Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Sea
Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton. About 45 percent of the sea's primary production of living material is contributed by diatoms. Much larger algae, commonly known as seaweeds, are important locally; "Sargassum" forms floating drifts, while kelp form seabed forests. Flowering plants in the form of seagrasses grow in "meadows" in sandy shallows, mangroves line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated salt marshes. All of these habitats are able to sequester large quantities of carbon and support a biodiverse range of larger and smaller animal life.