Brown Algae

Brown algae, the Phaeophyceae (or Fucophyceae; Christensen, 1978), are a class (or division, Phaeophyta; Papenfuss, 1951) of algae consisting mainly of complex, macroscopic seaweeds whose brown color comes from a carotenoid pigment, fucoxanthin, and in some species, various phaeophycean tannins. Of perhaps 2000 species (in 265 genera) of brown algae (Van den Hoek et al., 1995), less than 1% are known from freshwater habitats, although some marine species may colonize brackish waters (Wilce, 1966; Dop, 1979; West and Kraft, 1996). Various authors cite between 3 and 7 genera, and up to 12 species of freshwater brown algae worldwide (see Sect. 21.II.A). Members of the group have many features in common with chrysophytes, synurophytes (Chrysophyta), and diatoms (Bacillariophyta), including chloroplast structure (thylakoids in stacks of three, girdle lamella, chloroplast endoplasmic reticulum), motile stage heterokont (unequal flagella), major pigments (chlorophylls a, c1, and c2, β-carotene, violaxanthin, diatoxanthin, and large amounts of fucoxanthin), as well as the storage reserve laminarin (Craigie, 1974; Goodwin, 1974; Pueschel and Stein, 1983; Lee, 1989). However, no members of the Phaeophyceae are unicellular or colonial in the vegetative phase—the predominant morphology in other golden-brown groups. Brown algae have cell walls composed of cellulose, which is often supplemented with the mucopolysaccharide alginic acid. In seaweeds, this material is produced in sufficient quantities in some species to be harvested for commercial purposes, but in freshwater species, alginates appear to be less prevalent. Further descriptions of the group can be found in reviews by Papenfuss (1951), Van den Hoek et al. (1995), and Graham and Wilcox (2000). Freshwater species of brown algae have been known for more than 100 years [Pleurocladia was described by Braun (1855), Heribaudiella by Gomont (1896); Bourrelly (1981)], but today most are still known only from scattered locations. Freshwater phaeophytes occur in a variety of streams and rivers, as well as in the littoral zone of lakes, but their biology has largely remained obscure for most phycologists and freshwater ecologists. This is unfortunate, because some species, most notably Heribaudiella fluviatilis, can at times be one of the dominant species of benthic algae in smaller rivers (Kann, 1978a). One reason for their obscurity may be that most species form crusts or