What is the basis of classification of algae?

Hint: Algae are the chlorophyll bearing thalloid plants. Algae are classified into three classes. They are Chlorophyceae, Phaeophyceae and Rhodophyceae. They are classified into these three main classes based on certain criteria.

Complete Answer:
- Algae are classified into three classes based on several characteristics. Major pigments present in them is the main reason for division. Chlorophyceae or green algae contain pigments like chlorophyll a and chlorophyll b.
- Phaeophyceae are also called brown algae. They contain pigments like chlorophyll a, chlorophyll c, and fucoxanthin. Red algae or Rhodophyceae has pigments like Chlorophyll a, chlorophyll d, and phycoerythrin.
- Stored food is also different in these organisms. Green algae store their food as starch just like higher plants. Brown algae store their food in the form of mannitol, laminarin, etc. At the same time, red algae store their food and Floridian starch which is similar to glycogen in structure.
- The cell wall of these three classes is also different. Green algae have a cellulosic cell wall. The cell wall of brown algae is cellulosic, but also contains a substance called algin with it. Red algae have their cellulose made up of cellulose, pectin, and polysulphate esters.
- Green algae have flagella in their body. There will be 2-8 equal numbers of flagella present in them and the position of flagella will be apical. In brown algae, two unequal flagella are seen in the lateral part of the organism. Flagella are absent in red algae. These are the main criteria used for classifying algae.

Note: Red algae contain a red pigment called r-phycoerythrin in their body and hence their name. Green algae are mostly green in colour due to the abundance of green pigments for photosynthesis.