1. Food: Many green algae such as Ulva, Caulerpa, Enteromorpha, Chlorella etc. used as food. Chlorella has highest percentage of protein or even more than in other vegetables or egg. It is said to contain all vitamins from A to D.
2. Antibiotics: An antibiotics called Chlorellin is extracted from Chlorella. It has marked effects against both Gram positive and Gram negative bacteria especially E. coli etc. Even Cladophora and Caulerpa also yield antibiotics. The alga like Chara and Nitella have larvicidal property.
3. Sewage treatment: Green unicellular algae such as Chlorella and Chlamydomonas are used in sewage treatment ponds. They show photosynthesis, remove carbon dioxide and release oxygen and make the sewage water habitable for many fishes and aerobic bacteria.
4. Green algae in space research: In recent years biologists used green alga(Chlorella) extensively in space flight trips. The algae can reuse carbon dioxide during the process of photosynthesis and release oxygen for the use of astronauts.
5. Parasitic algae: Cephaleuros virescens, a filamentous green alga is parasitic on tea and coffee plants and causes red disease on the leaves. It reduces the yield.