Geosmin

Geosmin is an irregular sesquiterpene, produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step Mg2+-dependent reaction.[1] Geosmin, along with the irregular monoterpene 2-Methylisoborneol, together account for the majority of biologically-caused taste and odor outbreaks worldwide.[2] Geosmin has a distinct earthy or musty odor, which most people can easily smell. The odor detection threshold of geosmin is very low, ranging from 0.006 to 0.01 micrograms per liter in water.[3] Geosmin is also responsible for the earthy taste of beetroots and a contributor to the strong scent (petrichor) that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed.[4]

Geosmin
Names
IUPAC name
(4S,4aS,8aR)-4,8a-dimethyloctahydronaphthalen-4a(2H)-ol
Other names
(4S,4aS,8aR)-4,8a-Dimethyl-1,2,3,4,5,6,7,8-octahydronaphthalen-4a-ol; 4,8a-Dimethyl-decahydronaphthalen-4a-ol; Octahydro-4,8a-dimethyl-4a(2H)-naphthalenol
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.039.294
UNII
Properties
C12H22O
Molar mass 182.307 g·mol−1
Boiling point 270 to 271 °C (518 to 520 °F; 543 to 544 K)
Hazards
Flash point 104 °C (219 °F; 377 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

In chemical terms, it is a bicyclic alcohol with formula C12H22O, a derivative of decalin. Its name is derived from the Ancient Greek γεω- geō- "earth" and ὀσμή osmḗ "smell". The word was coined in 1965 by the American biochemist Nancy N. Gerber (1929–1985) and the French-American biologist Hubert A. Lechevalier (1926–2015).[5][6]

Production

Geosmin is produced by various blue-green algae (cyanobacteria) and filamentous bacteria in the class Actinomyces, and also some other prokaryotes and eukaryotes. The main genera in the cyanobacteria that have been shown to produce geosmin include Anabaena, Phormidium, and Planktothrix, while the main genus in the Actinomyces that produces geosmin is Streptomyces.[7][8][9][10] Communities whose water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. Under acidic conditions, geosmin decomposes into odorless substances.[5]

In 2006, the biosynthesis of geosmin by a bifunctional Streptomyces coelicolor enzyme was unveiled.[11][12] A single enzyme, geosmin synthase, converts farnesyl diphosphate to geosmin in a two-step reaction.

Not all blue-green algae cyanobacteria produce geosmin. Identification of species that might produce geosmin is traditionally done through microscopic identification of algae as geosmin producers, a technique that is labor-intensive and requires specialized knowledge. Recent advances in molecular biology have enabled identification of a geosmin synthase gene, geoA, which is present in cyanobacterial species that produce geosmin, but is not present in other cyanobacterial species [13]. Amplification of this gene from water samples using real-time PCR may permit predictions of taste and odor events caused by cyanobacteria in fresh water.

Streptomyces coelicolor is the model representative of a group of soil-dwelling bacteria with a complex lifecycle involving mycelial growth and spore formation. Besides the production of volatile geosmin, it also produces many other complex molecules of pharmacological interest; its genome sequence is available at the Sanger Institute.[14]

Effects

The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.[15]

Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp and catfish.[16][17] Geosmin combines with 2-methylisoborneol, which concentrates in the fatty skin and dark muscle tissues. It breaks down in acid conditions; hence, vinegar and other acidic ingredients are used in fish recipes to reduce the muddy flavor.[18] Taste and odor compounds including geosmin lead to an unpleasant taste of drinking water which is perceived by consumers as an indication of poor water quality.[19]

This compound is also an issue for saltwater fish farmed in recirculating aquaculture systems, such as Atlantic salmon. These systems rely on biological filtration using cultured microbial communities to process the nitrogenous waste from the fish (ammonia) into less harmful compounds (nitrite and nitrate) that can be tolerated at higher concentrations. However geosmin-producing bacteria can also grow in these systems, and often require fish to be transferred to an additional "finishing" or "purge" system where they are not fed for several days prior to harvest to remove off-flavor compounds and empty the intestinal tract. This process is also known as depuration.

