Geosmin is an irregular sesquiterpene, produced from the universal sesquiterpene precursor farnesyl pyrophosphate, 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 odor detection threshold of geosmin is very low, ranging from 0.006 to 0.01 micrograms per liter in water. Geosmin is also responsible for the earthy taste of beetroots and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed. In chemical terms, it is a bicyclicalcohol with formula, a derivative of decalin. Its name is derived from the Ancient Greek wikt:γῆ geō- "earth" and wikt:ὀσμή osmḗ "smell". The word was coined in 1965 by the American biochemist Nancy N. Gerber and the French-American biologist Hubert A. Lechevalier.
Production
Geosmin is produced by various blue-green algae 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, Phormidium, and Planktothrix, while the main genus in the Actinomyces that produces geosmin is Streptomyces. Communities whose water 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 localwater supply. Under acidic conditions, geosmin decomposes into odorless substances. In 2006, the biosynthesis of geosmin by a bifunctional Streptomyces coelicolorenzyme was unveiled. A single enzyme, geosmin synthase, converts farnesyl diphosphate to geosmin in a two-step reaction. Not all blue-green algae cyanobacteria 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. Streptomyces coelicolor is the model representative of a group of soil-dwelling bacteria with a complex lifecycle involving mycelial growth and spore formation. Besides the production of volatile geosmin, it also produces many other complex molecules of pharmacological interest; its genome sequence is available at the Sanger Institute.
Effects
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 responsible for the muddy smell in many commercially important freshwater fish such as carp and catfish. 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 also an issue for saltwater fish farmed in recirculating aquaculture systems, such as Atlantic salmon. These systems rely on biological filtration using cultured microbial communities to process the nitrogenous waste from the fish into less harmful compounds 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.
