Geosmin

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. The human nose is exquisitely sensitive to geosmin, able to detect it at concentrations down to 5 parts per trillion.

Geosmin is produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteria especially though 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 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. Blue green algae produce geosmin and methylisoborneol which concentrate in the skin and dark muscle tissue. Geosmin breaks down in acid conditions, and 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. (2006, 2007). A single enzyme, the germacradienol/germacrene D Synthase converts farnesyl diphosphate to geosmin in a two steps 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 his genome sequence is available at the Sanger Institute.