What’s behind the unique scent of rain?

The smell of rain has a name of its own, it's called 'petrichor'. It's the pleasant smell that that frequently accompanies the first rain after a long period of warm, dry weather. 

Two Australian researchers coined the word 'petrichor' after an influential study in the 1960s suggested a particular oil is produced by certain plants during dry periods, and then released into the air when it rains.

A new study published in the journal 'Nature Microbiology' is suggesting that this smell is released by bacteria trying to attract a particular arthropod as a way to spread its spores. The smell is a 500-million-year-old example of chemical communication, evolved to help a particular type of bacteria spread.

One essential part of petrichor is an organic compound called geosmin. For some time, scientists have known that a common genus of bacteria, called Streptomyces, produce geosmin. Virtually all Streptomyces species emit geosmin as they die, but until now it has been unclear exactly why the bacteria develop this distinctive aroma.

"The fact that they all make geosmin suggested that it confers a selective advantage on the bacteria, otherwise they wouldn't do it," says an author on the new research, Mark Buttner. "So, we suspected they were signaling to something and the most obvious thing would be some animal or insect that might help distribute the Streptomyces spores."

The researchers discovered that geosmin actually attracts a form of tiny arthropod called a springtail. Studying the springtail antennae, the researchers found that the species can detect geosmin directly. They believe that both species evolved together, the Streptomyces acting as food for the springtails, while the springtails later dispersed bacterial spores helping to establish new colonies of Streptomyces.

"There is mutual benefit," explains Buttner. "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. This is analogous to birds eating the fruits of plants. They get food but they also distribute the seeds, which benefits the plants."

This symbiotic relationship is crucial to Streptomyces' existence, because it is understood that the bacteria create other antibiotic compounds that make it poisonous to certain species including fruit flies or nematodes. At the other hand, springtails generate a variety of novel enzymes which can detoxify the Streptomyces-produced antibiotics.

This persuasive new finding indicates a significant dimension of the classic wet earth fragrance being underpinned by a nearly 500 million-year-old bacteria-arthropod relationship, regulated by an incredibly precise form of chemical communication.

"We used to believe Streptomyces spores were distributed by wind and water but there is little room for wind or water to do anything in the small air compartments in the soil," says Buttner. "So, these small primitive animals have become important in completing the lifecycle of the Streptomyces, one of the most important sources of antibiotics known to science."