Qs people asked me on science and my replies to them - part 180 - GEOSMIN

Q: Why do we get a sweet smell when it rains?

Krishna: Geosmin! Heard about it?

Image source: Google images

Geosmin is an organic compound with a distinct earthy flavour and aroma produced by certain bacteria, and is responsible for the earthy taste of beetroots and a contributor to the strong scent (petrichor) that occurs in the air when rain falls after a dry spell of weather or when soil is disturbed.

Geosmin is the soil-based compound that gives the evocative, earthy smell so characteristic of spring. It's best appreciated after recent rainfall or while digging. The human nose is so sensitive to the compound that it is detectable at one hundred parts per trillion (ppt).

According to new research (1), it is because of an ancestral mutual relationship between the soil bacteria Streptomyces and primitive, six-legged creatures called springtails (Collembola).

Streptomyces are globally significant bacteria that produce a range of organic compounds, including chemical weapons to fight off enemies in the soil. These have been exploited forhuman use as some world's most effective antibiotics.

They also produce geosmin and a range of other volatile organic compounds (VOCs). While Streptomyces species vary drastically in the types of molecules they produce as antibiotics, they all, without exception, produce geosmin. Scientists suspected that this might attract something, otherwise this will not be produced without any purpose. So they tried to find out what it is. 

To find out what might be attracted to the smell of geosmin, researchers  laid out a network of traps baited with Streptomyces. Others were baited with control substances.

The experiments revealed that springtails were attracted to the Streptomyces bait. 

 Scientists used a method called electroantennography—which involves inserting tiny electrodes into the antennae of the 1.5 mm-long springtails—and observing their responses to a succession of molecules.

All these experiments added up to one clear observation: the springtails were powerfully drawn to geosmin and to another earthy smelling compound produced by Streptomyces called 2-methylisoborneol (2-MIB).

The study finds that geosmin and 2-MIB are chemical signals that guide springtails to Streptomyces as a privileged food source—one that kills other organisms such as nematodes and fruit flies—but which springtails can withstand because they have a battery of enzymes which detoxify the antibiotics produced by Streptomyces.

In return, springtails help disperse Streptomyces spores in two ways: by defecation and by distribution of spores that stick to their bodies. The study found that the spores survived passage through the springtail gut to give rise to new colonies of Streptomyces.

There is mutual benefit. The springtails eat the Streptomyces, so the geosmin is attracting them to a valuable food source. And, the springtails distribute the spores, both stuck on their bodies and in their faeces, which are full of viable spores, so the Streptomyces get dispersed.

These insights provide an explanation for the previously reported correlation between geosmin production and sporulation in Streptomyces.

So the sweet smell of the first rain drops is the result of  of chemical communication that started and  lasted for 450 million years.

CITATIONS:

1. Paul G. Becher et al. Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal, Nature Microbiology (2020). DOI: 10.1038/s41564-020-0697-x