Geosmin ( ) is an irregular sesquiterpenoid, produced from the universal sesquiterpene precursor

farnesyl pyrophosphate Farnesyl pyrophosphate (FPP), also known as farnesyl diphosphate (FDP), is an intermediate in the biosynthesis of terpenes and terpenoids such as sterols and carotenoids. It is also used in the synthesis of CoQ (part of the electron transport c ...

(also known as farnesyl diphosphate), in a two-step -dependent reaction. Geosmin, along with the irregular monoterpene 2-methylisoborneol, together account for the majority of biologically-caused taste and odor outbreaks in drinking water worldwide. Geosmin has 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 is also responsible for the earthy taste of

beetroot The beetroot is the taproot portion of a beet plant, usually known in North America as beets while the vegetable is referred to as beetroot in British English, and also known as the table beet, garden beet, red beet, dinner beet or golden bee ...

s and a contributor to the strong scent (

petrichor Petrichor () is the earthy scent produced when rain falls on dry soil. The word is constructed , the ethereal fluid that is the blood of the gods in Greek mythology. Origins Long before this phenomenon received its name in 1964, it had been n ...

) that occurs in the air when rain falls after a spell of dry weather or when soil is disturbed. In chemical terms, geosmin is a bicyclic

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

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. Its name is derived from the

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

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 by various blue-green algae (

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

) 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 ''Anabaena'' is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyan ...

'', '' Phormidium'', and '' Planktothrix'', while the main genus in the Actinomyces that produces geosmin is ''

Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positiv ...

''. Communities whose

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

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 In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...

ic conditions, geosmin decomposes into odorless substances. In 2006, the

biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecul ...

of geosmin by a bifunctional '' Streptomyces coelicolor''

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

was unveiled. A single enzyme,

geosmin synthase Geosmin synthase or germacradienol-geosmin synthase designates a class of bifunctional enzymes () that catalyze the conversion of farnesyl diphosphate (FPP) to geosmin, a volatile organic compound known for its earthy smell. The N-terminal half ...

, converts

farnesyl diphosphate Farnesyl pyrophosphate (FPP), also known as farnesyl diphosphate (FDP), is an intermediate in the biosynthesis of terpenes and terpenoids such as sterols and carotenoids. It is also used in the synthesis of CoQ (part of the electron transport c ...

to geosmin in a two-step reaction. 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 0.4 parts per billion. Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as

carp Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...

and

catfish Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...

. 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. 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. 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 Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are ...

, 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 compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

) into less harmful compounds (

nitrite The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also ...

and

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...

) 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 Depuration of seafood is the process by which marine or freshwater animals are placed into a clean water environment for a period of time to allow purging of biological contaminants (such as ''Escherichia coli'') and physical impurities (such as s ...

Geosmin remediation in drinking water

According to municipal water treatment facilities, geosmin cannot be removed from water using standard treatment processes. Treating for geosmin requires additional treatment, which may not be available to all municipal water suppliers. 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