These organisms comprise most of the living matter in the sea, and most organic matter flows within this group of organisms, called the microbial loop.
Phytoplankton are single-celled algae.
Like all plants, they use carbon dioxide and light to produce food in the process of photosynthesis. Phytoplankton also have major effects on the world's climate. By absorbing carbon dioxide, they contribute to the uptake of CO2 from the atmosphere, thereby reducing the global Greenhouse Effect. The Southern Ocean is one of the world's important 'sinks' for carbon dioxide. Some phytoplankton also produce chemicals which when released to the atmosphere form aerosol particles that can trigger the formation of clouds.