Geosmin

Geosmin is produced as a secondary metabolite by various blue-green algae (cyanobacteria), filamentous bacteria in the class Actinomyces, other prokaryotes, and various 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.{{cite journal | last1 = Izaguirre | first1= G. | last2 = Taylor | first2 = W. D. | year = 2004 | title = A guide to geosmin- and MIB-producing cyanobacteria in the United States | journal = Water Science and Technology | volume = 49 | issue = 9 | pages = 19–24 | pmid = 15237602| doi = 10.2166/wst.2004.0524 }}{{cite journal | last1 = Zaitlin | first1 = B. | last2 = Watson | first2 = S. B. | year = 2006 | title = Actinomycetes in relation to taste and odour in drinking water: Myths, tenets and truths | journal = Water Research | volume = 40 | issue = 9 | pages = 1741–1753 | pmid = 16600325 | doi = 10.1016/j.watres.2006.02.024| bibcode = 2006WatRe..40.1741Z }}{{cite journal | last1 = Suurnäkki | first1 = S. | last2 = Gómez Sáez | first2 = G. V. | last3 = Rantala-Ylinen | first3 = A. | last4 = Jokela | first4 = J. | last5 = Fewer | first5 = D. P. | last6 = Sivonen | first6 = K. | year = 2015 | title = Identification of geosmin and 2-methylisoborneol in cyanobacteria and molecular detection methods for the producers of these compounds | journal = Water Research | volume = 68 | issue = 1 | pages = 56–66 | pmid = 25462716 | doi = 10.1016/j.watres.2014.09.037| bibcode = 2015WatRe..68...56S }} In beetroots, geosmin is produced endogenously, not by environmental nor endophytic microbes, suggesting beets possess separate or acquired means of geosmin biosynthesis.{{Cite journal |last1=Hanson |first1=Solveig J |last2=Dawson |first2=Julie C |last3=Goldman |first3=Irwin L |date=2021-09-29 |title=Beta vulgaris ssp. vulgaris chromosome 8 shows significant association with geosmin concentration in table beet |url=https://academic.oup.com/g3journal/article/11/12/jkab344/6377785 |journal=G3: Genes, Genomes, Genetics |volume=11 |issue=12 |doi=10.1093/g3journal/jkab344 |issn=2160-1836 |pmc=8664477 |pmid=34586384}} 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.{{cite journal | pmid = 5866039 | volume=13 | issue=6 |date=November 1965 | pages=935–938 | last1 = Gerber | first1 = N. N. | last2 = Lechevalier | first2 = H. A. | title = Geosmin, an earthly-smelling substance isolated from actinomycetes | pmc = 1058374 | journal = Applied Microbiology| doi=10.1128/am.13.6.935-938.1965 }}

In 2006, geosmin was biosynthesized by a bifunctional Streptomyces coelicolor enzyme.{{Cite journal | doi = 10.1021/ja062669x | volume = 128 | issue = 25 | pages = 8128–8129 | last1 = Jiang | first1 = J. | first2 = X. | last2 = He | first3 = D. E. | last3 = Cane | title = Geosmin biosynthesis. Streptomyces coelicolor germacradienol/germacrene D synthase converts farnesyl diphosphate to geosmin | journal = Journal of the American Chemical Society | year = 2006 | pmid = 16787064}}{{Cite journal | doi = 10.1038/nchembio.2007.29 | volume = 3| last1 = Jiang | first1 = J. | first2 = X. | last2 = He | first3 = D. E. | last3 = Cane | pmc = 3013058 | title = Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme | journal = Nature Chemical Biology | year = 2007 | pages = 711–715 | pmid = 17873868 | issue = 11}} A single enzyme, geosmin synthase, converts farnesyl diphosphate to geosmin in a two-step reaction. This may have an evolutionary value in the dispersal of the producers. Geosmin and 2-methylisoborneol have been shown to attract springtails, Folsomia candida, which are believed to feed on and spread the spores of Streptomyces both through their faces and by attachment to their cuticle.{{Cite journal |last=Becher |first=Paul G. |last2=Verschut |first2=Vasiliki |last3=Bibb |first3=Maureen J. |last4=Bush |first4=Matthew J. |last5=Molnár |first5=Béla P. |last6=Barane |first6=Elisabeth |last7=Al-Bassam |first7=Mahmoud M. |last8=Chandra |first8=Govind |last9=Song |first9=Lijiang |last10=Challis |first10=Gregory L. |last11=Buttner |first11=Mark J. |last12=Flärdh |first12=Klas |date=2020-04-06 |title=Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal |url=https://www.nature.com/articles/s41564-020-0697-x |journal=Nature Microbiology |language=en |volume=5 |issue=6 |pages=821–829 |doi=10.1038/s41564-020-0697-x |issn=2058-5276}}

Not all blue-green algae 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.{{Cite journal | doi = 10.1016/j.jglr.2014.03.016 | volume = 40 | issue = 2 | last1 = Kutovaya | first1 = O. | last2 = Watson | first2 = S. | title = Development and application of a molecular assay to detect and monitor geosmin-producing cyanobacteria and actinomycetes in the Great Lakes | journal = Journal of Great Lakes Research | year = 2014 | pages = 404–414| bibcode = 2014JGLR...40..404K }} 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.

