Photosynthetic picoplankton or picophytoplankton is the fraction of the phytoplankton performing photosynthesis composed by cells between 0.2 and 2 µm. 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 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.
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
Because of its very small size, picoplankton is difficult to study by classic methods such as optical microscopy. More sophisticated methods are needed.
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.
Three major groups of organisms constitute photosynthetic picoplankton...
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. 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.
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 :
In temperate coastal environment, the genus Micromonas 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, 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.
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 per generation. This hypothesis was supported by the fact that the biomass 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 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 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 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 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. The complete genomes of two different Micromonas strains revealed that they were quite different - and had similarities with land plants. Several other cyanobacteria and of small eukaryotes are under sequencing. In parallel, genome analyses begin to be done directly from oceanic samples, allowing us to access to large sets of gene for uncultivated organisms.
