Arthrospira

''Arthrospira'' and ''Limnospira'' are two genera of free-floating filamentous cyanobacteria characterized by cylindrical, multicellular trichomes in an open left-hand helix. A dietary supplement is made from ''L. platensis'' and ''L. maxima'', known as spirulina.

The two species species were commonly treated as if they are in the genus ''Spirulina'' since 1932, even though they were originally proposed in ''Arthorospira'' in 1892 and 1917. The distinction was restored in the late 20th century. Although the introduction of the two separate genera ''Arthrospira'' and ''Spirulina'' is now generally accepted, there has been much dispute in the past and the resulting taxonomical confusion is tremendous. To add to the problem, it was shown in 2019 that the type species for ''Arthrospira'', ''A. jenneri'', was very distantly related to the species used in food production. This necessitated the creation of yet another genus, ''Limnospira'', to hold these economically-important species.

Taxonomy

The common name, spirulina, refers to the dried biomass of ''Arthrospira platensis'' (now ''L. platensis''), a type of Cyanobacteria, which are oxygenic (produce oxygen) photosynthetic bacteria. These photosynthetic organisms were first considered to be algae, a very large and diverse group of eukaryotic organisms, until 1962 when they were reclassified as prokaryotes and named Cyanobacteria. This designation was accepted and published in 1974 by '' Bergey's Manual of Determinative Bacteriology''. Scientifically, quite a distinction exists between the ''Spirulina'' and ''Arthrospira'' genera. Stizenberger, in 1852, gave the name ''Arthrospira'' based on the presence of septa, its helical form, and its multicellular structure, and Gomont, in 1892, confirmed the aseptate form of the genus ''Spirulina''. Geitler in 1932 reunified both members designating them as ''Spirulina'' without considering the septum. Research on microalgae was carried out in the name of ''Spirulina'', but the original species used to produce the dietary supplement spirulina belongs to the genus ''Arthrospira''. This misnomer has been difficult to correct.

As of 2010, taxonomy states that the name spirulina for strains which are used as food supplements is inappropriate, and agreement exists that ''Arthrospira'' is a distinct genus, consisting of over 30 different species, including ''A. platensis'' and ''A. maxima''.

A 2019 analysis of ''Arthrospira'' species using 16S rRNA gene sequence suggests that the type species of this genus (''A. jenneri'') is much closer to ''Planktothrix'' clade than previously thought. It also lacks characteristics of mass produced species (such as preference of alkaline habitats). As a result, researchers proposed a new genus closer to '' Limnoraphis'' and '' Neolyngbya'' called ''Limnospira'' comprising '' L. fusiformis'', '' L. maxima'' and '' L. indica''. A further 2024 study confirmed that ''L. platensis'' should be in this genus, but also found that ''L. fusiformis'' and ''L. indica'' are insufficiently different from ''L. maxima'' to be their own species.

Morphology

The genus ''Arthrospira'' comprises helical trichomes of varying size and with various degrees of coiling, including tightly-coiled morphology to a straight form.

The helical parameters of the shape of ''Arthrospira'' is used to differentiate between and even within the same species. These differences may be induced by changing environmental conditions, such as temperature. The helical shape of the trichomes is only maintained in a liquid environment. The filaments are solitary and reproduce by binary fission, and the cells of the trichomes vary in length from 2 to 12 μm and can sometimes reach 16 μm.

Biochemical composition

''Limnospira'' is very rich in proteins, and constitute 53 to 68 percent by dry weight of the contents of the cell.
Its protein harbours all essential amino acids. ''Limnospira'' also contain high amounts of polyunsaturated fatty acids (PUFAs), about 1.5–2 percent, and a total lipid content of 5–6 percent. These PUFAs contain the γ-linolenic acid (GLA), an omega-6 fatty acid. Further contents of ''Limnospira'' include vitamins, minerals and photosynthetic pigments.

Occurrence

Species of the genera ''Arthrospira'' and ''Limnospira'' have been isolated from alkaline brackish and saline waters in tropical and subtropical regions. Among the various species included in the genus, ''L. platensis'' is the most widely distributed and is mainly found in Africa, but also in Asia. ''L. maxima'' is believed to be found in California and Mexico. ''L. platensis'' and ''L. maxima'' occur naturally in tropical and subtropical lakes with alkaline pH and high concentrations of carbonate and bicarbonate. ''L. platensis'' occurs in Africa, Asia and South America, whereas ''L. maxima'' is confined to Central America. ''A. pacifica'' (name not found in LPSN) is endemic to the Hawaiian islands. Most cultivated spirulina is produced in open-channel raceway ponds, with paddle-wheels used to agitate the water. The largest commercial producers of spirulina are located in the United States, Thailand, India, Taiwan, China, Pakistan, Myanmar, Greece and Chile.

