Thursday, April 11, 2024

Why Spring Smells So Good

Raindrops fall on a plant

As the soil warms up in April and May and green plants spring forth once again, a delicate aroma hangs in the air, apart from any floral scent wafting on the breeze. It’s earthy and fresh, and I find it almost intoxicating. It turns out that spring’s special perfume has some fun and quirky root causes. 

Spring’s perfume has long intrigued humans, to the point that sixty years ago, Australian scientists gave it a name: “petrichor.” Greek in origin, this word means “smells like a rock.” Or something like that. Not long after the wondrous bouquet of spring got its official name, British scientists in the UK found its main source, a tertiary alcohol they dubbed “geosmin,” another Greek-based term. Roughly translated, it means “smells like a rock.” 

In other words, the springtime “smells like a rock” fragrance is mainly due to an alcohol that smells (at least to British scientists) like a rock. If you gave that kind of circular-reasoning answer on a science test, I doubt you’d get full credit for it.

To be fair, petrichor, the hallmark smell of spring, may involve other elements like plant oils, but the chief contributor is geosmin. We can smell geosmin at crazy-low levels, in the parts-per-trillion range. Apparently, we are 1,000 times better at detecting geosmin than a shark is at sensing blood at one part per million. You also get a whiff of petrichor just as a rainstorm breaks, and again right afterward. 

It took a while to figure out that geosmin is made by soil bacteria in the genus Streptomyces. If that name rings a bell, it’s because the vital antibiotic streptomycin comes from these bacteria.  In fact, Streptomyces bacteria give us more than two-thirds of all naturally-sourced antibiotics. It’s a little creepy to think that Streptomyces’ close relatives cause leprosy and tuberculosis, while other members of the family provide the cure. Talk about job security. 

Geosmin is one of several toxins that Streptomyces make to kill fungi, nematodes and other soil organisms that like to dine on them. Paradoxically, there’s one predator that they do want to come and devour them – getting eaten by this creature is the only way Streptomyces can survive.  

The “marauder” is tiny, about 1/16th to 1/8th of an inch long. Known as springtails, they crave alcohol. Specifically, geosmin, which they can safely digest without suffering ill effects. Found the world over, including the polar regions, springtails are hexapods (the non-robotic kind), related to insects. Sometimes they’re called snow fleas, as they like to bop around on the snow on mild winter days. Outside of winter, they burrow through soil in search of rotting stuff to eat. 

Springtails, which are some 8,000 times bigger than a bacterium, still find plenty of bacteria to eat. How do these critters, which presumably don’t use microscopes, find prey that’s too small to see? The answer is that Streptomyces form colonies of up to several inches across, resembling patches of mold. Easy pickings for the only living thing able to eat them and not die. 

The reason Streptomyces is in such a huff to use geosmin to lure springtails to gobble up their colonies is that springtails are the perfect vectors for bacterial spores. Essential to the formation of healthy topsoil, springtails can number around 100,000 individuals per cubic yard, and they range throughout the soil profile, from the surface to quite deep down. Their high numbers and roving natures help move Streptomyces away from older colonies, which die out as available food is depleted, to far-flung locales. Some of these new neighborhoods are just right to begin fresh bacterial outposts. 

Though the first bacteria appeared on Earth 3.5 billion years (give or take a couple, I imagine) ago, Streptomyces showed up about 450 million years back. Evidently, springtails date to around the same period, so these two organisms have had copious time to perfect their symbiotic ballet. Both of them are only active in the presence of moisture, which accounts for why we notice petrichor in spring when the ground is usually moist, as well as during and after rain events. 

Streptomyces has been quite generous to us in terms of the array of antibiotics derived from them, but springtails have gifted us with cool stuff, too. They make a unique protein-based antifreeze (that’s why they’re so active in winter) that may soon allow us to freeze human organs destined for transplants without damage to the organs. The same antifreeze compound is said to keep ice from forming on top of the ice cream in our freezer. Springtails are entertaining as well – they can bounce seven inches into the air, which is like one of us jumping roughly 500 feet straight up. Which would be fun, except for the return trip.

What it boils down to is that the pleasing balm of springtime (and rainstorms) is thanks to a group of bacteria that look like fungi, urgently want to be someone’s dinner, and make deadly poisons that we use as life-saving drugs. In turn, these bacteria rely on a coterie of insect-like critters who can’t say no to alcohol, excel in outdoor winter high-jumping, and enjoy subterranean rambles. A bit complicated, but at least it’s easy to get outside and enjoy the welcome spring petrichor.

A version of this essay first appeared in The Saturday Evening Post in April, 2023.

Photo at top: Wikimedia Commons photo.

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World




5 Responses

  1. Randy Fredlund says:

    Another entertaining and educational article. Thanks.

  2. Worth Gretter says:

    Incredible details of life on Earth, that we never think about!

  3. What an eye-opening read! Now that I know all this is going on while I stand and admire the spring rain smell, I’ll have a new, expanded appreciation for the symbiotic nature of our world. Thanks, Paul.

  4. Boreas says:

    Great article!

  5. Shawnee says:

    I almost passed by this article and that would have been my loss! I did not know any of this, all the while just taking the smell of spring for granted. Spring is richer to me, now. I hope you were or are a teacher/professor; you have a gift.

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