Photosynthetic picoplankton
Ecology
Ecology
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[thumb\|right\|Vertical distribution of picoplankton in the Pacific Ocean](/wiki/Image:Picoplankton_profile_Pacific.jpg "Picoplankton profile Pacific.jpg")
Each picoplanktonic population occupies a specific [ecological niche](/wiki/Ecological_niche "Ecological niche") in the oceanic environment.
* The *Synechococcus* cyanobacterium is generally abundant in [mesotrophic](/wiki/Mesotrophic_lake "Mesotrophic lake") environments, such as near the equatorial [upwelling](/wiki/Upwelling "Upwelling") or in coastal regions.
* The *Prochlorococcus* cyanobacterium replaces it when the waters becomes impoverished in nutrients (i.e., [oligotrophic](/wiki/Oligotroph "Oligotroph")). 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 (the "euphotic" 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](/wiki/Cell_division "Cell 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](/wiki/Biomass "Biomass") (estimated for example by the contents of [chlorophyll](/wiki/Chlorophyll "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](/wiki/Predator "Predator") 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](/wiki/DNA_replication "DNA replication") by microscopy. By replacing the microscope by a [flow cytometer](/wiki/Flow_cytometer "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](/wiki/Circadian_rhythm "Circadian rhythm").


How was the speed of division for micro-organisms previously thought to be in central oceanic ecosystems?