Brown algae

Brown algae (phylum Phaeophyta) are familiar to most people as brown or dark green seaweeds. Some brown algae are microscopic in size, but many are relatively large: One giant kelp measured 710 feet in length. All brown algae are multicellular.

Appearance and Distribution

Brown algae have a body, called a thallus, which is a fairly simple, undifferentiated structure. Some thalli consist of simple branched filaments. Some brown algae have more complex structures called pseudoparenchyma because they superficially resemble the more complex tissues of higher plants.

Giant kelp has a thallus that is differentiated into a holdfast, a stipe, and one or more flattened, leaflike blades. The holdfast functions as the name implies, and holds the rest of the organism to the substrate. It is a tough, sinewy structure resembling a mass of intertwined roots. The stalk that constitutes the stipe is often hollow, with a meristem (a zone of growing tissue) either at its base or at the blade junctions. Because the meristem produces new tissue at the base, the oldest parts of the blades are at the tips.

The blades, which, like most of the rest of the giant kelp body, are photosynthetic, may have gas-filled floats called bladders toward their bases, which may contain carbon monoxide gas. The function of this particular gas has not yet been determined.

The vast majority of species are marine, living in cold, shallow ocean waters, and may be the dominant plant life on rocky coastlines. The giant kelp can be found in waters around 100 feet deep. Only 4 of the 260 identified genera occur in freshwater. Brown algae of the order Fucales are commonly called rockweeds; kelp belongs to the order Laminariales.

Brown algae are less common in tropical and subtropical areas. However, in the Caribbean region, sargassum (large masses of brown algae having a branching thallus with lateral outgrowths differentiated into leafy segments, air bladders, or spore-bearing structures) make up large floating mats; they gave their name to the Sargasso Sea.

Brown algae seaweeds such as kelps are known for their high rate of growth. Some of the genera can grow as quickly as two feet per day, making them an economically renewable source of biofuel that requires no irrigation and otherwise low inputs as an agricultural crop. A 2016 report from the World Bank estimates that worldwide seaweed production could replace about 1.5% percent of fossil fuel used to run vehicles by 2050 and represent a paradigm shift in the perception and use of the world's oceans. Research continues into the viability of brown algae and kelp as a biofuel.

Pigments and Food Reserves

The color of the brown algae can vary from light yellow-brown to almost black. Its color reflects the presence of varying amounts of the brown xanthophyll pigment fucoxanthin, a carotenoid pigment, in addition to chlorophylls a and c. The main food reserve is a carbohydrate called laminarin, although giant kelp can also translocate mannitol. Algin (alginic acid) can be found in or on the cell walls and may comprise as much as 40 percent of the dry weight of some kelps.

Reproduction

Reproductive cells of brown algae are unusual in that their two flagella are located laterally, instead of at the ends. The only motile cells in the brown algae are the gametes or reproductive cells. In the common genus Fucus, separate male and female thalli are produced. Fertile areas called receptacles develop at the tips of the lobes of the thallus. Each receptacle has pores on the surface. These pores open into special spherical, hollow chambers called conceptacles, in which the gametes are formed. Eight eggs are produced in the female structure, while sixty-four sperm cells are produced in the male structure. Eventually, both eggs and sperm are released into the water, where fertilization takes place and the resulting zygotes develop into mature thalli.

Economic Uses

Brown algae have several uses and applications for humans. Giant kelp is eaten, and one species found in the Pacific Ocean has been used, in chopped-up form, as a poultice applied to cuts. Algin, a colloidal substance produced by brown algae, is used as a thickener or stabilizer in commercially produced ice cream, salad dressing, beer, jelly beans, latex paint, penicillin suspensions, paper, textiles, toothpastes, and floor polish. Brown algae, with its high concentration of the element iodine, has been used to treat goiter, an iodine-deficiency disease. Kelp, also high in nitrogen and potassium, has been used as fertilizer and as livestock feed.

Some types of brown algae, such as Fucus, contain either phenols or terpenes. Botanists believe these substances may discourage herbivory. These substances also have been shown to possess microbe- and cancer-fighting properties. Brown algae are the subject of continuing research in these areas of medicine.

Bibliography

Fehrenbacher, Katie. “Meet the New US Entrepreneurs Farming Seaweed for Food and Fuel.” The Guardian, 29 June 2017, www.theguardian.com/sustainable-business/2017/jun/29/seaweed-farms-us-california-food-fuel. Accessed 4 Apr. 2026.

Fonseca-Barahona, Irvin, et al. “Bioactives From Brown Algae: Antioxidant, Anti-Inflammatory, Anticancer, and Antimicrobial Potential.” ChemBioEng Reviews, vol. 12, no. 3, 19 Apr. 2025, doi:10.1002/cben.70007. Accessed 4 Apr. 2026.

Graham, Linda E., and Lee W. Wilcox. Algae. Prentice Hall, 2000.

Hemavathy, R. V., et al. “Biofuel Production from Marine Macroalgae: Pathways, Technologies, and Sustainable Energy Solutions.” Industrial Crops and Products, vol. 224, Feb. 2025, article 120282, doi:10.1016/j.indcrop.2024.120282. Accessed 4 Apr. 2026.

Meinesz, Alexandre. Killer Algae. University of Chicago Press, 1999.

Shapiro, Ari, and Monika Evstatieva. “Scientists Hope to Farm the Biofuel of the Future in the Pacific Ocean.” NPR, 22 Aug. 2017, www.npr.org/sections/thesalt/2017/08/22/542903378/scientists-hope-to-farm-the-biofuel-of-the-future-in-the-pacific-ocean. Accessed 4 Apr. 2026.

Sze, Philip. A Biology of the Algae. WCB/McGraw-Hill, 1998.