Geosmin
Encyclopedia
Geosmin, which literally translates to "earth smell", is an organic compound
with a distinct earthy flavour and aroma, and is responsible for the earthy taste of beet
s and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor
) or when soil is disturbed.
(especially streptomyces
), and released when these microbes die. Communities whose water
supply depends 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
ic conditions, geosmin decomposes into odorless substances.
In 2006, the biosynthesis
of geosmin by a bifunctional Streptomyces
coelicolor enzyme
was unravelled by Jiang et al. A single enzyme, the germacradienol/germacrene
D Synthase converts farnesyl diphosphate to geosmin in a two-step reaction.
Streptomyces coelicolor is the model representative of a group of soil-dwelling organisms with a complex lifecycle involving mycelial growth
and spore formation. Beside the production of volatile geosmin, it also produces many other complex molecules of pharmacological interest and its genome
sequence is available at the Sanger Institute
.
s and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor
) or when soil is disturbed. The human nose
is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.
Geosmin is also responsible for the muddy smell in bottom-dwelling freshwater fish such as carp
and catfish
. Cyanobacteria produce geosmin and 2-methylisoborneol
, which concentrate in the skin and dark muscle tissue. Geosmin breaks down in acid conditions; hence vinegar and other acidic ingredients in fish recipes help reduce the muddy flavor.
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
with a distinct earthy flavour and aroma, and is responsible for the earthy taste of beet
Beet
The beet is a plant in the Chenopodiaceae family which is now included in Amaranthaceae family. It is best known in its numerous cultivated varieties, the most well known of which is the purple root vegetable known as the beetroot or garden beet...
s and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor
Petrichor
Petrichor is the scent of rain on dry earth. The word is constructed from Greek, petra, meaning stone + ichor, the fluid that flows in the veins of the gods in Greek mythology....
) or when soil is disturbed.
Production
Geosmin is produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteriaActinobacteria
Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....
(especially streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
), and released when these microbes die. Communities whose water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
supply depends 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
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ic conditions, geosmin decomposes into odorless substances.
In 2006, the biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of geosmin by a bifunctional Streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
coelicolor enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
was unravelled by Jiang et al. A single enzyme, the germacradienol/germacrene
Germacrene
Germacrenes are a class of volatile organic hydrocarbons, specifically, sesquiterpenes. Germacrenes are typically produced in a number plant species for their antimicrobial and insecticidal properties, though they also play a role as insect pheromones...
D Synthase converts farnesyl diphosphate to geosmin in a two-step reaction.
Streptomyces coelicolor is the model representative of a group of soil-dwelling organisms with a complex lifecycle involving mycelial growth
Mycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
and spore formation. Beside the production of volatile geosmin, it also produces many other complex molecules of pharmacological interest and its genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
sequence is available at the Sanger Institute
Sanger Institute
The Wellcome Trust Sanger Institute is a non-profit, British genomics and genetics research institute, primarily funded by the Wellcome Trust....
.
Effects
Geosmin is responsible for the earthy taste of beetBeet
The beet is a plant in the Chenopodiaceae family which is now included in Amaranthaceae family. It is best known in its numerous cultivated varieties, the most well known of which is the purple root vegetable known as the beetroot or garden beet...
s and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor
Petrichor
Petrichor is the scent of rain on dry earth. The word is constructed from Greek, petra, meaning stone + ichor, the fluid that flows in the veins of the gods in Greek mythology....
) or when soil is disturbed. The human nose
Human nose
The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...
is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.
Geosmin is also responsible for the muddy smell in bottom-dwelling freshwater fish such as carp
Carp
Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain...
and catfish
Catfish
Catfishes 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 heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...
. Cyanobacteria produce geosmin and 2-methylisoborneol
2-Methylisoborneol
2-Methylisoborneol is an organic chemical with a strong odor. It is a derivate of borneol. Its odor detection threshold is very low and it is one of the chemicals with major influence on the quality of drinking water...
, which concentrate in the skin and dark muscle tissue. Geosmin breaks down in acid conditions; hence vinegar and other acidic ingredients in fish recipes help reduce the muddy flavor.