Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat.

[4] Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species.

The smallest members of the group grow as tiny, feathery tufts of threadlike cells no more than a few centimeters (a few inches) long.

They may consist of delicate felt-like strands of cells, as in Ectocarpus, or of 30-centimeter-long (1 ft) flattened branches resembling a fan, as in Padina.

However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms.

Whatever their form, the body of all brown algae is termed a thallus, indicating that it lacks the complex xylem and phloem of vascular plants.

Unlike a root system, the holdfast generally does not serve as the primary organ for water uptake, nor does it take in nutrients from the substrate.

In the most structurally differentiated brown algae (such as Fucus), the tissues within the stipe are divided into three distinct layers or regions.

These regions include a central pith, a surrounding cortex, and an outer epidermis, each of which has an analog in the stem of a vascular plant.

In some brown algae, the pith region includes a core of elongated cells that resemble the phloem of vascular plants both in structure and function.

Species such as Nereocystis luetkeana and Pelagophycus porra bear a single large pneumatocyst between the top of the stipe and the base of the blades.

Species of Sargassum also bear many blades and pneumatocysts, but both kinds of structures are attached separately to the stipe by short stalks.

[15][16] Besides fronds, there are the large in size parenchymatic kelps with three-dimensional development and growth and different tissues (meristoderm, cortex and medulla) which could be consider the trees of the sea.

[15] Specifically, the brown algal cell wall consists of several components with alginates and sulphated fucan being its main ingredients, up to 40% each of them.

Cellulose and alginate biosynthesis pathways seem to have been acquired from other organisms through endosymbiotic and horizontal gene transfer respectively, while the sulphated polysaccharides are of ancestral origin.

[21] Genetic and ultrastructural evidence place the Phaeophyceae among the heterokonts (Stramenopiles),[22] a large assemblage of organisms that includes both photosynthetic members with plastids (such as the diatoms) as well as non-photosynthetic groups (such as the slime nets and water molds).

Although some heterokont relatives of the brown algae lack plastids in their cells, scientists believe this is a result of evolutionary loss of that organelle in those groups rather than independent acquisition by the several photosynthetic members.

[23] Thus, all heterokonts are believed to descend from a single heterotrophic ancestor that became photosynthetic when it acquired plastids through endosymbiosis of another unicellular eukaryote.

[25] DNA sequence comparison also suggests that the brown algae evolved from the filamentous Phaeothamniophyceae,[26] Xanthophyceae,[27] or the Chrysophyceae[28] between 150[1] and 200 million years ago.

The occurrence of Phaeophyceae as fossils is rare due to their generally soft-bodied nature,[30] and scientists continue to debate the identification of some finds.

Among the brown algae, only species of the genus Padina deposit significant quantities of minerals in or around their cell walls.

[30] The Devonian megafossil Prototaxites, which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga.

Several fossils of Drydenia and a single specimen of Hungerfordia from the Upper Devonian of New York have also been compared to both brown and red algae.

[32] Fossils of Drydenia consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species of Laminaria, Porphyra, or Gigartina.

[45] Choristocarpaceae Discosporangiaceae Ishigeaceae Petrodermataceae Onslowiales Dictyotales Syringodermatales Lithodermataceae Phaeostrophiaceae Stypocaulaceae Cladostephaceae Sphacelariaceae Bachelotiaceae Desmarestiales Sporochnales Ascoseirales Ralfsiales Cutleriaceae Tilopteridaceae Phyllariaceae Nemodermatales Sargassaceae Durvillaeaceae Himanthaliaceae Fucaceae Asteronemataceae Splachnidiaceae Phaeosiphoniellaceae Akkesiphycaceae Pseudochordaceae Chordaceae Alariaceae Agaraceae Laminariaceae Asterocladales Adenocystaceae Scytosiphonaceae Petrospongiaceae Ectocarpaceae Acinetosporaceae Chordariaceae This is a list of the orders in the class Phaeophyceae:[45][46] Most brown algae, with the exception of the Fucales, perform sexual reproduction through sporic meiosis.

Meiosis takes place within several unilocular sporangium along the algae's blade, each one forming either haploid male or female zoospores.

Gametes are formed in specialized conceptacles that occur scattered on both surfaces of the receptacle, the outer portion of the blades of the parent plant.

Egg cells and motile sperm are released from separate sacs within the conceptacles of the parent algae, combining in the water to complete fertilization.

[26] In addition to alginates, fucoidan and cellulose, the carbohydrate composition of brown algae consists of mannitol, laminarin and glucan.

[56] Brown algae including kelp beds also fix a significant portion of the earth's carbon dioxide yearly through photosynthesis.