Did it rain where you are today?
Did you smell it? That delightful earthy smell?
It’s wonderful, right?
Well, you can thank microbes for making a chemical called geosmin.
The name geosmin, given in 1965, comes from the Greek for earth, “ge” and odor, “osme.” But the earthy smell has been studied for over 100 years and was certainly enjoyed for much longer. The 1965 publication starts off this way: “Freshly plowed soil has a typical odor which was undoubtedly detected even by primeval men and extolled in all tongues by bucolic poets.”
Geosmin is one of my favorite gifts from the microbial world that I like to share with people. While waiting at the vet recently, I heard the receptionist say how she loved the smell that comes after rain, which caught my attention because, heck yeah, me too! So I got to tell her that it comes from microbes and that they’re usually not bad (disease-causing) microbes! She seemed excited. It’s little moments like those that are fun because I get to share the good aspects of the microbial world since, many times, the pathogens get all of the attention.
That’s all great, but what really is geosmin, what microbes make it, and why? Here, I’ll answer those questions, share a few places you may encounter geosmin without realizing it, and hopefully not disappoint you too much when I reveal the context where geosmin can be a problem. After reading this post, you’ll be armed with detailed knowledge about this microbially-produced perfume that you can share with your friends so they, too, can find delight in the microbial world.
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What is geosmin?
Geosmin is the pleasant smell of soil and dirt — that earthy scent you notice after rain. It is one of the chemicals that makes the smell of petrichor. Out of the petrichor chemicals, I choose to highlight geosmin, though, because microbes produce it.
Here’s a great infographic from the American Chemical Society that explains petrichor.
But why does this smell from microbes come out when it rains? As raindrops hit the ground, they create small air pockets in the soil that trap geosmin and other volatile compounds. When these pockets burst, they release geosmin into the air as tiny droplets or aerosols. These aerosols can travel through the air, spreading and eventually hitting our noses.
What microbes make geosmin?
Now we know that microbes produce this delightful smell, but which ones do it? Geosmin is often associated with a group of soil bacteria called Actinomycetes, including Streptomyces spp. and Nocardia spp. However, other bacteria and microbes, in general, make it, too. Here are a few of the others: myxobacteria Myxococcus xanthus and Stigmatella aurantica, cyanobacteria (aka blue-green algae), fungi (mushrooms and Penicillium spp.), and amoeba (Vannella spp.).
What’s slightly disappointing for us microbe enthusiasts is that it appears that plants can produce it as well, including liverwort and beets. So, microbes don’t completely own this one.
Where do we find geosmin?
Geosmin can be found in many places besides soil, including lakes, rivers, drinking water, wine, fish, beets, mushrooms, and carrots. If a vegetable or fruit smells like dirt, you can pretty safely assume it’s geosmin.
Why do microbes make geosmin?
Until very recently, scientists didn’t know why microbes make geosmin. We know it’s there; we know we can surely smell it, but why would they spend the energy to produce this odorous chemical?
In 2020, scientists in Sweden made a discovery about geosmin that revealed a benefit for Streptomyces bacteria: they produce geosmin to attract small invertebrates called springtails in the soil to carry their spores. So, we’re not the only ones who can detect geosmin: springtails do it, too. And in this particular relationship, attracting springtails helps the bacteria reproduce because they disperse their spores to new locations.
Other organisms can detect geosmin, too. It attracts mosquitos but repels fruit flies. For mosquitoes, it helps them find a breeding area, and for the fruit flies, it helps them avoid eating and breeding near microbes making toxins that could harm them. However, whether there are benefits to these relationships from the microbe’s standpoint, it is not clear yet.
Where would we encounter geosmin in our daily lives?
Humans can smell geosmin at an extremely low level. There are times when we enjoy the smell and times when we don’t, all depending on how we sense it.
Geosmin as a delight
Most people enjoy the earthy aroma of geosmin when smelling soil in the garden or rain. So, the scent of geosmin can be a delight. A synthetic version is added to perfumes, and you can even find it in candles. I remember buying a car air freshener that was supposed to smell like fallen leaves, and I am now certain that geosmin was included in the formulation.
Geosmin as a problem
As much as we love the smell of geosmin, we can’t stand it when we taste it in our food, fish in particular, and drinks, like drinking water and wine. In fact, geosmin poses a significant problem for the food, beverage, aquaculture, and water industries. Many studies have been dedicated to finding ways to remove geosmin. If you type the word geosmin into Google Scholar, you’ll see mostly articles about how to control and eliminate it. And it makes sense if you’ve ever tasted water that has a soil-like flavor (I know I have). Plus, I certainly wouldn’t want wine that tastes like dirt, either.
There is a way to get rid of that musty, earthy flavor in your food at home, though. All you have to do is add acid. The reaction of geosmin with acid yields odorless argosmin. So, if you like to cook with citrus juice (citric acid) or vinegar (acetic acid), you’re all set.
Geosmin, a new favorite smell?
Geosmin is both a delight and a frustration to our senses. Most of my encounters with geosmin have been by smell rather than taste, so I remain a dedicated fan of it. I hope you do, too, after reading this article.
Links & Resources
This post contains affiliate links — see my disclosure policy.
- Wanna discover other microbial smells? Check out this article about The Delightful and Repulsive Smells of Microbes
- Go deeper with this review article: Biochemical and Ecological Control of Geosmin and 2-Methylisoborneol in Source Waters
- Learn more about geosmin in A Field Guide to Bacteria by Betsey Dexter Dyer
- 7 Ways to Enjoy Microbes in Your Daily Life
- Geosmin sticker and other geosmin goodies on RedBubble
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Space Microbiology: Marta Cortesão
