Photosynthetic picoplankton

History

• 1952: Description of the first truly picoplanktonic species, Chromulina pusilla, by Butcher. This species was renamed in 1960 to Micromonas 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 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.W Li and colleagues, including Trevor 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. 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 microscopy allows researchers to detect certain groups of cells possessing fluorescent pigments such as Synechococcus which possess phycoerythrin.
• Flow cytometry measures the size 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 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 researchers to sort out specific populations 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 allows researchers to determine the various groups of algae present in a sample.
• Molecular biology techniques:

Composition

• Cyanobacteria belonging to the genus Synechococcus of a size of 1 μm were first discovered in 1979 by J. Waterbury. They are quite ubiquitous, 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 and R.J. Olson. 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 a eukaryote of very small size, Ostreococcus tauri, dominating the phytoplanktonic biomass of a French brackish lagoon, 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.

• Prasinophyceae
• Haptophyta
• Cryptophyta

Ecology

• The Synechococcus cyanobacterium is generally abundant in mesotrophic environments, such as near the equatorial upwelling or in coastal regions.
• The Prochlorococcus cyanobacterium replaces it when the waters becomes impoverished in nutrients. On the other hand, in temperate regions such as the North Atlantic Ocean, Prochlorococcus is 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. In coastal regions, certain sorts of picoeukaryotes such as Micromonas dominate. As with larger plankton, their abundance follows a seasonal cycle with a maximum in summer.

Genomics