The classification of algae into Chlorophyceae (green), Phaeophyceae (brown) and Rhodophyceae (red) rests on differences in pigmentation, stored food, cell wall composition, and flagellar features.

Algae, though a highly diverse group, are traditionally divided into three main classes based on several structural and biochemical criteria:

1. Type of pigments present: Chlorophyceae (green algae) contain chlorophyll a and b, giving them a grass-green colour; Phaeophyceae (brown algae) contain chlorophyll a, c and the brown pigment fucoxanthin; Rhodophyceae (red algae) contain chlorophyll a, d and the red pigment r-phycoerythrin.
2. Chemical nature of the stored food: green algae store starch, brown algae store mannitol and laminarin (complex carbohydrates), and red algae store floridean starch.
3. Cell wall composition: e.g., cellulose in green algae, cellulose plus algin in brown algae, cellulose plus pectin (and sometimes calcium carbonate deposits) in red algae. …

(a) Green algae, (b) Phaeophyceae, (c) Red algae, (d) Mannitol and laminarin.

Algae are classified into three major classes based on their pigmentation and the type of food they store:

• Chlorophyceae, commonly called (a) green algae, store food mainly as starch, just like higher plants.
• (b) Phaeophyceae, commonly called brown algae, contain the pigment fucoxanthin along with chlorophyll a and c, and store food as complex carbohydrates, mainly (d) mannitol and laminarin. …

Red and brown algae are both mainly marine but differ in their characteristic pigments, food reserves, and cell wall composition.

Red algae (class Rhodophyceae):

• Get their colour mainly from the red pigment phycoerythrin, along with chlorophyll a, d and phycocyanin.
• Store food as floridean starch, a polymer very similar to amylopectin/glycogen, deposited outside the chloroplast in the cytoplasm.
• Cell walls typically contain cellulose along with polysaccharides such as agar and carrageenan (economically important, e.g., Gelidium, Gracilaria).
• Mostly found attached to the sea bed at greater depths in calmer marine waters, since phycoerythrin efficiently absorbs the blue-green light that penetrates deeper water.

Brown algae (class Phaeophyceae):

• Get their colour mainly from the brown/olive pigment fucoxanthin, along with chlorophyll a and c.
• Store food as complex carbohydrates — mainly laminarin (a polysaccharide) and mannitol (a sugar alcohol). …