Picoeukaryote

Photosynthetic picoplankton off the Marquesas Islands observed by epifluorescence microscopy (blue exciting light). Orange fluorescing dots correspond to Synechococcus cyanobacteria, red fluorescing dots to picoeukaryotes.

Picoeukaryotes are picoplanktonic eukaryotic organisms 3.0 μm or less in size. They are distributed throughout the world's marine and freshwater ecosystems and constitute a significant contribution to autotrophic communities. Though the SI prefix pico- might imply an organism smaller than atomic size, the term was likely used to avoid confusion with existing size classifications of plankton.