Everyone knows that smell. It appears every time the first large drops of rain hit the ground - a distinctive earthy scent that fills the air, a scent that signals the change of seasons and promises relief from the stifling summer heat. The smell of rain also has a name: petrikor, a poetic word formed from the Greek words petra (stone) and ichor (blood of the gods in Greek mythology).
Petricore: the smell of rain and what causes it
The name "petrichor" was coined by Australian scientists Isabella Bear and Robert Thomas in 1964 and mentioned in a paper that was perhaps the first serious scientific attempt to explain the phenomenon. They used the word to refer to oil that had been distilled from samples of soil and vegetation, outdoors and exposed to sunlight throughout the year, but protected from rain. They found that the oil contained a complex mixture of volatile organic compounds.
One question left unanswered by Behar and Thomas was the origin of these compounds, and subsequent research focused on one particular compound, a volatile bicyclic alcohol called geosmin. This compound was isolated a year after Behar and Thomas' paper was published, and its name literally means "earth odor."
Along with another volatile organic compound called 2-methylisoborneol or 2-MIB, geosmin is mainly responsible for the characteristic earthy odor, and both compounds greatly influence the smell of rain.
Ryan Busby, an ecologist with the U.S. Army Corps of Engineers, told Popular Science magazine that these compounds are present in soil around the world and are released into the air with any soil exposure.
"The compounds accumulate in soil pores," Busby explains. "They probably bind to soil particles. And studies have shown that when exposed to the soil surface, volatiles are released into the atmosphere."
Where geosmin and 2-MIB come from
Busby says that while the source of both compounds remains the subject of active research, scientists currently agree that they are secreted by soil bacteria. Different ratios of the two compounds may explain why the odor differs slightly from place to place.
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"Geosmin is fairly stable in the environment, while 2-MIB is more variable. Where 2-MIB is present, it is released in much higher concentrations, so there are areas where its concentration is very high and there are areas where it is not," Busby says.
The other components that make up petrichor - the many less potent volatiles associated with plants, and perhaps the characteristic acrid ozone odor that accompanies lightning - vary from place to place.
Humans are particularly sensitive to the odor of geosmin. It can be found in water in concentrations as low as 4 ng/L, which equates to about one teaspoon per 200 Olympic swimming pools. Busby says there are several theories to explain why this is so.
"One theory has to do with finding water sources," he explains. "Geosmin appears to be more common in moist, fertile soils." The presence of moist soil means the presence of water, and it's easy to see how the ability to pick up the odor of geosmin on the wind and trace it to a water source could provide a valuable evolutionary advantage.
According to Busby, humans aren't the only ones who can rely on the smell of these volatile compounds to find water: "Berbils can detect geosmin and find oases in the desert about 80 kilometers away. Mosquitoes use it to find stagnant ponds to lay eggs, and raccoons use it to find turtle nests and buried eggs."
But while the smell of geosmin and 2-MIB is appealing to us, their taste is the exact opposite. "It's funny," Busby reflects. "We love the smell but hate the taste."
In water, these compounds are responsible for the musty, moldy taste that indicates the water is not drinkable. Busby says, "Whenever you drink water and think, 'Oh, that tastes like lake water,' it's because these compounds are dissolved in what you're drinking."
Again, there's likely an evolutionary explanation for this: it's one thing for the soil around a water source to smell like bacteria, but if the water itself has the characteristic musty odor of geosmin and 2-MIB, it's also likely to cause gastrointestinal discomfort. Busby says this explains why geosmin and 2-MIB are "the major odor contaminants of drinking water worldwide."
Why bacteria produce geosmin and 2-MIB
As Busby notes, while it is clear that "geosmin has a number of applications for us, we do not know exactly why bacteria produce it in such quantities. The production of such a chemical is energy-intensive." So why do soil bacteria produce geosmin and 2-MIB? What do they get out of it?
A paper published in Nature Microbiology in 2020 suggested a possible explanation. The study examined interactions between Streptomyces, a type of bacteria that produces geosmin and 2-MIB, and small creatures called columbols.
Collembolas (or legwalkers) are one of three species of six-legged arthropods that are not considered insects and feed on bacteria.
Importantly, the scientists found that in the bacteria studied, geosmin and 2-MIB were only produced by colonies that also produced reproductive spores. In fact, they could only be produced by these particular colonies: "The geosmin and 2-MIB synthase genes are under direct control of spore-specific transcription factors, which limits odor production to spore-producing colonies," the paper explains.
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Collembolae are attracted to geosmin and 2-MIB, so it is not surprising that when they arrived at odor-emitting colonies, they enjoyed a tasty microbial snack. In doing so, they also consumed the bacterial spores. The spores then passed through the digestive tract and came out the other end, ready for action.
Busby says this may also explain why the smell of rain is strongest when it falls on dry soil. "When the soil dries out, the bacteria go into a dormant state, and there seems to be a massive release of bacteria at that point. So from that perspective, the compounds are a way to attract something that will perhaps move the bacteria to a more favorable environment for growth."
It might seem that the poetic appeal of petrichorus is somewhat diminished when you learn that the rain's evocative, pleasant smell likely exists to encourage a bunch of tiny arthropods to secrete bacterial spores. But ultimately, it's another example of nature finding a way out - a "co-evolutionary relationship" reminiscent of the relationship between
bees and pollen that benefits humans.
So the next time rain hits dry ground, it's worth remembering the tiny bacteria that lead people to water and keep them from drinking from sources that could harm them.