Chlorella is agenus of about thirteen species of single-celled orcolonialgreen algae of the divisionChlorophyta. The cells are spherical in shape, about 2 to 10μm in diameter, and are withoutflagella. Theirchloroplasts contain the green photosynthetic pigmentschlorophyll-a and-b. In ideal conditions cells ofChlorella multiply rapidly, requiring onlycarbon dioxide,water,sunlight, and a small amount ofminerals to reproduce.[2]
The nameChlorella is taken from theGreek χλώρος,chlōros/ khlōros, meaning green, and theLatin diminutive suffix -ella, meaning small. Germanbiochemist and cell physiologistOtto Heinrich Warburg, awarded with theNobel Prize in Physiology or Medicine in 1931 for his research oncell respiration, also studied photosynthesis inChlorella. In 1961,Melvin Calvin of theUniversity of California received theNobel Prize in Chemistry for his research on the pathways ofcarbon dioxide assimilation in plants usingChlorella.
Chlorella has been considered as a source of food and energy because itsphotosynthetic efficiency can reach 8%,[3] which exceeds that of other highly efficient crops such assugar cane.
Chlorella consists of small, rounded cells which are spherical, subspherical, or ellipsoidal, and may be surrounded by a layer ofmucilage. The cells contain a singlechloroplast which is parietal (lying against the inner side of the cell membrane), with a singlepyrenoid that is surrounded by grains of starch.[1]
Reproduction occurs by the formation ofautospores;zoospores or gametes are not known to be produced inChlorella.[1] In autosporulation, the contents of the cell divide into two, four or sometimes eightprotoplasts. Each daughter protoplast rounds off, and are liberated by the rupture of the parent cell wall. On release, each autospore grows to become a new individual.[citation needed] The daughter cell may remain attached to the parent cell wall, thereby forming colonies of cells.[1] The presence ofsulphur in the culture medium is considered essential for cell division. It takes place even in the dark with sulphur alone as the source material but under light conditions nitrogen also required in addition.[citation needed] Pearsall and Loose (1937)[4] reported the occurrence of motile cells inChlorella. Bendix (1964)[5] also observed thatChlorella produces motile cells which might be gametes. These observations have an important bearing on the concept of the life cycle ofChlorella, which at present is considered to be strictly asexual in character.[citation needed]
Asexual reproduction inChlorella ellipsoides has been studied in detail and the following four phases have been observed during the asexual reproduction.
Chlorella was first described byMartinus Beijerinck in 1890. Since then, over a hundred taxa have been described within the genus. However, biochemical and genomic data has revealed that many of these species were not closely related to each other, even being placed in a separate classChlorophyceae. In other words, the "green ball" form ofChlorella appears to be a product ofconvergent evolution and not a natural taxon.[6] IdentifyingChlorella-like algae based on morphological features alone is generally not possible.[7]
Some strains of "Chlorella" used for food are incorrectly identified, or correspond to genera that were classified out of trueChlorella. For example,Heterochlorella luteoviridis is typically known asChlorella luteoviridis which is no longer considered a valid name.[8]
When first harvested,Chlorella was suggested as an inexpensive protein supplement to the human diet. According to theAmerican Cancer Society, "available scientific studies do not support its effectiveness for preventing or treating cancer or any other disease in humans".[9]
Under certain growing conditions,Chlorella yields oils that are high inpolyunsaturated fats—Chlorella minutissima has yieldedeicosapentaenoic acid at 39.9% of total lipids.[10]
Following global fears of an uncontrollable human 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 time thought hunger would be an overwhelming problem and sawChlorella as a way to end this crisis by providing large amounts of high-quality food for a relatively low cost.[11]
Many institutions began to research the algae, including theCarnegie Institution, theRockefeller Foundation, theNIH,UC Berkeley, theAtomic Energy Commission, andStanford University. FollowingWorld War II, many Europeans were starving, and manyMalthusians attributed this not only to the war, but also to the inability of the world to produce enough food to support the increasing population. According to a 1946FAO report, the world would need to produce 25 to 35% more food in 1960 than in 1939 to keep up with the increasing population, while health improvements would require a 90 to 100% increase.[11] 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. TheUSDA calculated that, 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.
