A class of algae, common in cold waters along continental coasts. Species colour varies from dark brown to olive green, depending upon the proportion of brown pigment (fucoxanthin) to green pigment (chlorophyll).

Brown algae vary in form and size from small filamentous epiphytes (Ectocarpus) to complex giant kelps that range in size from 1 to more than 100 metres (3.3 to 330 feet; Laminaria, Macrocystis, Nerocystis). Other brown algae can be found attached to rocky coasts in temperate zones (Fucus, Ascophyllum) or floating freely (Sargassum). Freshwater species are rare. Brown algae multiply by asexual and sexual reproduction; both the motile zoospores and gametes have two unequal flagella. Some seaweed species have gas-filled bladders (pneumatocysts), which keep photosynthetic parts of the algal thallus floating on or near the surface of the water.

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Britannica Editors. "brown algae". Encyclopedia Britannica, 16 Aug. 2024, https://www.britannica.com/science/brown-algae. Accessed 16 August 2026.