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- What causes the characteristic smell of soil?
What causes the characteristic smell of soil?
by Distinguished Research Professor R. Meganathan
Smell of soil is due to the smell of two small molecules produced by small organisms. These small molecules are known as geosmin and 2-methylisoborneol (MIB). These compounds are mostly produced by bacteria belonging to the genus Streptomyces. In this connection it is worth remembering that the majority of antibiotics we use are produced by streptomycetes. A Petri dish with colonies of Streptomyces and the structures of the two compounds are shown in the Figure at right. The smell of these compounds can cause reduced quality of drinking water. Geosmin and MIB also have been found to reduce the quality of fish in freshwater aquacultures as the odours penetrate and accumulate in the fish, thereby lowering the commercial value.
Human’s can smell concentrations as low as 10 parts per trillion of Geosmin and MIB in water. The characteristic odor of soil was first investigated by Berthelot in 1891 (1). In 1965 the structure of the responsible compound geosmin was determined by Gerber (2). It was not until 1981 that the pathways for the biosynthesis of these compounds determined. Prof. Bentley and I determined on the basis of isotopic labeling experiments that geosmin is a degraded sesquiterpene and MIB is a methylated monoterpene (3). According to Prof. David E Cane of Brown University “Although there have been more than 800 papers dealing with the production, detection and remediation of geosmin and other volatile metabolites in water supplies, aquaculture products and wine, there were no further reports on the mechanism of microbial geosmin biosynthesis until five years ago” (4) when the work was repeated (5).
Recently, with availability of the genome sequence of Streptomyces coelicolor Cane and associates have done extensive research on the ezymology and the reaction mechanisms of the geosmin and MIB biosynthetic pathways (4, 6).
As pointed above, the human’s olfactory system can detect extremely low concentrations of these compounds. It is interesting to note that the same compounds contribute to the well being and survival of the Camel.
The wild Bactrian camels (two humped camel; Camelus bactrianus) are reputed to be able to find water up to 50 miles away. In the desert, Streptomyces gives off the signature smell and that scent is carried on a breeze, and it can be picked up by the camel's well-tuned nostrils. In fact, it could be a matter of life or death for the camel. What does the Streptomyces get out of its scent? According to Professor Keith Chater, the smell could be a way of luring animals into carrying its spores. "You could imagine that the camels would disperse the spores as they take a drink, either they would eat or drink the spores, or they would get stuck on them and then get dispersed wherever the camel moves to" (7)
- Berthelot, M. & Andre. G. Compt. Rend. 112, 598–599 (1891).
- Gerber, N.N. CRC Crit. Rev. Microbiol. 7, 191–214 (1979).
- Bentley, R. & Meganathan, R. FEBS Lett. 125, 220–222 (1981).
- Jiang, J; He, X.; Cane, D. E. Nat. Chem. Biol. 3, 711–715 (2007).
- Spiteller, D., Jux, A., Piel, J. & Boland, W. Phytochemistry 61, 827–834 (2002).
- Wang, C. & Cane, D. E. JACS. 130, 8908–8909 (2008). 7. Simons, P. The Guardian, March ,6, (2003).