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 beets 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 produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteria (especiallystreptomyces), 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 acidic conditions, geosmin decomposes into odorless substances.
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.
Recently, the biosynthesis of geosmin by a bifunctional Streptomyces coelicolor enzyme has been 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 growthand 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.