Giant kelp isn't a plant, and its closest kin are microscopic
Brown seaweeds such as giant kelp can top 50 metres, the largest algae on Earth, forming undersea forests rich in life. Yet they are not related to plants or to green and red seaweeds. They belong to a separate branch of life, and acquired their chloroplasts second-hand by absorbing another alga.
The brown algae, class Phaeophyceae, number roughly 1,500 to 2,000 species and dominate the seaweeds of cold temperate and polar coasts. They sit among the stramenopiles, a group mostly made up of single-celled organisms. Their chloroplasts are wrapped in four membranes, evidence they came from an ancient partnership with a red or possibly green alga. Genetic studies name Schizocladia isciensis as their nearest relative, followed by the yellow-green algae.
Two traits set them apart. Their olive-to-brown tint comes from the pigment fucoxanthin, and the amount determines the shade. And every known species is multicellular, making them the only major seaweed group without single-celled or colonial members, though that may reflect how scientists draw the boundaries rather than evolution itself. They still reproduce with swimming, flagellated cells much like their tiny relatives.
Sizes vary enormously. Some are feathery tufts a few centimetres long, and some life stages are microscopic. The sea palm Postelsia stands about 60 centimetres; the giant kelp Macrocystis pyrifera grows beyond 50 metres. Because they lack the water-conducting tissues of true plants, their parts get their own names. A holdfast grips the rock, but unlike a root it takes up neither water nor nutrients. A stalk called a stipe may be flexible in strong currents or rigid in species exposed at low tide, and in Fucus it has layers comparable to a plant stem. Leaflike blades carry the reproductive structures, and some are coated in slime to discourage hitchhikers and grazers.
To reach the light, many kelps and rockweeds grow gas-filled floats called pneumatocysts. Bull kelp carries one large float where stipe meets blades, while giant kelp has a small float at the base of every blade. In tropical waters, free-floating Sargassum forms great mats that give the Sargasso Sea its name and shelter many species. Kelps are eaten by people, and brown algae also matter for the carbon they lock away.
Source: Brown algae
