In my novel A Diary in the Age of Water, the diarist, Lynna Dresden, writes in her entry for September 9, 2048:

Petrichor: A pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather. The rain helps release plant oils into the air and chemicals produced by soil-dwelling bacteria called actinomycetes. The term arose from the Greek petra (“stone”) and ichor, the fluid that flows in the veins of the gods.

I know that musty, barky aroma of fresh rain on the dry earth. I know it well. It was the scent in the air when I gave birth to Hildegard seven years ago. She was born on the first rain in forty days. It wouldn’t rain for another forty.

Despite the nurses at the hospital discouraging me, I took my baby girl outside and let the fresh rain spatter our faces and soak us through. Little Hilde loved it. She cooed and watched with the awe of innocence. She’s still so innocent and still loves water.

She’s in Grade Two now and doesn’t comprehend that it isn’t God who controls the weather and the rain; it’s CanadaCorp.

Raindrops in a creek, ON (photo by Nina Munteanu)

Rich Hardy of New Atlas adds that a recent study in Nature Microbiology demonstrates that the bacteria evolved to release this chemical to attract a particular arthropod as a way to spread their spores. “The smell is a 500-million-year-old example of chemical communication, evolved to help a particular type of bacteria spread,” writes Hardy.

The term petrichor was created by Isabel Joy Bear and Richard G. Thomas, two Australian researchers after an article in Nature, in which authors described how the smell originates from an oil produced by plants during dry periods. The oil is released into the air along with geosmin, a metabolic by-product of certain actinobacteria when it rains.

One major component of petrichor is an organic compound called geosmin. Scientists already knew that a common genus of bacteria, known as Streptomyces, produce geosmin. Virtually all species of Streptomyces release geosmin when they die; but until now it was unclear why they generated this distinctive scent.

“The fact that they all make geosmin suggested that it confers a selective advantage on the bacteria, otherwise they wouldn’t do it,” says Mark Buttner, one of the researchers. “We suspected they were signaling to something and the most obvious thing would be some animal or insect that might help distribute the Streptomyces spores.”

Researchers discovered that geosmin specifically attracts springtails—a type of tiny arthropod—that use their antennae to sense chemical. The Streptomyces serve as food for the springtails; the springtails in turn help spread the bacterial spores. This suggests that the two organisms co-evolved in a symbiotic relationship. “This is analogous to birds eating the fruits of plants; they get food but they also distribute the seeds, which benefits the plants,” said Buttner.

Streptomyces—one of the most important sources of antibiotics to humans—relies on the springtail, given that its antibiotic compounds make it toxic to other potential distributors such as fruit flies or nematodes. Springtails generate a number of enzymes that can detoxify the antibiotics produced by Streptomyces. This suggests a form of aggressive symbiosis in a co-evolution between these two organisms—an interaction that is more common in Nature than previously thought.

The earthy scent of rain on dry soil evokes wonderful memories of playful childhood, freedom and awestruck wonder. Why not? Petrichor is, after all, the fluid that flows in the veins of the gods…

For more on “ecology” and a good summary and description of environmental factors like aggressive symbiosis, co-evolution, and other ecological relationships, read my book “The Ecology of Story: World as Character” (Pixl Press, 2019).

Glossary of Terms: 

Aggressive Symbiosis: a common form of symbiosis where one or both symbiotic partners demonstrates an aggressive and potentially harmful effect on the other’s competitor or potential predator (Ryan, 1997). 

Co-evolution: when two or more species reciprocally affect each other’s evolution through the process of natural selection and other processes. 

Petrichor: A pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather. The rain helps release plant oils into the air and chemicals produced by soil-dwelling bacteria called actinomycetes. The term arose from the Greek petra (“stone”) and ichor, the fluid that flows in the veins of the gods. 

Symbiosis: Greek for “companionship” describes a close and long term interaction between two organisms that may be beneficial (mutualism), beneficial to one with no effect on the other (commensalism), or beneficial to one at the expense of the other (parasitism). (Munteanu, 2019).

References:

Frazer, Jennifer. 2015. “Root Fungi Can Turn Pine Trees Into Carnivores—or at Least Accomplices.” Scientific American, May 12, 2015. Online: https://blogs. scientificamerican.com/artful-amoeba/root-fungi-can-turn-pine-trees-into- carnivores-8212-or-at-least-accomplices/

Hardy, Rich. 2020. “The 500-million-year-old reason behind the unique scent of rain” New Atlas.

Munteanu, N. 2019. “The Ecology of Story: World as Character.” Pixl Press, Vancouver, BC. 198pp. (Section 2.7 Evolutionary Strategies)

Munteanu, N. 2020. “A Diary in the Age of Water.” Inanna Publications, Toronto. 300pp.

Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press (Vancouver) was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” will be released by Inanna Publications (Toronto) in June 2020.