]

Ecology

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Vertical distribution of picoplankton in the Pacific Ocean

Each picoplanktonic population occupies a specificecological nichein the oceanic environment.

  • TheSynechococcuscyanobacterium is generally abundant inmesotrophicenvironments, such as near the equatorialupwellingor in coastal regions.
  • TheProchlorococcuscyanobacterium replaces it when the waters becomes impoverished in nutrients (i.e.,oligotrophic). On the other hand, in temperate regions such as the North Atlantic Ocean,Prochlorococcusis absent because the cold waters prevent its development.
  • The diversity of eukaryotes derives from their presence in a large 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 asMicromonasdominate. As with larger plankton, their abundance follows a seasonal cycle with a maximum in summer.

Thirty years ago, it was hypothesized that the speed ofdivisionfor micro-organisms in central oceanic ecosystems was very slow, of the order of one week or one month per generation. This hypothesis was supported by the fact that thebiomass(estimated for example by the contents ofchlorophyll) 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, smallpredatorsof 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 nearly always at equilibrium and results in a quasi-constant picoplankton biomass. This close equivalence between production and consumption makes it extremely difficult to measure precisely the speed at which the system turns over.

In 1988, two American researchers, Carpenter and Chang, suggested estimating the speed of cell division of phytoplankton by following the course ofDNA replicationby microscopy. By replacing the microscope by aflow cytometer,it is possible to follow the DNA content of picoplankton cells over time. This allowed researchers to establish that picoplankton cells are highly 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 aninternal biological clock.

Genomics

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In the 2000s,genomicsallowed to cross a supplementary stage. Genomics consists in determining the complete sequence ofgenomeof an organism and to list everygenepresent. 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 ofProchlorococcus[21][22]andSynechococcus,[23]and of a strain ofOstreococcus[24]have been determined. The complete genomes of two differentMicromonasstrains revealed that they were quite different (different species) and had similarities with land plants.[15]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 metagenomics),[25]allowing us to access to large sets of gene for uncultivated organisms.

Genomes of photosynthetic picoplankton strains
that have been sequenced to date
Genus Strain Sequencing center Remark
Prochlorococcus MED4 JGI
SS120 Genoscope
MIT9312 JGI
MIT9313 JGI
NATL2A JGI
CC9605 JGI
CC9901 JGI
Synechococcus WH8102 JGI
WH7803 Genoscope
RCC307 Génoscope
CC9311 TIGR [26]
Ostreococcus OTTH95 Genoscope
Micromonas RCC299 and CCMP1545 JGI [15]

