Phytoplankton

Phytoplankton obtain their energy through photosynthesis, as trees and other plants do on land. This means phytoplankton must receive sunlight, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes. Compared with terrestrial plants, phytoplankton are distributed over a larger surface area, experience less seasonal variation, and have markedly faster turnover rates (days versus decades). As a result, phytoplankton respond rapidly on a global scale to climate variations.

Phytoplankton form the base of marine and freshwater food webs and are key players in the global carbon cycle. They account for about half of global photosynthetic activity and at least half of the oxygen production, despite amounting to only about 1% of the global plant biomass.

Phytoplankton are very diverse, comprising photosynthesizing bacteria (cyanobacteria) and various unicellular protist groups (notably the diatoms).

Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in sufficient numbers, some varieties may appear as colored patches on the water surface due to chlorophyll in their cells and, in some species, accessory pigments (such as phycobiliproteins or xanthophylls).