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Name derivation:
M.F. Dunal (1789 - 1856) became interested in the reddish coloration of the salt crystals in Mediterranean salt fields. In 1837 he examined samples of red brine with a microscope and saw many red-colored biflagellate cells that he thought were Haematococcus. Joly (1873) collected samples in the same area and observed flagella and rapid motion in presumed protococci, and named them Monas Dunalii in honor of his Professor M.F. Dunal. Teodoresco (1905) examined similar cells from salt ponds in Rumania and found they lacked cell walls, so he gave them a new generic name, Dunaliella, in honor of Dunal.
Classification:
Dunaliella Teodoresco 1905; 23 of 28 species descriptions are currently accepted taxonomically (Guiry and Guiry 2013). Order Chlamydomonadales; Family Dunaliellaceae
Morphology:
Biflagellate Unicells varying in color from green, yellow, orange, and red depending on the salinity of the environment. The saltier it gets the more β-carotene is produced that leads to a redder color (Brock, 1975).
Reproduction:
Details of conjugation of biflagellate cells followed by stages of merger to form an immobile zygote with the use of mating pairs of cells low in carotenoids (green cells) with those high in carotenoids (red cells) (Lerch 1937).
Similar genera:
Haematococcus also produces excessive amounts of β-carotene and colors its surrounding water bright red, as is often seen in concrete bird baths.
Osmoregulation:
Dunaliella is euryhaline, a halophile that grows best at >0.6 M to 2.5 M NaCl but can survive at much lower levels. The optimal range is hypersaline relative to seawater (~0.5 M NaCl). As it is subjected to variable salinity, and because it lacks a pressure-resistant cell wall, it must adjust its cytoplasmic solute concentration to minimize shrinkage in high salinity or explosion in low salinity. The ‘compatible solution’ it produces and reduces is glycerol (Ben-Amotz and Avron 1972), unique among almost all life forms. The small 3C molecule normally leaks across the cell membrane in most organisms, so Dunaliella apparently has evolved either a unique membrane or some other mechanism to prevent leakage in hypersaline water. Interestingly leakage increases rapidly when salinity is decreased from 0.6 to 0.0 M NaCl. When subjected to a rapid decrease in salinity from 1.5 M to 0.6 M, intracellular glycerol content decreased, and when salinity was increased up to 1.3 M, glycerol increased. The time to rerach equilibrium was 90 minutes in each direction, and was independent of light level (Ibid.). Leakage has also been monitored at 2.0 M NaCl (Chow et al. 2013). The biochemical mechanism used by Dunaliella to minimize intracellular loss of glycerine by leakage across its cytoplasmic membrane is yet to be determined.
Industrial potential:
Dunaliella is important economically because of its large production of β-carotene as well as a source of biofuels (Oren 2005). The β-carotene concentration increases under stress conditions such as high salinity, high light intensity, nutrient deprivation and extreme temperatures, as reviewed by Lamers et al. (2012). Apparently it may also be a source for industrial use of glycerol (Chow et al 2013).
Habitat:
Joly (1873) described changing color of cells during aging from green to brick-red and then blood-red in Mediterranean "salt fields". He also noted an apparent relationship of color to salinity, including the reddest water color after a record dry summer in 1839. Dunaliella is usually the only photoautotroph in Great Salt Lake (Utah, USA) and the Dead Sea (Israel, West Bank, and Jordan) where salinity is >8x oceanic water. Dunaliella can be found where salinity varies from near freshwater to very salty brine, as well as soils. In the saltier water Dunaliella produces β-carotene and glycerol to enable water absorption even in extremely saline environments (Brock, 1975). Many hypersaline lakes where Dunaliella is the dominant plankton turn milky pink to bright red. Some industries grow Dunaliella, optimally at 30 degrees Celsius, in extreme salinities to stimulate the production of β-carotene because of its uses as feed for live aquaria (Garcia et. al. 2007).
References:
Ben-Amotz, A., and M. Avron 1972. The role of glycerol in the osmotic regulation of the halophilic alga Dunaliella parva. Plant Physiology 51:875-878. Brock, T.D. 1975. Salinity and the Ecology of Dunaliella from the Great Salt Lake. Chow, Y.Y.S., S.J.M. Goh, Z. Su, D.H.P Ng, C.Y.Lim, N.Y.N. Lim, H.Lin,L. Fang and Y.K. Lee 2013. Continual production of glycerol from carbon dioxide by Dunaliella tertiolecta. Bioresource Technology 136:550-555. Garcia, F., Freile-Pelegrin, Y. and Robledo, D. 2007 Physiological characteristics of Dunaliella sp. From Yucatan, Mexico. Bioresource Technology 98:1359-1365. online Guiry, M.D. and G.M. Guiry 2013. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 10 September 2013. Joly, N. 1873. Water turned to blood (transl.). Popular Science Monthly 4:202-208. Lamers, P.P., M. Janssen, R.C.H. De Vos, R.J. Bino and R.H. Wijffels 2012. Carotenoid and fatty acid metabolism in nitrogen-starved Dunaliella salina, a unicellular green microalga. Journal of Biotechnology 162:21-27. Lerche, W. 1937. Untersuchungen uber Entwicklung und Fortpflanzung in der Gattung Dunaliella. Archiv für Protistenkunde 88:236-268. Oren, A. 2005. A hundred years of Dunaliella research: 1905-2005. Saline Systems 1:2. Teodoresco, E.C. (1905). Organisation et développement du Dunaliella, nouveau genre de Volvocacée - Polyblepharidée. Beihefte zum Botanischen Centralblatt 18(Abt. 1): 215‑232.
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