See also

References
  1. , "Watson, W. and Juttner, F. 2019. IWA Publishing 2019. Taste and Odour in Source and Drinking Water: Causes, Controls, and Consequences".
  2. Biochemical and Ecological Control of Geosmin and 2-Methylisoborneol in Source Waters, "Applied and Environmental Microbiology", Volume 73, July 2007.
  3. Biochemical and Ecological Control of Geosmin and 2-Methylisoborneol in Source Waters, "Applied and Environmental Microbiology", Volume 73, July 2007.
  4. The earth's perfume, Protein Spotlight, Issue 35, June 2003.
  5. Gerber, NN; Lechevalier, HA (November 1965). "Geosmin, an earthly-smelling substance isolated from actinomycetes". Applied Microbiology. 13 (6): 935–8. PMC 1058374. PMID 5866039.
  6. Merriam-Webster dictionary: geosmin
  7. Biochemical and Ecological Control of Geosmin and 2-Methylisoborneol in Source Waters, "Applied and Environmental Microbiology", Volume 73, July 2007.
  8. Izaguirre G, Taylor WD. (2004). "A guide to geosmin- and MIB-producing cyanobacteria in the United States". Water Sci. Technol. 49 (9): 19–24. doi:10.2166/wst.2004.0524. PMID 15237602.CS1 maint: uses authors parameter (link)
  9. Zaitlin B, Watson SB. (2006). "Actinomycetes in relation to taste and odour in drinking water: Myths, tenets and truths". Water Res. 40 (9): 1741–53. doi:10.1016/j.watres.2006.02.024. PMID 16600325.CS1 maint: uses authors parameter (link)
  10. Suurnäkki S, Gomez-Saez GV, Rantala-Ylinen A, Jokela J, Fewer DP, Sivonen K. (2015). "Identification of geosmin and 2-methylisoborneol in cyanobacteria and molecular detection methods for the producers of these compounds". Water Res. 68 (1): 56–66. doi:10.1016/j.watres.2014.09.037. PMID 25462716.CS1 maint: uses authors parameter (link)
  11. Jiang, J.; He, X.; Cane, D.E. (2006). "Geosmin biosynthesis. Streptomyces coelicolor germacradienol/germacrene D Synthase converts farnesyl diphosphate to geosmin". J. Am. Chem. Soc. 128 (25): 8128–8129. doi:10.1021/ja062669x. PMID 16787064.
  12. Jiang, J.; He, X.; Cane, D.E. (2007). "Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme". Nat. Chem. Biol. 3 (11): 711–5. doi:10.1038/nchembio.2007.29. PMC 3013058. PMID 17873868.
  13. Kutovaya, Olga; Watson, Susan (2014). "Development and application of a molecular assay to detect and monitor geosmin-producing cyanobacteria and actinomycetes in the Great Lakes". Journal of Great Lakes Research. 40 (2): 404–414. doi:10.1016/j.jglr.2014.03.016.
  14. "The genome of Streptomyces coelicolor A3(2) producing geosmin". Sanger Institute. Retrieved 2009-01-31.
  15. Polak, E.H.; Provasi, J. (1992). "Odor sensitivity to geosmin enantiomers". Chemical Senses. 17: 23–26. doi:10.1093/chemse/17.1.23.
  16. Vallod, D.; Cravedi, J. P.; Hillenweck, A.; Robin, J. (2007-04-03). "Analysis of the off-flavor risk in carp production in ponds in Dombes and Forez (France)". Aquaculture International. 15 (3–4): 287–298. doi:10.1007/s10499-007-9080-7. ISSN 0967-6120.
  17. Lovell, Richard T.; Lelana, Iwan Y.; Boyd, Claude E.; Armstrong, Martin S. (1986-05-01). "Geosmin and Musty-Muddy Flavors in Pond-Raised Channel Catfish". Transactions of the American Fisheries Society. 115 (3): 485–489. doi:10.1577/1548-8659(1986)115<485:gamfip>2.0.co;2. ISSN 0002-8487.
  18. Winkler, Lawrence (2012-08-06). Westwood Lake Chronicles. Lawrence Winkler. ISBN 9780991694105.
  19. Bristow, R.L.; Young, I.S.; Pemberton, A.; Williams, J.; Maher, S. (2019). "An extensive review of the extraction techniques and detection methods for the taste and odour compound geosmin (trans-1, 10-dimethyl-trans-9-decalol) in water". Trends in Analytical Chemistry. 110: 233–248. doi:10.1016/j.trac.2018.10.032.

Further reading
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