Chlorella is a genus of single-celled green algae, belonging to the phylum Chlorophyta. It is spherical in shape, about 2 to 10 μm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast. Through photosynthesis it multiplies rapidly requiring only carbon dioxide, water, sunlight, and a small amount of minerals to reproduce Fact: date=July 2008.

The name Chlorella is taken from the Greek word chloros meaning green and the Latin diminutive suffix ella meaning "small". German biochemist Otto Heinrich Warburg received the Nobel Prize in Physiology or Medicine in 1931 for his study on photosynthesis in Chlorella. In 1961 Melvin Calvin of the University of California received the Nobel Prize in Chemistry for his research on the pathways of carbon dioxide assimilation in plants using Chlorella. In recent years, researchers have made less use of Chlorella as an experimental organism because it lacks a sexual cycle and, therefore, the research advantages of genetics are unavailable.

Many people believed Chlorella could serve as a potential source of food and energy because its photosynthetic efficiency can, in theory, reach 8%, comparable with other highly efficient crops such as sugar cane. It is also an attractive food source because it is high in protein and other essential nutrients; when dried, it is about 45% protein, 20% fat, 20% carbohydrate, 5% fiber, and 10% minerals and vitamins. However, because it is a single-celled algae, harvest posed practical difficulties for its large-scale use as a food source. Fact: date=May 2008 Mass-production methods are now being used to cultivate it in large artificial circular ponds.

Aquarium

Chlorella can create green and opaque water problems in aquaria. Chlorella can grow due to high nitrate and phosphate levels or direct sunlight. Decreasing phosphate and nitrate by partial water change and moving the aquarium to shade can help alleviate the problem.

Chlorella as a food source

Following global fears of an uncontrollable population boom, during the late 1940s and the early 1950s Chlorella was seen as a new and promising primary food source and as a possible solution to the then current world hunger crisis. Many people during this era thought that world hunger was a growing problem and saw Chlorella as a way to end this crisis by being able to provide large amounts of high quality food for a relatively low cost. .

Many institutions began to research the algae, including the Carnegie Institution, the Rockefeller Foundation, the NIH, UC Berkeley, the Atomic Energy Commission, and Stanford University. Following WWII, many Europeans were starving and many Malthusians attributed this not only to the war but to the inability of the world to produce enough food to support the currently-increasing population. According to a 1946 FAO report, the world would need to produce 25 to 35 percent more food in 1960 than in 1939 to keep up with the increasing population, while health improvements would require a 90 to 100 percent increase. Because meat was costly and energy-intensive to produce, protein shortages were also an issue. Increasing cultivated area alone would go only so far in providing adequate nutrition to the population. The USDA calculated to feed the U.S. population by 1975, it would have to add 200 million acres (800,000 km²) of land, but only 45 million were available. One way to combat national food shortages was to increase the land available for farmers, yet the American frontier and farm land had long since been extinguished in trade for expansion and urban life. Hopes rested solely on new agricultural techniques and technologies. Because of these circumstances, an alternative solution was needed.