Phytoplankton are essential because they form the foundation of the marine food chain. Phytoplankton are the primary food source for everything from microscopic organisms to whales. Phytoplankton also remove vast amounts carbon dioxide from the atmosphere, which helps regulate global surface temperatures. Even small changes in phytoplankton populations may impact the atmosphere and climate.
Phytoplankton are photosynthesizing microorganisms found in the upper sunlit layers of oceans, lakes and rivers. Phytoplankton use the carbon dioxide dissolved in the water to produce organic compounds, forming the basis of the marine food web. These organisms account for half of all photosynthetic activity on Earth and transfer about 10 gigatons of carbon from the atmosphere to the ocean's floor each year.
The term phytoplankton refers to all photoautotrophic microorganisms found in aquatic ecosystems. Phytoplankton are diverse, with about 5,000 known species of eubacterial and archaebacterial prokaryotes. Diatoms, dinoflagellates and cyanobacteria are the most important groups of phytoplankton. These organisms also release significant quantities of an important atmospheric gas known as dimethyl sulfide, an oxidized sulfate that plays a role in cloud formation. Most phytoplankton are unable to be seen with the naked eye but may create colored patches visible on water surfaces when present in high enough numbers.