The term "plankton" encompasses many different creatures, so the food plankton eat is varied. Phytoplankton, zooplankton and bacterioplankton all use different sources of energy for food.
One of the most important types of plankton is phytoplankton. Like land-based food chains, marine food chains begin with creatures capable of producing food from nutrients and sunlight. Phytoplankton do not eat in a traditional sense: They produce energy through photosynthesis. These tiny organisms provide a foundation for aquatic food chains. Zooplankton, however, do consume external food sources. The zooplankton group is broader than many would expect. It includes a variety of single-celled organisms but also includes creatures such as jellyfish. These creatures will consume other plankton or sea life. For instance, a jellyfish can eat plankton, fish eggs, small fish or even other jellyfish. Many zooplankton consume their food by enveloping it and slowly digesting it. The final group of plankton works in the same way land bacteria does. Bacterioplankton decompose organic matter and convert it into usable energy. They traditionally do not eat living creatures, but instead break down matter that has died or is unusable by other sea life. Like land bacteria, bacterioplankton provide a valuable service by removing waste from the water and adding a variety of nutrients.