The human olfactory system is extremely sensitive to geosmin and is able to detect it at concentrations as low as anywhere from 0.4 parts per billion{{cite journal | last1 = Polak | first1 = E.H. | last2 = Provasi | first2 = J. | title = Odor sensitivity to geosmin enantiomers | journal = Chemical Senses | volume = 17 | pages = 23–26 | year = 1992 | doi = 10.1093/chemse/17.1.23 }} to 5 parts per trillion.{{Cite web |title=Geosmin |url=https://www.acs.org/molecule-of-the-week/archive/g/geosmin.html#:~:text=Geosmin%20is%20a%20natural%20bicyclic,as%20low%20as%205%20ppt |access-date=2024-10-15 |website=American Chemical Society |language=en}} It is proposed that this extreme sensitivity to geosmin evolved in humans or their ancestors to aid them in their search for scarce water, but this is not conclusive. Similarly, many insects possess a sensitivity to geosmin such that it may act as an attractant, signaling the presence of microbial prey, or as a repellant, where its presence may coincide with that of toxin-producing microbes.{{Cite journal |last1=Garbeva |first1=Paolina |last2=Avalos |first2=Mariana |last3=Ulanova |first3=Dana |last4=van Wezel |first4=Gilles P. |last5=Dickschat |first5=Jeroen S. |date=Sep 2023 |title=Volatile sensation: The chemical ecology of the earthy odorant geosmin |url=https://sfamjournals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.16381 |journal=Environmental Microbiology |language=en |volume=25 |issue=9 |pages=1565–1574 |doi=10.1111/1462-2920.16381 |pmid=36999338 |issn=1462-2912|hdl=1887/3719502 |hdl-access=free }} Geosmin is also implicated in the migratory patterns of eels, where its higher concentration in freshwater systems guides the fish from the ocean into estuaries and rivers.{{Cite journal |last1=Tosi |first1=Laura |last2=Sola |first2=Carla |date=1993-01-12 |title=Role of Geosmin, a Typical Inland Water Odour, in Guiding Glass Eel Anguilla anguilla (L.) Migration |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1439-0310.1993.tb00468.x |journal=Ethology |language=en |volume=95 |issue=3 |pages=177–185 |doi=10.1111/j.1439-0310.1993.tb00468.x |issn=0179-1613|url-access=subscription }} Geosmin is also likely involved in raccoon predation of turtle nests, where burrowing disturbs the soil and ejects the chemical and other volatile indicators into the air.{{Cite journal |last=Edmunds |first=Sarah E. |last2=Kasparov |first2=Christine N. |last3=Yoon |first3=Jae Byeok |last4=Kanonik |first4=Alexandra K. |last5=Burke |first5=Russell L. |date=2018 |title=Twelve Years Later: Reassessing Visual and Olfactory Cues Raccoons Use to Find Diamondback Terrapin Nests |url=http://www.bioone.org/doi/10.1670/17-029 |journal=Journal of Herpetology |language=en |volume=52 |issue=3 |pages=307–312 |doi=10.1670/17-029 |issn=0022-1511}}

Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp, catfish, and tilapia.{{Cite journal|last1=Vallod|first1=D.|last2=Cravedi|first2=J. P.|last3=Hillenweck|first3=A.|last4=Robin|first4=J.|date=April 2007|title=Analysis of the off-flavor risk in carp production in ponds in Dombes and Forez (France)|journal=Aquaculture International|language=en|volume=15|issue=3–4|pages=287–298|doi=10.1007/s10499-007-9080-7|bibcode=2007AqInt..15..287V |s2cid=6669343 |issn=0967-6120}}{{Cite journal|last1=Lovell|first1=R. T.|last2=Lelana|first2=I. Y.|last3=Boyd|first3=C. E.|last4=Armstrong|first4=M. S.|date=May 1986|title=Geosmin and Musty-Muddy Flavors in Pond-Raised Channel Catfish|journal=Transactions of the American Fisheries Society|volume=115|issue=3|pages=485–489|doi=10.1577/1548-8659(1986)115<485:gamfip>2.0.co;2|bibcode=1986TrAFS.115..485L |issn=0002-8487}}{{cite journal | last1=Yamprayoon | first1=Jirawan | last2=Noomhorm | first2=Athapol | title=Geosmin and Off-Flavor in Nile Tilapia (Oreochromis niloticus) | journal=Journal of Aquatic Food Product Technology | volume=9 | issue=2 | date=2000 | issn=1049-8850 | doi=10.1300/J030v09n02_04 | pages=29–41| bibcode=2000JAFPT...9b..29Y | s2cid=85598428 }} Geosmin combines with 2-methylisoborneol, which concentrates in the fatty skin and dark muscle tissues. It breaks down in acidic conditions; hence, vinegar and other acidic ingredients are used in fish recipes to reduce the muddy flavor.{{Cite book|url=https://books.google.com/books?id=3tKAfqxkFAIC|title=Westwood Lake Chronicles|last=Winkler|first=L.|date=2012|publisher=Lawrence Winkler|isbn=9780991694105|language=en}} 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.{{cite journal | last1 = Bristow | first1 = R. L. | last2 = Young | first2 = I. S. | last3 = Pemberton | first3 = A. | last4 = Williams | first4 = J. | last5 = Maher | first5 = S. | title = 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 | journal = Trends in Analytical Chemistry | volume = 110 | pages = 233–248 | year = 2019 | doi = 10.1016/j.trac.2018.10.032 | doi-access = free }} Despite its negative effects on the taste and odor of fish and drinking water, geosmin is nontoxic to humans.{{Cite journal |last=Mochida |first=Kyo |date=2009 |title=Evaluation of the Cytotoxicity of Geosmin and 2-Methylisoborneol Using Cultured Human, Monkey, and Dog Cells |url=https://www.jstage.jst.go.jp/article/bio/14/1/14_1_35/_article |journal=Biocontrol Science |volume=14 |issue=1 |pages=35–38 |doi=10.4265/bio.14.35|pmid=19344097 |url-access=subscription }}

This compound is reported to be an issue for saltwater fish farmed in recirculating aquaculture systems, such as Atlantic salmon,{{Cite journal |last1=Burr |first1=Gary S. |last2=Wolters |first2=William R. |last3=Schrader |first3=Kevin K. |last4=Summerfelt |first4=Steven T. |date=2012-09-01 |title=Impact of depuration of earthy-musty off-flavors on fillet quality of Atlantic salmon, Salmo salar, cultured in a recirculating aquaculture system |journal=Aquacultural Engineering |language=en |volume=50 |pages=28–36 |doi=10.1016/j.aquaeng.2012.03.002 |issn=0144-8609|doi-access=free |bibcode=2012AqEng..50...28B }}{{citation needed|date=August 2020}} but there are also studies that show that the presence in seawater is significantly lower than that found in freshwater which is why many people consider freshwater fish to taste muddy compared to marine fish. 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.{{citation needed|date=August 2020}} This process is also known as depuration.

Geosmin cannot be removed from water using standard treatment processes,{{Cite journal |last1=Bruce |first1=Darlene |last2=Westerhoff |first2=Paul |last3=Brawley-Chesworth |first3=Alice |date=2002-06-01 |title=Removal of 2-methylisoborneol and geosmin in surface water treatment plants in Arizona |url=https://doi.org/10.2166/aqua.2002.0016 |journal=Journal of Water Supply: Research and Technology-Aqua |volume=51 |issue=4 |pages=183–198 |doi=10.2166/aqua.2002.0016 |issn=0003-7214|url-access=subscription }} requiring a combination of integrated solutions which may not be available to all municipal water suppliers. Possible advanced oxidation methods include photocatalysis, ozonation, and ultrasonication.{{Cite journal|doi=10.1007/s10661-021-08980-9|title=A critical review on geosmin and 2-methylisoborneol in water: sources, effects, detection, and removal techniques|year=2021|last1=Mustapha|first1=S.|last2=Tijani|first2=J.O.|last3=Ndamitso|first3=M.|last4=Abdulkareem|first4=A.S.|last5=Shuaib|first5=D.T.|last6=Mohammed|first6=A.K.|journal=Environmental Monitoring and Assessment|volume=193|issue=204|pages=1-34|pmid=33751262}} Activated carbon filters may be able to help somewhat, but few studies have been completed to address this issue.{{Cite journal |last=Kim |first=Keug Tae |date=27 February 2021 |title=Geosmin and 2-MIB Removal by Full-Scale Drinking water Treatment Processes in the Republic of Korea |url=https://www.mdpi.com/2073-4441/13/5/628/pdf |journal=Water |volume=13 |issue=5 |pages=628 |doi=10.3390/w13050628 |via=mdpi.com|doi-access=free }}

• Dimethyl sulfide – One of the molecules responsible for the odour of the sea
• {{Annotated link|Petrichor}}

{{reflist}}

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• {{ cite journal | volume = 207 | issue = 5005 | pages = 1415–1416 | last1 = Bear | first1 = I. J. |first2=R. G. |last2=Thomas | title = Petrichor and plant growth | journal = Nature | year = 1965 | doi = 10.1038/2071415a0 | bibcode = 1965Natur.207.1415B | s2cid = 4174301 }}

Category:Tertiary alcohols

Category:Flavors

Category:Soil

Category:Decalins

Category:Foul-smelling chemicals