Phytoplankton (fido-plank-tin) are the autotrophic component of the plankton community.  Most phytoplankton are too small to be individually seen with the unaided eye.  However, when present in high enough numbers, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells (although the actual color may vary with the species of phytoplankton present due to varying levels of chlorophyll or the presence of accessory pigments such as phycobiliproteins, xanthophylls, etc.).  Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water.  Phytoplankton account for half of all photosynthetic activity on Earth.