Often, as storms approach my house, I smell them coming. And, after the rains fall on a warm spring day, I can pick up that sweet smell of rain in the back garden. Evidently, humans aren’t the only creatures loving that smell. Bugs, reptiles, and other mammals also respond to a waft of geosmin emanating from the newly wet soil. Geosmin is the chemical compound responsible for the smell of rain.
Human sensitivity to geosmin is impressive since our olfactory powers are supposedly pitiful when compared to dogs or cats. But, we can detect geosmin in amounts as low as one part per trillion. Wine lovers get a whiff of its earthy smell when they uncork a bottle of good red wine.
Streptomyces
Geosmin does not come directly from the soil, but rather it emanates from bacteria in the soil. The particular bacteria producing that delightful smell of rain belong to the genus Streptomyces. There are 500 known species of Streptomyces bacteria, and they are famous for the range of unusual and unique compounds they produce. Some species produce antifungal compounds, and other species produce anticancer compounds, but nearly all of the species produce the compound geosmin.
After a fresh rainfall, the bacteria release geosmin as part of their reproductive strategy. The smell of this compound pleases humans but also attracts springtails. These tiny, six-legged arthropods can’t get enough of it, so they rush in and eat the bacteria. Thus, the bacteria’s spores end up in the springtail’s gut or stuck to its body. When dinner is over, then the springtail heads off to new hunting grounds where the spores are pooped out, or they fall off the creature’s body. Either method does the trick for the bacteria since the spores then seed a new population for the species.
This symbiotic relationship between the bacteria and springtails points to a long evolutionary relationship. The springtails seem to be a preferred carrier for the spores. Streptomyces produce a range of compounds that kill many fungi and insects. But springtails branched off from the insects about 500 million years ago and evolved to cope with the more toxic Streptomyces compounds.
Humans benefit from this relationship also. Streptomyces produce many of the antibiotics that support our modern medical practices. Springtails help the bacteria thrive but they also keep alive a species that, in turn,
Sources:
Streptomyces (By J. Parker, in Encyclopedia of Genetics) – https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/streptomyces Also:
Reducing cork taint in wine (By R. Jung, V. Schaefer, in Managing Wine Quality: Oenology and Wine Quality) – https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/geosmin Also:
How Rain Evolved Its Distinct Scent—and Why Animals and Humans Love It (By Alex Fox, smithsonianmag.com) – https://www.smithsonianmag.com/smart-news/smell-rain-explained-180974692/ Also:
Feature Image: Springtail (Modified) – By Katja Schulz from Washington, D. C., USA – Springtail, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=81226973