To cope with the upcoming postwar population boom in the United States and elsewhere, researchers decided to tap into the unexploited sea resources. Initial testing by theStanford Research Institute showedChlorella (when growing in warm, sunny, shallow conditions) could convert 20% of solar energy into a plant that, when dried, contains 50% protein.[11] In addition,Chlorella contains fat and vitamins. The plant's photosynthetic efficiency allows it to yield more protein per unit area than any plant—one scientist predicted 10,000 tons of protein a year could be produced with just 20 workers staffing a 1000-acre (4-km2)Chlorella farm.[11] The pilot research performed at Stanford and elsewhere led to immense press from journalists and newspapers, yet did not lead to large-scale algae production.Chlorella seemed like a viable option because of the technological advances in agriculture at the time and the widespread acclaim it got from experts and scientists who studied it. Algae researchers had even hoped to add a neutralizedChlorella powder to conventional food products, as a way to fortify them with vitamins and minerals.[11]
When the preliminary laboratory results were published, the scientific community at first backed the possibilities ofChlorella.Science News Letter praised the optimistic results in an article entitled "Algae to Feed the Starving". John Burlew, the editor of theCarnegie Institution of Washington bookAlgal Culture-from Laboratory to Pilot Plant, stated, "the algae culture may fill a very real need",[12] whichScience News Letter turned into "future populations of the world will be kept from starving by the production of improved or educated algae related to the green scum on ponds". The cover of the magazine also featuredArthur D. Little's Cambridge laboratory, which was a supposed future food factory. A few years later, the magazine published an article entitled "Tomorrow's Dinner", which stated, "There is no doubt in the mind of scientists that the farms of the future will actually be factories."Science Digest also reported, "common pond scum would soon become the world's most important agricultural crop." However, in the decades since those claims were made, algae have not been cultivated on that large of a scale.
Since the growing world food problem of the 1940s was solved by better crop efficiency and other advances in traditional agriculture,Chlorella has not seen the kind of public and scientific interest that it had in the 1940s.Chlorella has only a niche market for companies promoting it as a dietary supplement.[11]
The experimental research was carried out in laboratories, rather than in the field, andscientists discovered thatChlorella would be much more difficult to produce than previously thought. To be practical, the algae grown would have to be placed either inartificial light or in shade to produce at its maximum photosynthetic efficiency. In addition, for theChlorella to be as productive as the world would require, it would have to be grown incarbonated water, which would have added millions to the production cost. A sophisticated process, and additional cost, was required to harvest the crop and forChlorella to be a viable food source, its cell walls would have to be pulverized. The plant could reach its nutritional potential only in highly modified artificial situations. Another problem was developing sufficiently palatable food products fromChlorella.[13]
Although the production ofChlorella looked promising and involved creative technology, it has not to date been cultivated on the scale some had predicted. It has not been sold on the scale ofSpirulina,soybean products, or whole grains. Costs have remained high, andChlorella has for the most part been sold as a health food, for cosmetics, or asanimal feed.[13] After a decade of experimentation, studies showed that following exposure to sunlight,Chlorella captured just 2.5% of the solar energy, not much better than conventional crops.[11]Chlorella, too, was found by scientists in the 1960s to be impossible for humans and other animals to digest in its natural state due to the tough cell walls encapsulating the nutrients, which presented further problems for its use in American food production.[11]
In 1965, the RussianCELSS experimentBIOS-3 determined that 8 m2 of exposedChlorella could remove carbon dioxide and replace oxygen within the sealed environment for a single human. The algae were grown in vats underneath artificial light.[14]
Chlorella is consumed as adietary supplement. Some manufacturers ofChlorella products have falsely asserted that it has health benefits,[15] including an ability to treat cancer,[16] for which theAmerican Cancer Society stated "available scientific studies do not support its effectiveness for preventing or treating cancer or any other disease in humans".[16] The United StatesFood and Drug Administration has issuedwarning letters to supplement companies for falsely advertising health benefits of consuming chlorella products, such as one company in October 2020.[17]
There is some support from animal studies of chlorella's ability to detoxifyinsecticides.Chlorella protothecoides accelerated the detoxification of rats poisoned withchlordecone, a persistent insecticide, decreasing the half-life of the toxin from 40 to 19 days.[18] The ingested algae passed through the gastrointestinal tract unharmed, interrupted the enteric recirculation of the persistent insecticide, and subsequently eliminated the bound chlordecone with the feces.
A 2002 study showed thatChlorella cell walls containlipopolysaccharides,endotoxins found inGram-negative bacteria that affect theimmune system and may causeinflammation.[19][20][21] However, more recent studies have found that the lipopolysaccharides in organisms other than Gram-negative bacteria, for example in cyanobacteria, are considerably different from the lipopolysaccharides in Gram-negative bacteria.[22]
