The Phaeophyceae, or brown algae, derive their characteristic color from the large amounts of the carotenoid fucoxanthin (Fig. 21.2) in their chloroplasts as well as from any phaeophycean tannins that might be present. The chloroplasts also have chlorophylls a, c 1, and c 2. There are two membranes of chloroplast E.R., which are usually continuous with the outer membrane of the nuclear envelope. The storage product is laminarin (Fig. 21.2). There are no unicellular or colonial organisms in the order, and the algae are basically filamentous, pseudoparenchymatous, or parenchymatous. They are found almost exclusively in the marine habitat, there being only four genera containing freshwater species, that is, Heribaudiella, Pleurocladia, Bodanella, and Sphacelaria (Fig. 21.1) (Schloesser and Blum, 1980). A number of marine forms penetrate into brackish water, where they often form an important part of the salt marsh flora. These brackish water plants have almost totally lost the ability to reproduce sexually, and propagate by vegetative means only. Most of the Phaeophyceae grow in the intertidal belt and the upper littoral region. They dominate these regions in colder waters, particularly in the Northern Hemisphere, where the number of phaeophycean species is less than that of the Rhodophyceae, but the number of phaeophycean plants is much greater. In the tropics, the only place where large numbers of Phaeophyceae are found is the Sargasso Sea of the Atlantic.

The Phaeophyceae probably evolved about 260 Ma ago during the Permian Period (Brown, 2010 ; Kawai et al., 2015) from an organism in the Phaeothamniophyceae, which have motile cells similar to those in the Phaeophyceae, but lack the characteristic unilocular and plurilocular sporangia of the Phaeophyceae (Bailey et al., 1998).

Cell Structure

The cell structure (Fig. 21.3) is in many ways similar to that of the Chrysophyceae, Prymnesiophyceae, Bacillariophyceae, and Xanthophyceae, which are closely related to the Phaeophyceae. The main difference lies in the large amounts of extracellular polysaccharides surrounding the protoplast.

Cell Walls

There are no unicellular members of the Phaeophyceae, so cell walls joining cells are characteristic of the group (Yamagishi et al., 2014). Phaeophycean cell walls are generally composed of at least two layers, with cellulose making up the main structural skeleton (Kloareg and Quatrano, 1988).