Ochrophyta

All brown algae are multicellular and belong to the class Phaeophyceae, phylum Ochrophyta. The phylum and class names come from the color of this distinctive group of algae. Almost all brown algal species are marine, and the largest and most morphologically complex marine algae are brown algae.

Brown algae are also distinguished by the two different types of flagella borne by the swimming reproductive cells. The forward-directed flagellum bears stiff tubular hairs called mastigonemes and is usually longer than the smooth, backwarddirected flagellum. The only flagellated cells are reproductive cells (there are no free-living flagellated unicells in the class Phaeophyceae), and the flagella are inserted laterally or subapically.

Brown algal chloroplasts are usually golden brown in color because of the accessory pigment fucoxanthin. Cell walls are composed of a fibrillar cellulose network and an alginate mucilage.

The chloroplast is surrounded by four membranes: the double membrane of the chloroplast envelope, and the chloroplast endoplasmic reticulum, which is often continuous with the nuclear envelope. Thylakoids within the chloroplast are stacked in groups of three, and the primary pigments are chlorophylls a, c¹ , and c². The main storage product of photosynthesis is laminarin, a β-1,3-linked polymer of glucose; it is stored outside the chloroplast.