Algae

The pigmentation-based grouping of algae in NCERT Class 11, Chapter 3 (Plant Kingdom) recognises three classes, Chlorophyceae, Phaeophyceae and Rhodophyceae, distinguished mainly by pigments, stored food and flagellar features. Brown algae belong to Phaeophyceae and owe their greenish-brown colour to a xanthophyll pigment called fucoxanthin, present in varying amounts, which masks the green chlorophylls a and c. Fucoxanthin is therefore the correct pigment. Phycoerythrin is a red pigment characteristic of red algae, Rhodophyceae, giving them their colour, so it does not apply here. Phycocyanin is a blue pigment found in cyanobacteria and red algae, again not responsible for the brown colour. Chlorophyll b is present in green algae and higher plants but is absent in brown algae, which instead contain chlorophyll c. Brown algae store food as the complex carbohydrate laminarin and also as mannitol, and their cell walls bear an outer layer of algin. Genera such as Laminaria, Fucus, Ectocarpus and the giant kelp Sargassum illustrate the group. A consistency check confirms the logic: a brown tint demands a brown-yellow accessory pigment overpowering green chlorophyll, a role fulfilled precisely by fucoxanthin in Phaeophyceae.