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
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.