Chlorophyceae – These are called green algae, due to the presence of pigments CHLOROPHYLL a and b. Examples are Chlamydomonas, Spirogyra, and CharaPhaeophyceae – Also called as brown algae, they are predominantly marine. They have chlorophyll a, c, carotenoids and xanthophyll pigments. Examples are Dictyota, Laminaria, and SargassumRhodophyceae – They are the RED algae because of the presence of the red PIGMENT, r-phycoerythrin. Examples are Porphyra, Gracilaria, and Gelidium.Explanation:According to some scientists, algae produce half of the earth’s oxygen. And this production of oxygen is by far the most important contribution of algae. As mentioned in the beginning they are a source of crude oil. These algal biofuels could well be a replacement for the fossil fuels.Algae also play an effective role in keeping the atmospheric carbon DIOXIDE STABLE and also using it efficiently. The food industry also uses some algae. Agar is obtained from Gelidium and Gracilaria and is making ice-creams and jellies. The other food supplements that are algae and which are widely used are Chlorella and Spirulina.