Most people love the smell of rain and would you be surprised to know that Streptomyces bacteria are associated with that sweet, earthy smell of rain.
Perhaps you could have made a remarkable observation that the smell of rain is more prominent for the first rainfall of the season.
Furthermore, you could have sensed that earthy smell when water hits the long-dried soil, while you are gardening.
And there is a reason for this too!
Note: Rain doesn’t have any odor, what we are discussing here is the odor produced when the first rain hits the dry soil.
Scientists unveiled that the smell of rain is called petrichor and it is due to a chemical called geosmin, produced by streptomyces bacteria found in the soil.
What is the smell of rain called?
There is a name for the smell of rain, it’s called petrichor!
The Oxford English dictionary described petrichor as “A pleasant, distinctive smell frequently accompanying the first rain after a long period of warm, dry weather in certain regions.”
The word “petrichor” was coined in 1964 by Australian scientists, Isabel Bear and Richard Thomas.
Read the first article described petrichor, published in the Nature journal of March 7, 1964, by Isabel Bear and Richard Thomas titled Nature of Argillaceous Odour.
The word petrichor comes from the Greek word “petra” meaning stone and “ichor” describing the mythological blood that flows in the veins of the gods. Therefore, literally meaning, petrichor is the blood of the stone.
The petrichor scent had been recognized and applied in the production of perfumes in Uttar Pradesh, India. It was mentioned as “mitti ka attar” meaning “earth perfume”.
Petrichor is the smell produced when the first rain of the season hits the dry ground. You can even sense the smell of petrichor while gardening, dealing with dry soil, and spraying water on it for the first time.
However, petrichor is not the smell of the rainwater. It is the smell of geosmin produced by the soil bacteria streptomyces.
Role of Streptomyces in the smell of rain
The natural habitat of streptomyces bacteria is the soil. They are abundant in the soil and a major contributor to the petrichor smell.
The streptomyces bacteria are widely distributed in the soil and can be observed all over the world, thus explaining the ubiquitous earthy smell of the rain.
These bacteria require damp and moist conditions for their active growth and multiplication. When the soil dries out, streptomyces produce spores and release geosmin.
What are spores?
During the adverse environmental conditions including dehydration, bacteria (certain bacteria) produce spores.
Spores are the dormant structures of bacteria and they have minimum metabolism, respiration, and reduced enzyme production. Spores do not reproduce.
Bacterial spores are highly resistant life forms and they are extremely tolerant against stresses such as dehydration, high temperatures, chemicals, and various other severe physical conditions.
Moreover, the extremely dormant nature of the spores makes them survive for many years.
Spore formation is one of the most common survival mechanisms for bacteria to protect themselves against adverse environmental conditions.
Under favorable environmental conditions, the spores can get back to their vegetative state.
Because the bacteria thrive in moist soil and produce spores when the soil dries out, the smell of rain would be intense for the first fresh rain after a prolonged period of dry spell.
The longer the soil goes dry, the more spores would be released and the more intense would be the smell of rain.
When the rainfall hits the surface of the soil with consistent velocity, these tiny spores go up in the air along with the aerosols. And these aerosols carry the spores to us and as we breathe, we could sense the distinctive earthy odor we often associate with the smell of rain.
Aerosols are tiny liquid droplets suspended in the air.
However, the smell of rain is not actually caused by the actinomycetes spores itself. Rather it is caused by a particular chemical called geosmin released during the production of spores.
The smell of rain (petrichor) is due to a molecule called geosmin, produced by soil bacteria streptomyces during the spore formation.
Our noses are exceptionally sensitive to geosmin such that we can detect it in low parts per trillion (ppt) concentrations.
Almost all the species of Streptomyces bacteria release geosmin when they produce spores generating distinctive smell of rain.
Why streptomyces produce geosmin?
In the latest study which included several fields and lab experiments, researchers discovered why Streptomyces produce geosmin.
The study revealed that the geosmin confers a selective advantage on the bacteria.
Streptomyces uses geosmin as a signaling molecule to specifically attract an arthropod called springtail.
Both organisms have mutual benefits, as the Streptomyces serves as food for the springtail, while springtail spread the Streptomyces spores.
Moreover, the antibiotics produced by the Streptomyces do not affect the springtails while they are toxic to other organisms such as fruit flies.
For the information of microbiology enthusiasts, there are other microorganisms capable of producing geosmin.
Other microorganisms responsible to produce geosmin in the soil are bacteria belonging to actinomycetes – Norcardia cummidelens; N. fluminea; Luridiscabiei; S. albidoflavus – since they all are soil-dwelling bacteria.
Certain fungi including Penicillium discolour, P. crustosum, P. expansum, Botrytis cinerea, and Chaetomium sp also produce geosmin.
Geosmin does not taste good
The smell of rain feels good to many people, and geosmin is the causative chemical of that earthy, calming aroma.
However, unlike the smell, it doesn’t taste good to most of the people.
Yes, you heard it right! You have tasted geosmin. The beets contain geosmin and it is the geosmin that makes the beets taste earthy.
Furthermore, the presence of geosmin in water is also not appealing as it gives the water a muddy, earthy flavor.
However, the flavors of some wines are enhanced by geosmin.
Other reasons for the smell of rain
Since I am a microbe lover, I would love to explain the smell of rain in the microbiological perspective, though there are other factors contributing to the smell of rain besides the bacteria.
For example, the volatile oils that plants and trees release also contribute to the smell of rain.
- The smell of rain has a name. It is called Petrichor.
- A chemical called geosmin is the cause of petrichor scent.
- Geosmin is produced by Streptomyces during the spore formation.
- Streptomyces use geosmin as a signaling molecule to attract an arthropod called springtail.
Next time, when you step outside to enjoy the smell of rain, you know it is having a name and it’s caused by a chemical released by the bacteria.
In which month do you anticipate the smell of rain depending on where you live?
And do you appreciate that earthy smell?
I would like to know from you. Please let me know in the comments.