The smell of rain may sound like something purely ethereal and symbolic that resides in the heart and mind rather than an actual aroma you take in through your nostrils. But it’s actually a very real smell (termed petrichor), and more specifically, the smell of ozone, water, plant oils, and geosmin.
Geosmin is a natural bicyclic terpene that stems from the Greek words for “earth” (“geo”) and “odor” (“osme). It can also be found in beets, shellfish, freshwater fish, and drinking water, and it’s credited for their distinctively potent, earthy, musty aromas.
While the characteristic scent lends itself very nicely to perfumes, it does not necessarily to foods and drinks, which is why people often seek for ways to get rid of it.
Even though geosmin was first investigated much earlier, it was in 1965 when geosmin was first isolated. The discovery came at the hands of Nancy N. Gerber and Hubert A. Lechevalier from Rutgers University. They isolated geosmin from Streptomyces which is a species of bacteria.
Streptomyces are found in the soil, which makes sense, since rain tends tо stir soil up unlocking its scents as a result. So, the smell of rain might actually come less from the rain itself and more from the awakened soil as a byproduct. [1]
But why do steptomyces produce geosmin?
As we know, there are no accidents in nature; everything really does happen for a reason, serving a larger purpose within a larger machine. And survival, the perseverance of life, is basically the very essence of nature. And who is a bigger survivor than bacteria?
Streptomyces produce spores during dire straits to transport themselves to more favorable environments and conditions. And geosmin does its share by attracting organisms that disperse spores, such as Folsomia candida, a type of hexapod called a springtail. [2]
“Geosmin and 2-MIB [2-methylisoborneol] in these odours induce electrophysiological responses in the antennae of the model springtail Folsomia candida, which is also attracted to both compounds. Moreover, the genes for geosmin and 2-MIB synthases are under the direct control of sporulation-specific transcription factors, constraining emission of the odorants to sporulating colonies.”
The scientists conclude that geosmin is an “integral part” of the sporulation process and Streptomyces’ life cycle. Curiously and rather morbidly, the geosmin emitted by the bacteria essentially provides an open invitation to the springtails to come feast on the bacteria, thereby ingesting Streptomyces spores to be spread far and wide via springtail excrement.
Perhaps the smell of rain doesn’t sound as romantic anymore, but nevertheless, geosmin, like nature altogether, is fascinating.
References:
[1] Jiang J, He X, Cane DE. Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme. Nat Chem Biol. 2007;3(11):711-715. [Journal Impact Factor = 15.04; Times Cited = 246][2] Becher PG, Verschut V, Bibb MJ, et al. Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal. Nat Microbiol. 2020;5(6):821-829. [Journal Impact Factor = 14.3; Times Cited = 54]
