Brown algae

Brown algae are a group of multicellular, photosynthetic organisms classified as the class Phaeophyceae. They belong to the Stramenopiles, a clade within the SAR supergroup, which means they are not closely related to red algae, green algae, or green plants (all of which are archaeplastids). Most brown algae are marine seaweeds found in colder waters of the Northern Hemisphere, and they dominate the seaweeds of temperate and polar regions. Many, like those in the order Fucales, are common on rocky seashores. They serve important ecological roles as both food and habitat. For example, the kelp *Macrocystis* (order Laminariales) can grow up to 60 meters (200 feet) long and forms underwater kelp forests that support high biodiversity. Another example is *Sargassum*, which creates floating mats in the tropical Sargasso Sea, providing habitat for many species. Humans also use some brown algae, such as kelps, as food.

Worldwide, between 1,500 and 2,000 species are known. Some, like *Ascophyllum nodosum*, are heavily researched due to their commercial value. They also contribute to environmental carbon fixation. Their chloroplasts have four surrounding membranes, indicating they were acquired secondarily through a symbiotic relationship between a stramenopile and possibly a red or green alga. Most brown algae contain the pigment fucoxanthin, which gives them their characteristic greenish-brown color. They are unique among Stramenopiles for developing into multicellular forms with differentiated tissues, though they still reproduce using flagellated spores and gametes similar to other Stramenopiles. Genetic studies show their closest relative is *Schizocladia isciensis*, followed by the yellow-green algae.

**Morphology** Brown algae vary greatly in size and shape. The smallest grow as tiny, feathery tufts of threadlike cells only a few centimeters long. Some species have a life stage consisting of just a few cells, making the entire alga microscopic. Others are much larger: rockweeds and leathery kelps are often the most conspicuous algae in their habitats. Kelps range from the 60-centimeter-tall (2 ft) sea palm *Postelsia* to the giant *Macrocystis pyrifera*, which exceeds 50 meters (150 ft) and is the largest of all algae. Forms include small crusts, cushions, leafy floating mats (as in *Sargassum*), delicate felt-like strands (as in *Ectocarpus*), or 30-centimeter-long (1 ft) flattened branches resembling a fan (as in *Padina*).

Two features set brown algae apart from all other algae. First, their color ranges from olive green to various shades of brown, depending on fucoxanthin content. Second, all brown algae are multicellular—no known species exist as single cells or colonies. However, this may be a result of classification, since their closest relatives include single-celled or colonial forms. They can also change color depending on salinity, ranging from reddish to brown.

**Visible structures** The body of a brown alga is called a thallus, meaning it lacks the xylem and phloem of vascular plants. This does not mean they lack specialized structures, but because true stems, leaves, and roots are defined by the presence of those tissues, botanists use different terms for similar structures in brown algae. Structurally complex species typically have one or more characteristic parts.

A holdfast is a rootlike structure at the base. It anchors the alga to the substrate, preventing it from being swept away by currents. Unlike a plant root, it does not absorb water or nutrients from the substrate. Its appearance varies—it may be heavily branched or cup-like. Most algae have just one holdfast, though some species have multiple stipes growing from a single holdfast.

A stipe is a stalk or stemlike structure. It may be short near the base (as in *Laminaria*) or large and complex, running throughout the body (as in *Sargassum* or *Macrocystis*). In the most structurally differentiated brown algae (like *Fucus*), the stipe’s tissues are divided into three layers: a central pith, a surrounding cortex, and an outer epidermis—each analogous to parts of a vascular plant stem. In some brown algae, the pith contains elongated cells resembling phloem in both structure and function. In others (like *Nereocystis*), the stipe’s center is hollow and filled with gas to keep the alga buoyant. The stipe can be flexible and elastic.