While a bit cold and wet and stressful when your umbrella has decided to invert itself, there’s something undeniably relaxing about rain. Perhaps best enjoyed from the safety of indoors (good news for you lockdown’ers), everyone loves the fresh smell that fills the air after a good downpour. And, as it happens, there could be a scientific reason for that. A group of researchers in Australia have uncovered that a group of bacteria produce a compound that releases the “smell of rain”, which is intended as a bait to lure animals and help spread the bacteria. Those cheeky chappies.
The study, published in the journal Nature Microbiology, was investigating the effect of the presence of Streptomyces on the behavior of animals within sniffing distance such as insects. Streptomyces is the largest group of Actinobacteria with over 500 species within the genus. They’re commonly found in soil and rotting vegetation, releasing that earthy smell we all just can’t get enough of, which is actually a compound they release called geosmin. It's more noticeable to us after it rains as the fall of water thrusts the compound into the air.
Lead researcher Klas Flärdh and colleagues at Monash University decided to investigate the effects of this alluring smell using a combination of field and laboratory experiments. They wanted to explore if the smell of Streptomyces was attractive to arthropods in the soil. Field trap analysis revealed that springtails were lured in by the earthy scent of geosmin. They investigated the attraction further under lab settings and found that the springtails were using their antennae to pick up on the fragrance.
During their investigations, they noticed that insects feeding on the bacteria ended up with bacterial spores stuck to their bodies. As they trudged off into the soil, they took these spores with them, spreading them from their surface and by defecating ingested spores as they went. The authors suggest that this dispersal is an ecological benefit for the bacteria in producing the geosmin and the enticing smell that comes with it. By attracting animals to feed on and accidentally coat themselves in the bacteria, the Streptomyces can spread far and wide, riding into the sunset on the back (or in the digestive tract) of a springtail.
So, if you’re feeling a little cooped up and separated from nature, next time it’s raining make sure to crack open the window and inhale deeply.
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