See also

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Notes and references

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  1. ^Butcher, R. (1952). Contributions to our knowledge of the smaller marine algae.Journal of the Marine Biological Association of the UK31:175-91.
  2. ^Manton, I. & Parke, M. (1960). Further observations on small green flagellates with special reference to possible relatives ofChromulina pusillaButcher.Journal of the Marine Biological Association of the UK39:275-98.
  3. ^abWaterbury, J. B.et al.(1979). Wide-spread occurrence of a unicellular, marine planktonic, cyanobacterium.Nature277:293-4.
  4. ^Johnson, P. W. &Sieburth, J. M.(1979). Chroococcoid cyanobacteria in the sea: a ubiquitous and diverse phototrophic biomass.Limnology and Oceanography24:928-35.
  5. ^Johnson, P. W. & Sieburth, J. M. (1982). In-situ morphology and occurrence of eucaryotic phototrophs of bacterial size in the picoplankton of estuarine and oceanic waters.Journal of Phycology18:318-27.
  6. ^Li, W. K. W.et al.(1983). Autotrophic picoplankton in the tropical ocean.Science219:292-5.
  7. ^abChisholm, S. W.et al.(1988). A novel free-living prochlorophyte occurs at high cell concentrations in the oceanic euphotic zone.Nature334:340-3.
  8. ^Chisholm, S. W.et al.(1992).Prochlorococcus marinusnov. gen. nov. sp.: an oxyphototrophic marine prokaryote containing divinyl chlorophyllaandb.Archives of Microbiology157:297-300.
  9. ^abCourties, C.et al.(1994). Smallest eukaryotic organism.Nature370:255.
  10. ^abLópez-García, P.et al.(2001). Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton.Nature409:603-7.
  11. ^abMoon-van der Staay, S. Y.et al.(2001). Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryotic diversity.Nature409:607-10.
  12. ^Rappe, M.et al.(1998). Phylogenetic diversity of ultraplankton plastid Small-Subunit rRNA genes recovered in environmental nucleic acid samples from the Pacific and Atlantic coasts of the United States.Applied and Environmental Microbiology64294-303.
  13. ^Kim, E., Harrison, J., Sudek, S.et al.(2011). Newly identified and diverse plastid-bearing branch on the eukaryotic tree of life. Proceedings of the National Academy of Sciences USA. Vol. 108: 1496-1500.
  14. ^abNot, F.et al.(2004). A single speciesMicromonas pusilla(Prasinophyceae) dominates the eukaryotic picoplankton in the western English Channel.Applied and Environmental Microbiology70:4064-72.
  15. ^abcWorden, A.Z.,et al.(2009). Green evolution and dynamic adaptations revealed by genomes of the marine picoeukaryotesMicromonas.Science324:268-272.
  16. ^Johnson, Z. I.et al.(2006). Niche partitioning amongProchlorococcusecotypes along ocean-scale environmental gradients.Science311:1737-40.
  17. ^Partensky, F.et al.(1999).Prochlorococcus,a marine photosynthetic prokaryote of global significance.Microbiology and Molecular Biology Reviews63:106-27.
  18. ^Andersen, R. A.et al.(1993). Ultrastructure and 18S rRNA gene sequence forPelagomonas calceolatagen. and sp. nov. and the description of a new algal class, the Pelagophyceae classis nov.Journal of Phycology29:701-15.
  19. ^Guillou, L.et al.(1999).Bolidomonas:a new genus with two species belonging to a new algal class, the Bolidophyceae (Heterokonta).Journal of Phycology35:368-81.
  20. ^Worden, A.Z. & Not, F.(2008) Ecology and Diversity of Picoeukaryotes. Book Chapter in: Microbial Ecology of the Ocean, 2nd Edition. Ed. D. Kirchman. Wiley.
  21. ^Rocap, G.et al.(2003). Genome divergence in twoProchlorococcusecotypes reflects oceanic niche differentiation.Nature424:1042-7.
  22. ^Dufresne, A.et al.(2003). Genome sequence of the cyanobacteriumProchlorococcus marinusSS120, a nearly minimal oxyphototrophic genome.Proceedings of the National Academy of Sciences of the United States of America100:10020-5.
  23. ^Palenik, B.et al.(2003). The genome of a motile marineSynechococcus.Nature424:1037-42.
  24. ^Derelle, E.et al.(2006). Genome analysis of the smallest free-living eukaryoteOstreococcus tauriunveils many unique features.Proceedings of the National Academy of Sciences of the United States of America103:11647-52.
  25. ^Venter, J. C.et al.(2004). Environmental genome shotgun sequencing of the Sargasso Sea.Science304:66-74.
  26. ^Palenik, B.et al.(2006). Genome sequence ofSynechococcusCC9311: Insights into adaptation to a coastal environment.PNAS103:13555-9.

Bibliography

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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 ofBathycoccusgen. nov. andB. prasinossp. 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 taurigen 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.
  • Not, F., Valentin, K., Romari, K., Lovejoy, C., Massana, R., Töbe, K., Vaulot, D. &Medlin, L. K.2007. Picobiliphytes, a new marine picoplanktonic algal group with unknown affinities to other eukaryotes. Science 315:252-4.
  • Vaulot, D., Eikrem, W., Viprey, M. & Moreau, H. 2008. The diversity of small eukaryotic phytoplankton (≤3 μm) in marine ecosystems. FEMS Microbiol. Rev. 32:795-820.
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 ofProchlorococcusto 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.
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