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Humans love the smell after good rain, though we may not be the the target of the pleasing aroma. There's evidence the characteristic post-rain scent is used to lure arthropods to bacteria.

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It's fair to say that a lot of us love the fresh smell that comes after a rain shower, but scientists have actually uncovered a reason why the smell of rain is so attractive -- at least if you're a bug.

That characteristic earthy smell is partly a result of the compounds geosmin and 2-methylisoborneol or 2-MIB. Both compounds are made by soil bacteria belonging to the genus Streptomyces and are released into the air when it rains.

But why the bacteria make these chemicals has been a bit of a mystery; that is until a 2020 paper published in the journal Nature Microbiology in which scientists noticed that the aroma seemed to lure in small six-legged arthropods closely related to insects called springtails. That made them think that the compounds might be serving as bait for the springtails so that they'd help spread the bacteria making the smell. So, the scientists put tiny electrodes onto springtails' antennae in order to register when the antennae pinged something.

They then wafted smells at the springtails to see which ones they reacted to. And lo and behold, geosmin and 2-MIB along with their chemical building blocks got the antennae firing. The springtails feed on leaf litter and bacteria in the soil, including Streptomyces. The researchers saw that the bugs were picking up bacteria on little hair-like structures on their abdomens as they scuttled around.

Then they analyzed springtail poop and found that bacterial spores could survive being eaten by the bugs. So, all those findings together made the scientists think that springtails and Streptomyces may have co-evolved in a similar way to flowers and their pollinators. So the earthy smell helps lure springtails to a source of food and the bacteria spores get the chance to be carried along on or inside a springtail.

And just like a bee carrying pollen to a new flower, those spores can be spread a lot further by an animal than they could travel on their own, and that's pretty important for these bacteria who have to move to a new patch of soil when they run out of nutrients or the soil conditions become too harsh to live in.

It's not clear yet what geosmin or 2-MIB do on their own, but researchers know that the compounds are more powerful together, and it seems that this bacteria spreading role doesn't really explain why we humans find rain smell attractive -- it's not like rain showers make us hungry or anything. In fact, if it's not part of that pleasant rain smell cocktail, geosmin actually smells and tastes "off" to us.

It could be serving as a warning that water is contaminated with bacteria that could make us sick, but at least for springtails and potentially other arthropods, that rain smell is what tells them that there's a meal around.

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