Scytonemin Explained

Scytonemin is a secondary metabolite and an extracellular matrix (sheath) pigment synthesized by many strains of cyanobacteria, including Nostoc, Scytonema, Calothrix, Lyngbya, Rivularia, Chlorogloeopsis, and Hyella.[1] Scytonemin-synthesizing cyanobacteria often inhabit highly insolated terrestrial, freshwater and coastal environments such as deserts, semideserts, rocks, cliffs, marine intertidal flats, and hot springs.[2]

The pigment was originally discovered in 1849 by Swiss botanist Carl Nägeli,[3] although the structure remained unsolved until 1993.[4] It is an aromatic indole alkaloid built from two identical condensation products of tryptophanyl- and tyrosyl-derived subunits linked through a carbon-carbon bond. Depending on the redox conditions it can exist in two inter-convertible forms: a more common oxidized yellow-brown form which is insoluble in water and only slightly soluble in organic solvents, such as pyridine, and a reduced form with bright red color that is more soluble in organic solvents.[5] Scytonemin absorbs very strongly and very broadly across the UV-C-UV-B-UV-A-violet-blue spectral region, with an in vivo maximum absorption at 370 nm and an in vitro maximum absorption at 386 and 252 nm, and with smaller peaks at 212, 278 and 300 nm.[6]

It is believed that scytonemin acts as a highly efficient protective biomolecule (sunscreen) that filters out damaging high frequency UV rays while at the same time allowing the transmittance of wavelengths necessary for photosynthesis.[7] Its biosynthesis in cyanobacteria is mostly triggered by exposure to UV-A and UV-B wavelengths.[8] [9]

Recently, Couradeau and coworkers found that cyanobacterial soil crusts warm the soil surface by as much as 10 °C through the production and accumulation of scytonemin pigments.[10] This effect is due to the dissipation of the absorbed photons by the scytonemin molecules into heat.

Biosynthesis

The biosynthesis in Lyngbya aestuarii was recently explored by Balskus, Case, and Walsh. It proceeds by the conversion of L-tryptophan to 3-indole pyruvic acid, followed by coupling to p-hydroxyphenylpyruvic acid. Cyclization of the resultant β-ketoacid yields a tricyclic ketone. Oxidation and dimerization yields the completed natural product. Three scytonemin biosynthetic enzymes are necessary, denoted as ScyA-C.[11]

References

  1. Sinha, Hader. 2008-03-01. UV-protectants in cyanobacteria. Plant Science. 174. 3. 278–289. 10.1016/j.plantsci.2007.12.004. 0168-9452.
  2. Book: Ecology of Cyanobacteria II - Their Diversity in Space and Brian A. Whitton Springer. en. 978-94-007-3854-6. Springer. 2012.
  3. Book: Nägeli, Carl. Gattungen einzelliger Algen physiologisch und systematisch bearbeitet. 1849. Zürich, Friedrich Schulthess. MBLWHOI Library.
  4. 825–9 . 10.1007/BF01923559 . The structure of scytonemin, an ultraviolet sunscreen pigment from the sheaths of cyanobacteria . 1993 . Proteau . P. J. . Gerwick . W. H. . Garcia-Pichel . F. . Castenholz . R. . Experientia . 49 . 9 . 8405307. 22975257 .
  5. Garcia-Pichel. Ferran. Castenholz. Richard W.. 1991-06-01. Characterization and Biological Implications of Scytonemin, a Cyanobacterial Sheath Pigment1. Journal of Phycology. en. 27. 3. 395–409. 10.1111/j.0022-3646.1991.00395.x. 1991JPcgy..27..395G . 84058783. 1529-8817.
  6. Sinha. Rajeshwar. Klisch. M. Vaishampayan. Akhouri. Häder. Donat. 1999-11-01. Biochemical and spectroscopic characterization of the cyanobacterium Lyngbya sp. inhabiting mango (Mangifera indica) trees: Presence of an ultraviolet-absorbing pigment, scytonemin. Acta Protozoologica. 38. 291–298.
  7. Ekebergh. Andreas. Sandin. Peter. Mårtensson. Jerker. 2015-11-25. On the photostability of scytonemin, analogues thereof and their monomeric counterparts. Photochemical & Photobiological Sciences. en. 14. 12. 10.1039/C5PP00215J. 26452010. 1474-9092. 2179–2186. 23558706 .
  8. Sorrels. Carla M.. Proteau. Philip J.. Gerwick. William H.. 2009-07-15. Organization, Evolution, and Expression Analysis of the Biosynthetic Gene Cluster for Scytonemin, a Cyanobacterial UV-Absorbing Pigment. Applied and Environmental Microbiology. en. 75. 14. 4861–4869. 10.1128/AEM.02508-08. 0099-2240. 19482954. 2708446. 2009ApEnM..75.4861S .
  9. Rastogi. Rajesh P.. Incharoensakdi. Aran. 2014-01-01. Characterization of UV-screening compounds, mycosporine-like amino acids, and scytonemin in the cyanobacteriumLyngbyasp. CU2555. FEMS Microbiology Ecology. en. 87. 1. 244–256. 10.1111/1574-6941.12220. 24111939. 0168-6496. free. 2014FEMME..87..244R .
  10. Couradeau. Estelle. Karaoz. Ulas. Lim. Hsiao Chien. Rocha. Ulisses Nunes da. Northen. Trent. Brodie. Eoin. Garcia-Pichel. Ferran. 2016-01-20. Bacteria increase arid-land soil surface temperature through the production of sunscreens. Nature Communications. En. 7. 10373. 10.1038/ncomms10373. 26785770. 4735820. 2016NatCo...710373C.
  11. Balskus. Emily P.. Case. Rebecca J.. Walsh. Christopher T.. 2011. The biosynthesis of cyanobacterial sunscreen scytonemin in intertidal microbial mat communities. FEMS Microbiology Ecology. 77. 2. 1–11. 10.1111/j.1574-6941.2011.01113.x. 21501195. 3134115. 2011FEMME..77..322B .

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