Brown Algae - Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name.

Phaeophyceae: Brown Algae - The brown colour of these algae results from the dominance of the xanthophyll pigment fucoxanthin, which masks the other pigments, Chlorophyll a and c (no Chlorophyll b), beta-carotene and other xanthophylls. Food reserves are typically complex polysaccharides, sugars and higher alcohols. The principal carbohydrate reserve is laminaran, and true starch is absent (compare with the green algae). The walls are made of cellulose and alginic acid, a long-chained heteropolysaccharide.

Phaeophyta (Brown algae) - The 1,500 species of brown algae are almost exclusively found in marine habitats. The browns include the largest of the seaweeds, Durvillea and the kelps, found in cold waters. The kelps present the most complex anatomy of all the algae with trumpet hyphae and sievelike cells that contribute to translocation. The brown algae have chlorophyll a and c, as well as carotenes and xanthophylls. Cell walls are composed of cellulose layered with polysaccharide (such as the valuable alginic acid.) Brown algal cells have a single nuclei and thylakoids in the chloroplast occur in bands of three.