Kelp

Kelp Temporal range: Lutetian–Present PreꞒ Ꞓ O S D C P T J K Pg N [1]
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Stramenopiles
Phylum:	Gyrista
Subphylum:	Ochrophytina
Class:	Phaeophyceae
Order:	Laminariales Migula, 1909[2]
Families
Agaraceae Akkesiphycaceae Alariaceae Aureophycaceae Chordaceae Laminariaceae Lessoniaceae Pseudochordaceae

Kelps are large brown algae or seaweeds that make up the order Laminariales. There are about 30 different genera.^[3] Despite its appearance, kelp is not a plant but a stramenopile (a group containing many protists).^[4]

Kelp grows in "underwater forests" (kelp forests) in shallow oceans. Kelps were previously thought to have appeared in the Miocene, 5 to 23 million years ago based on fossils from California.^[5] New fossils of kelp holdfasts from early Oligocene rocks in Washington State show that kelps were present in the northeastern Pacific Ocean by at least 32 million years ago.^[6] The organisms require nutrient-rich water with temperatures between 6 and 14 °C (43 and 57 °F). They are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 metres (100 to 260 ft).^[7]

Through the 19th century, the word "kelp" was closely associated with seaweeds that could be burned to obtain soda ash (primarily sodium carbonate). The seaweeds used included species from both the orders Laminariales and Fucales. The word "kelp" was also used directly to refer to these processed ashes.^[8]