(Class Phaeophyceae)
Recent investigations of plant and plant-like organisms using DNA data have altered our understanding of their relationships and therefore their classification at the highest levels. The brown algae, previously treated as Phylum Phaeophyta, are now Class Phaeophyceae in the Phylum Ochrophyta.
The golden-brown pigment fucoxanthin masks the green of chlorophyll. Consequently brown algae range in colour from yellow through golden brown to almost black. Energy required by these organisms is stored as a polysaccharide called laminarin. Brown algal cell walls are made up of cellulose, which is often combined with fucans or alginates. This provides structural flexibility and prevents rapid drying.
All of the approximately 2,000 species of brown algae are multicellular. The greatest diversity occurs in cool, temperate marine waters: 150 or more species inhabit the Pacific Northwest, the centre of diversity for kelps, such as Macrocystis pyrifera (SW59), shown in the photograph. Two kelps in particular, Nereocystis luetkeana (SW60) and Macrocystis pyrifera (SW59), are very important components of nearshore, rocky coastal ecosystems. All kelps are brown algae but not all brown algae are kelps—a fact that has led to some species being incorrectly named kelp.