page_title: Marine primary production
section_title: Marine plants
hierarchical_section_title: Marine primary production / Marine plants
caption_attribution_description: English: Submerged macrophyte evolution. Precambrian: First plant life (phytoplankton shown here) Silurian: Algal evolution Appearance of the first land plants Diversification of land plants: developed higher plant characteristics such as woodiness and sexual reproduction. Cretaceous: marine angiosperms evolved, characterised by mangroves and salt-marsh plants in intertidal zones seagrasses as the dominant submerged macrophyte Recoloured version of this image
context_page_description: Marine primary production is the chemical synthesis in the ocean of organic compounds from atmospheric or dissolved carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of inorganic chemical compounds as its source of energy. Almost all life on Earth relies directly or indirectly on primary production. The organisms responsible for primary production are called primary producers or autotrophs.
  Most marine primary production is generated by a diverse collection of marine microorganisms called algae and cyanobacteria. Together these form the principal primary producers at the base of the ocean food chain and produce half of the world's oxygen. Marine primary producers underpin almost all marine animal life by generating nearly all of the oxygen and food marine animals need to exist. Some marine primary producers are also ecosystem engineers which change the environment and provide habitats for other marine life.
  Primary production in the ocean can be contrasted with primary production on land.
context_section_description: Back in the Silurian, some phytoplankton evolved into red, brown and green algae. These algae then invaded the land and started evolving into the land plants we know today. Later, in the Cretaceous, some of these land plants returned to the sea as mangroves and seagrasses.
  Plant life can flourish in the brackish waters of estuaries, where mangroves or cordgrass or beach grass  might grow. Flowering plants grow in sandy shallows in the form of seagrass meadows, 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. Marine plants can be found in  intertidal zones and shallow waters, such as seagrasses like eelgrass and turtle grass, Thalassia. These plants have adapted to the high salinity of the ocean environment.
  Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow. The surface layers are often deficient in biologically active nitrogen compounds. The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen, its assimilation, nitrification, anammox and denitrification. Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal.
caption: Evolution of mangroves and seagrasses
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