Picoplankton
Encyclopedia
Picoplankton is the fraction of plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 composed by cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 between 0.2 and 2 μm that can be either :
  • photosynthetic 
  • heterotrophic 

Some species can also be mixotrophic.
Picoplankton are responsible for the most primary productivity in oligotrophic gyres, and are distinguished from nanoplankton and microplankton. Because they are small, they have a greater surface to volume ratio, enabling them to obtain the scarce nutrients in these ecosystems. Note that the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 prefix pico- is used quite loosely here, as nanoplankton and microplankton are only 10 and 100 times larger, respectively.
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