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Love The Smell of Rain? There's an Ulterior Motive Behind The Lure of Petrichor

7 APRIL 2020

If you've lived outside a major city, the smell is instantly recognisable – the earthy scent of petrichor as rain hits dry soil. Now, new research has uncovered why it's not just us humans who are attracted to this incredibly pleasant odour.

 

That luscious smell we can detect after rain comes from an organic compound called geosmin, which is produced by microbes, including the bacteria genus Streptomyces.

We also know that Streptomyces releases geosmin when they die, and that humans and other creatures are particularly attuned to it. The question, of course, is - why does this happen?

An international team of researchers set out to explain why bacteria produce geosmin, and whether any other creatures were able to enjoy the smell as much as we do.

"To investigate possible roles of geosmin and other Streptomyces volatile organic compounds in the context of soil ecosystems, we asked whether the smell of Streptomyces spp. might attract soil-dwelling arthropods," the international team of researchers wrote in their new paper.

And that's exactly what they found.

"In a network of field traps baited with Streptomyces coelicolor colonies, we found significant attraction of springtails (Collembola) compared with control traps."

The team did a number of experiments both in the field and in the lab to see the effects of geosmin and another compound called 2-methylisoborneol (2-MIB) on forest creatures, particularly springtails - tiny arthropods with a tail-like appendage, which live in organic materials such as leaf litter on a forest floor.

 

Turns out that springtails are big fans of geosmin. They can sense it with their antennae, are attracted to it, and will feed on the Streptomyces producing it.

But why would a bacterium go to so much effort just to be slurped up by an arthropod? Although producing a nice smell to get eaten might sound like a bad time for most, Streptomyces actually has a plan.

Streptomyces acts, in a lot of ways, like a fungus. It looks a lot like a filamentous fungus, and when it is ready to reproduce, it creates spores, which can spread newborn bacteria far and wide.

But it does need a vector for that spread, which is where the springtails come in.

"[A springtail] feeds on the Streptomyces colonies and disseminates spores both via faecal pellets and through adherence to its hydrophobic cuticle," the team explains.

"The results indicate that geosmin and 2-MIB production is an integral part of the sporulation process, completing the Streptomyces life cycle by facilitating dispersal of spores by soil arthropods."

Next time you smell the rain, you can enjoy the fact that what you're smelling is an entire circle of life in its own little way.

The research has been published in Nature.