Geosmin

Geosmin

Geosmin is a bicyclic compound with brute or molecular formula: C12H22O.
The name of this compound, whose official IUPAC name is: (4S,4aS,8aR)-4,8a-Dimethyloctahydronaphthalen-4a(2H)-ol, comes from from ancient Greek γῆ (ghê), i.e. earth and ὀσμή (osmè), i.e. smell; therefore earthy smell. The word was introduced into the literature in 1965 by the American biochemist Nancy N. Gerber (1929–1985) and the French biologist Hubert A. Lechevalier (1926–2015).
In fact, geosmin has the particularity of imparting, even at very low concentrations, a strong hint of earth, fungus and mould.
Geosmin, in addition to being an organic compound with a distinct earthy flavor and aroma, is responsible for the earthy taste of beets; contributes to the strong odor in the air when rain falls after a period of dry weather (petrichor) or the soil is disturbed.
Furthermore, geosmin is also responsible for the muddy smell in freshwater bottom fish, such as carp and catfish. Cyanobacteria produce geosmin and 2-methylisoborneol, which concentrate in the skin and dark muscle tissue. Because geosmin breaks down under acidic conditions, producing odorless substances, using vinegar, lemon and other acidic ingredients in fish recipes helps reduce the muddy taste.
The human nose is extremely sensitive to geosmin and can detect it at concentrations as low as 5 parts per trillion.
In nature, geosmin is produced by several classes of microbes, including cyanobacteria (blue-green algae) and Actinobacteria (especially Streptomyces), and is released when the bacteria die.
It was recently demonstrated by Jiang et al. the possibility of biosynthesizing geosmin from a bifunctional enzyme of Streptomyces coelicolor. A single enzyme, germacradienol/germacrene D synthase convertsfarnesyl diphosphate to geosmin in a two-step reaction.

Biosynthesis –
Geosmin is a derivative of norbornane and is synthesized, as mentioned, mainly by some soil bacteria and fungi. The synthesis of geosmin occurs through a series of biochemical steps involving specific enzymes; this starts with mevalonic acid, which is a common precursor in the biosynthesis of many terpenes and steroids.
Mevalonic acid is converted to isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). These compounds condense to form geranylgeranyl pyrophosphate (GGPP), a key intermediate in the biosynthesis of diterpenes.
A specific enzyme, geosmin synthase, catalyzes the conversion of GGPP to geosmin. Geosmin synthase is a multifunctional protein that plays a crucial role in the cyclization and oxidation reactions that lead to the formation of geosmin.
During the synthesis process, GGPP undergoes a series of structural modifications that include cyclizations and molecular rearrangements, which ultimately lead to the formation of the characteristic norbornane skeleton of geosmin.
The synthesis of geosmin is an interesting example of terpene biosynthesis, showing how simple organisms such as bacteria and fungi can produce complex organic compounds with specific and distinctive properties.
Among these simpler organisms we include various blue-green algae (cyanobacteria) and filamentous bacteria of the class Actinomyces, as well as some other prokaryotes and eukaryotes. The major genera of cyanobacteria that have been shown to produce geosmin include Anabaena, Phormidium, and Planktothrix, while the major genus of Actinomyces that produces geosmin is Streptomyces.
Under acidic conditions, geosmin decomposes into odorless substances.
Furthermore, in 2006, geosmin was biosynthesized by a bifunctional enzyme Streptomyces coelicolor. A single enzyme, geosmin synthase, converts farinasyl diphosphate to geosmin in a two-step reaction.

Possible uses –
Although it is often associated with an unpleasant odor in drinking water, geosmin has several important uses and meanings; among these we report:
– Environmental Indicators: the presence of geosmin can be used as an indicator of soil and water health. An increase in geosmin in a body of water may indicate the proliferation of cyanobacteria, which often occurs under eutrophic conditions.
– Flavors and Fragrances: in small quantities, geosmin can be used in the creation of perfumes and flavors to impart an earthy and musky note. It is sometimes used to recreate natural odors in perfumery.
– Food sector: even if not directly used as an ingredient, geosmin can influence the taste and aroma of some foods and drinks. For example, its odor can be detected in some types of wine and vegetables such as beets.
– Study of Microbial Interactions: geosmin is of interest to microbiologists who study the interactions between microorganisms and their environment. Understanding how and why microorganisms produce geosmin can provide useful information on soil biology and microbial ecology.
– Agriculture and Gardening: The presence of geosmin in soil can indicate rich microbial activity, which is often associated with healthy and fertile soil. Some gardeners and farmers can use this information to evaluate soil quality.
– Olfactory Signaling in Animals: Some animals are attracted to the smell of geosmin, which can help them find water sources. For example, camels in the desert can smell geosmin and use it to locate watering holes.
– Research and Development of Water Filters: Geosmin is a compound that drinking water engineers try to remove or reduce, as it can negatively affect the taste of water. Research into how to remove geosmin is important for improving water filtration and treatment systems.

Warning: The information provided is not medical advice and may not be accurate. The contents are for illustrative purposes only and do not replace medical advice.