Geosmin ( ) is an irregular sesquiterpenoid with a distinct earthy or musty odor, which most people can easily smell. The geosmin odor detection threshold in humans is very low, ranging from 0.006 to 0.01 micrograms per liter in water. Geosmin, along with the irregular monoterpene

2-methylisoborneol 2-Methylisoborneol (MIB) is an irregular monoterpene derived from the universal monoterpene precursor geranyl pyrophosphate. MIB and the irregular sesquiterpene geosmin together account for the majority of biologically-caused taste and odor outbr ...

, together account for the majority of biologically-caused taste and odor outbreaks in drinking water worldwide and in farmed fish. Geosmin is also responsible for the earthy taste of beetroots and a contributor to the strong scent, known as petrichor, that occurs when rain falls after a spell of dry weather or when soil is disturbed. In chemical terms, geosmin is a

bicyclic A bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all of the ring ...

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

with

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

, a derivative of decalin. It is produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step -dependent reaction. Its name is derived from the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

words (), meaning "earth", and (), meaning "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).

Production

Geosmin is produced as a secondary metabolite by various blue-green algae (

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

), 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''. In beetroots, geosmin is produced endogenously, not by environmental nor endophytic microbes, suggesting beets possess separate or acquired means of geosmin biosynthesis. Communities whose

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

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

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

ic conditions, geosmin decomposes into odorless substances. In 2006, geosmin was biosynthesized by a bifunctional '' Streptomyces coelicolor''

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

. 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. 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. 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.

Effects

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 to 5 parts per trillion. 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. 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 An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

and

rivers A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

. 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. Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as

carp The term carp (: carp) is a generic common name for numerous species of freshwater fish from the family (biology), family Cyprinidae, a very large clade of ray-finned fish mostly native to Eurasia. While carp are prized game fish, quarries and a ...

catfish Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...

, and tilapia. Geosmin combines with

, 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. 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. Despite its negative effects on the taste and odor of fish and drinking water, geosmin is nontoxic to humans. This compound is reported to be an issue for saltwater fish farmed in recirculating aquaculture systems, such as

Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Hucho taimen, Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlan ...

, 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 Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

) into less harmful compounds ( nitrite and

nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...

) 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.

Geosmin remediation in drinking water

Geosmin cannot be removed from water using standard treatment processes, requiring a combination of integrated solutions which may not be available to all municipal water suppliers. Possible advanced oxidation methods include photocatalysis,

ozonation Ozone () (or trioxygen) is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the low ...

, and ultrasonication. Activated carbon filters may be able to help somewhat, but few studies have been completed to address this issue.

Production

Effects

Geosmin remediation in drinking water

See also

References

Further reading