Геосмін

Geosmin is a bicyclic alcohol with the formula C₁₂H₂₂O, a derivative of decalin. Its molecular weight is 182.30. Its IUPAC name is (4S,4aS,8aR)-4,8a-dimethyl-1,2,3,4,5,6,7,8-octahydronaphthalen-4a-ol. It is an irregular sesquiterpenoid. Geosmin is a tertiary alcohol that exists as (+) and (-) enantiomers. Its name comes from the Ancient Greek words geō-, meaning "earth", and osmḗ, meaning "smell".

Geosmin is produced as a secondary metabolite by various cyanobacteria, filamentous bacteria in the class Actinomyces, other prokaryotes, and various eukaryotes. The main cyanobacterial genera shown to produce it include Anabaena, Phormidium and Planktothrix, while the main genus in the Actinomyces that produces it is Streptomyces. Geosmin is produced by a number of microorganisms, including most Streptomyces and several species of cyanobacteria, myxobacteria and fungi. In beetroots, geosmin is produced endogenously and not by environmental or endophytic microbes. Geosmin is responsible for the earthy taste of beetroots. The related molecule dehydrogeosmin, together with geosmin, gives some plants a musty smell, especially certain cacti. Geosmin has been found to repel the nematode Caenorhabditis elegans, reducing its contact with Streptomyces. Geosmin and 2-methylisoborneol have been shown to attract the springtail Folsomia candida, which is believed to feed on and spread the spores of Streptomyces.

Geosmin is easily smelled by most people, because its odour detection threshold is very low. Human sensitivity differs between the two enantiomers. Across 50 subjects, the mean odour detection threshold was 9.5 ng/L for naturally occurring (-)-geosmin and 78 ng/L for (+)-geosmin. This makes the natural form roughly eight times more potent. The extreme sensitivity of humans to geosmin is proposed to have evolved to help them or their ancestors find scarce water, but this is not conclusive.

Geosmin and 2-methylisoborneol are the major biological causes of muddy odour and flavour in drinking water worldwide and in farmed fish. Bottom-feeding freshwater fish can absorb geosmin that cyanobacteria (blue-green algae) and actinobacteria release when they die. In acidic conditions geosmin decomposes into odourless substances. In 2002, French wine growers were concerned about a musty smell in some young wines, and geosmin, specifically the (-) isomer, was identified as its cause. Communities that depend on surface water can periodically have episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the supply.

Geosmin is a colourless neutral oil with an optical rotation [α]D25 of -16.5° and a composition of 79.06% carbon, 12.16% hydrogen and 8.78% oxygen. Besides Streptomyces, it is produced by Penicillium and Aspergillus species and by a liverwort, Symphyogyna brongniartii. Geosmin levels in beetroot vary between cultivars, so selective breeding can produce varieties with a bland taste. Titanium dioxide photocatalysts can destroy geosmin in water, but geosmin resists oxidation and is difficult to remove with standard water treatments. A geosmin synthase gene, geoA, is present in geosmin-producing cyanobacteria but not in other cyanobacterial species, so detecting it in water samples may help predict taste and odour events.

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