Scientists have discovered the cause of a unique smell after rain, mediated by a 500 million-year-old chemical “AC” model.

A new study by an international team of scientists suggests that the smell of soil that can be identified immediately after rain is released by bacteria trying to attract specific arthropods as a way to spread spores. The smell is a 500 million-year-old example of chemical “communication” that evolved to help a particular type of bacteria spread.

Scientists have long been fascinated by the unique smell of rain. This smell is especially pronounced when the first rain of a season falls on dry soil. Two Australian researchers named the smell “tidal oil” (petrichor, the earthy smell of a long drought. Because an influential study in the 1960s showed that during dry spells, certain plants produce a special kind of oil that is then released into the air when it rains.

One of petrichor’s main ingredients is an organic compound called geosmin. Scientists have known that a common genus of bacteria, streptomyces, produces soil odors. Almost all kinds of streptomycin release skunk when they die, but until now, it was not clear exactly why the bacteria produced this unique odor.

“In fact, the fact that they all produce soil odors suggests that it gives the bacteria a selective advantage that would otherwise not have done so,” said Mark Buttner, one of the authors of the new study. “So we suspect they’re signaling something, most notably that some animals or insects may help distribute streptomyces. “

Through a series of laboratory and field experiments, researchers have found that soil odors are particularly attractive to a small arthropod called a bullet-tailed worm. By studying the tentacles of the pellets, the researchers found that the creature could sense the soil odor directly. The researchers believe the two creatures evolved together, with streptomycin being the food for the azilar, who then spread bacterial spores to help sow new streptomyces.

“The two are mutually beneficial,” Buttner explains. “The bullet-tailed worms eat streptomyces, so the soil odor attracts them to valuable food sources. Moreover, the azimuths distribute the spores in their bodies and feces, which are glued to them, and the faeces are filled with living spores, so streptomycins are dispersed. This is similar to birds eating the fruits of plants. They can also distribute seeds while getting food, which is good for plants. “

This symbiotic relationship is key to the survival of streptomycin, as it is known to produce certain antibiotic compounds that make it toxic to other organisms such as flies or nematodes. On the other hand, the pellets produce novel enzymes that detoxify antibiotics produced by streptomycin. This convincing new finding suggests that a major factor in this iconic wetland odor is the nearly 500 million-year-old relationship between bacteria and arthropods, mediated by an unusually special pattern of chemical communication.

“We used to think that spores of streptomycin are spread through wind and water, but in the small chamber of the soil, there is little room for wind or water,” Buttner said. “So these small primitive animals have become an important part of completing the life cycle of streptomycin, one of the most important sources of antibiotics known to the scientific community. “

The new study is published in the journal Nature Microbiology.