Chlorella is single-celled green alga which is placed under the phylum Chlorophyta. It is spherical, without flagella and about 2-10 µm in diameter. Chlorella bear a green pigment for carrying out photosynthesis known as chlorophyll. The chloroplast bears chlorophyll a and b. With the help of photosynthesis it multiplies rapidly and requires only carbon dioxide, water, sunlight and little amount of minerals for reproduction. Chlorella have derived its name from a Greek word chloros which means green. Since it lacks sexual cycle in its life history genetic studies are not possible in these algae so researches have declined the use of these algae as an experimental model in the recent years. Melvin Calvin of University of California was awarded Noble Prize in Chemistry in 1961 for working out the pathway of carbon dioxide assimilation in these algae. Many people believe that this alga is an efficient source of food and energy as its photosynthetic efficiency can reach up to 8% which is much higher as compared to that of other crops like sugarcane. It is a very attractive food source as it is highly rich in proteins and other essential minerals. When it is dried it contains about 45% protein, 20% fat, 20% carbohydrate, 5% fiber and 10% minerals and vitamins. As it is a single celled alga its harvesting on mass scale has posed a number of difficulties so it cannot be used as food source on large scale. Mass cultivation methods are now practiced in artificial circular ponds.

They are known to cause green and opaque problems in aquaria. They grow rapidly when water is rich in phosphates and nitrates or in direct sunlight. Decreasing the phosphate and nitrate levels in water and shifting the aquaria to shade can help to eliminate this problem. Due to the problems of hunger and malnutrition Chlorella is now cultivated on mass scale and marketed so that it can act as an effective food source. It is now clear from the studies that this alga has healing properties. According to one study when this alga was fed to rats it showed anti-tumour activities. It also increased vascular function in the hypertensive rats when given orally. It is a complete package of protein but also contains fats, vitamins and calories. It can act as a protein supplement for humans. It also yields oil which is rich in polyunsaturated fats. Several studies have suggested that Chlorella supplementation can be helpful in reducing the dioxin levels in the breast milk and it also has a beneficial effect on the nursing infants by increasing the IgA level in breast milk. Chlorella can also reduce high blood pressure, lower cholesterol levels, increase wound healing and enhance immune functions in humans. However, there are a number of controversies against Chlorella to be used as a food supplement.