Phytoplankton belong to a larger group of marine life forms known as plankton, which includes zooplankton (microscopic animals), and bacterioplankton, varieties of marine bacteria. These tiny plants inhabit the upper layers of water, where the sun provides energy for photosynthesis. Phytoplankton depend on minerals such as nitrates and phosphate and convert energy from carbon dioxide into organic compounds that fuel aquatic food webs.
Because phytoplankton create organic compounds, they constitute the first strand in the aquatic food web, providing food for larger organisms, which in turn become food for others. Marine food webs can include many levels or only a few: the smallest web includes only phytoplankton, the krill that feed on them, and the baleen whales that feed on krill. The functioning of the marine food web depends on the overall health of the aquatic ecosystem, which can be damaged by toxins and development.
Numerous varieties of phytoplankton exist in waters around the globe. Types of phytoplankton include cyanobacteria, dinoflagellates and diatoms, whose fossilized exoskeletons create diatomaceous earth, used for a variety of industrial and health purposes. Another form of phytoplankton, the coccolithoporids, release dimethyl sulfides into the atmosphere; these sulfate molecules act as the nuclei for cloud condensation, encouraging cloud cover and rainfall.
Phytoplankton play an essential role in the functioning of aquatic ecosystems. In addition to their contribution to marine food webs, phytoplankton are a major source of photosynthetic activity, releasing significant amounts of oxygen into the atmosphere. However, imbalances in aquatic ecosystems can lead to an overgrowth of phytoplankton -- the "algae bloom" that covers water surfaces. When the phytoplankton die and sink to the bottom, they decompose and deplete the water of oxygen, causing the death of some deepwater marine creatures.