Present and future uses

Spirulina is widely known as a food supplement, but there are other possible uses for this cyanobacterium. As an example, it is suggested to be used medically for patients for whom it is difficult to chew or swallow food, or as a natural and cheap drug delivery system. Further, promising results in the treatment of certain cancers, allergies and anemia, as well as hepatotoxicity and vascular diseases were found. Spirulina may also be used as a healthy addition to animal feed if the price of its production can be further reduced

Spirulina can be used in technical applications, such as the biosynthesis of silver nanoparticles, which allows the formation of metallic silver in an environmentally friendly way. In the creation of textiles it harbors some advantages, since it can be used for the production of antimicrobial textiles and paper or polymer materials.

They also may have an antioxidant effect and may maintain the ecological balance in aquatic bodies and reduces various stresses in the aquatic environment.

Cropping systems

Growth of ''L. platensis'' depends on several factors. To achieve maximum output, factors such as the temperature, light and photoinhibition, nutrients and carbon dioxide level, need to be adjusted.

In summer the main limiting factor of spirulina growth is light. When growing in water depths of 12–15 cm, self-shading governs the growth of the individual cell. However, research has shown, that growth is also photoinhibited, and can be increased through shading. The level of photoinhibition versus the lack of light is always a question of cell concentration in the medium.

The optimal growth temperature for ''L. platensis'' is 35–38 Â°C. This poses a major limiting factor outside the tropics, confining growth to the summer months. ''L. platensis'' has been grown in fresh water, as well as in brackish water and sea water. Apart from mineral fertilizer, various sources such as waste effluents, and effluents from fertilizer, starch and noodle factories have been used as a nutrient source.

Waste effluents are more readily available in rural locations, allowing small scale production.
One of the major hurdles for large scale production is the complicated harvesting process which accounts for 20–30% of the total production costs. Due to their small cell size, and diluted cultures (mass concentration less than 1 g/L) with densities close to that of water microalgae, they are difficult to separate from their growing medium.

Cultivation systems

Open pond

Open pond systems are the most common way to grow ''L. platensis'' due to their comparatively low cost. Typically, channels are built in form of a raceway from concrete or PVC coated earth walls, and water is moved by paddle wheels. The open design, however allows contamination by foreign algae and/or microorganisms.

Another problem includes water loss due to evaporation. Both of these problems can be addressed by covering the channels with transparent polyethylene film.

Closed system

Closed systems have the advantage of being able to control the physical, chemical and biological environment. This allows for increased yield, and more control of the nutrient level. Typical forms such as tubes or polyethylene bags, also offer a larger surface-to-volume ratios than open pond systems, thus increasing the amount of sunlight available for photosynthesis. These closed systems help expanding the growing period into the winter months, but often lead to overheating in summer.

Market potentials and feasibility

Cultivation of ''Limnospira'' has occurred for a long period of time, especially in Mexico and around Lake Chad on the African continent. During the 21st century however, its beneficial properties were rediscovered and therefore studies about ''Limnospira'' and its production increased. In the past decades, large-scale production of the cyanobacterium developed. Japan started in 1960, and in the following years Mexico and several other countries over all continents, such as China, India, Thailand, Myanmar and the United States started to produce on large-scale. In little time, China has become the largest producer worldwide. A particular advantage of the production and use of spirulina is that its production can be conducted at a number of different scales, from household culture to intensive commercial production over large areas.

Especially as a small-scale crop, ''Limnospira'' still has considerable potential for development, for example for nutritional improvement. New countries where this could happen, should dispose of alkaline-rich ponds on high altitudes or saline-alkaline-rich groundwater or coastal areas with high temperature. Otherwise, technical inputs needed for new spirulina farms are quite basic.

The international market of spirulina is divided into two target groups: the one includes NGO’s and institutions focusing on malnutrition and the other includes health conscious people. There are still some countries, especially in Africa, that produce at a local level. Those could respond to the international demand by increasing production and economies of scale. Growing the product in Africa could offer an advantage in price, due to low costs of labour. On the other hand, African countries would have to surpass quality standards from importing countries, which could again result in higher costs.

Phylogeny

As of 2025, the only sequenced species is ''Limnospira platensis''. As a result the GTDB does not produce an informative tree. Existing sources generally use a single locus, usually 16S rRNA, ITS, or the phycocyanin operon ''cpcBA''.

16S rRNA tree, trimmed to show important genera (1800 bp fragment from Nowicka-Krawczyk et al. 2019):

References

External links

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{{Taxonbar, from=Q987596, from2=Q98815276

Cyanobacteria genera
Oscillatoriales