It's a rainy day, so you stick out your tongue to catch a sip of nature's hydration. As the misty air hits your taste buds, millions of tiny soil bacteria suddenly cry out in terror and are suddenly silenced.
Gif: Gizmodo/Joung et al. Nature Communications
Thanks to the dirt beneath your feet, you just committed mass murder in your quest to taste the rain. Soil is one of the most vital substances on Earth; a digestive stew where trillions upon trillions of microorganisms break down plants and animals into tiny molecules, recycling their nutrients so that the circle of life can churn on.
But being a denizen of the dirt is not without hazard. For every raindrop that hits the ground, up to thousands of bacteria are catapulted into the air, trapped inside tiny beads of water. Some of these unwitting aeronauts fall right back to Earth, but other bacteria are swept skyward, potentially moving vast distances before finding a new home, according to Cullen Buie, a mechanical engineer at the Massachusetts Institute of Technology who has spent years studying the surprisingly beautiful physics of rainfall.
Buie's latest work, published this week in Nature Communications, finds that light rainstorms could be catapulting bacteria into the air en masse. If that sounds astoundingly esoteric and a little unsettling, just know that the discovery could have applications in everything from agricultural pest control to understanding the spread of infectious diseases.
It all started a few years back, when Buie and his colleagues used high-speed cameras to visualise raindrops striking a porous surface. They observed that when water droplets fall at speeds similar to those of a light rain, they trap tiny bubbles upon impact. Those bubbles then burst, releasing a cloud of even smaller water particles, called aerosols. "When you pour yourself a soda, you see that fizz, above the surface," Buie told Gizmodo. "That's the same process, only with CO2 bubbles bursting and releasing kinetic energy."
This initial study garnered a fair amount of attention when it was published back in 2015 -- not because of the public's deep interest in the physics of water droplets, but because it helped explain another phenomenon: that earthy aroma that hits your nose after a rainstorm. Buie and his colleagues hypothesized that this familiar scent, called petrichor, is the result of aromatic compounds from plants and soil bubbling skyward in the champagne-like effervescence of a rainstorm.
"After we published that [study], a researcher in the UK asked if we'd ever looked at bacteria," Buie said. "This person was familiar with a tropical disease caused by a soil bacteria, and they'd found that after rainfall, this disease had a higher prevalence."
"We were inspired by their work," to take a closer look, he added.
To see whether rainfall disperses bacteria the same way it disperses other molecules, Buie and his colleagues conducted a series of experiments, dropping water at varying speeds on soils with different levels of sand and clay, spiked with three species of bacteria marked with fluorescent tags. Sampling plates positioned above the surface were used to catch any aerosols, including bacteria, which were then examined under microscopes, and cultivated in petri dishes.
The researchers wanted to see whether bacteria survived the ordeal of being catapulted into the air. Most of the time, they did.
"We found that after an hour, many microbes were still cultivable," Buie said. "If they were swept into the wind for an hour, they could land in a new location and colonise that surface." Extrapolating from his results, Buie estimates that rainfall could be responsible for kicking up 800,000 trillion soil bacteria into the sky ever year, which is not too far from one astronomer's estimate of the number of stars in the observable universe.
The next step is figuring out how far airborne bacteria travel in nature, and plugging this information into models to solve problems. In addition to understanding how tropical diseases spread, Buie hopes agricultural researchers might use his insights to figure out the best ways to quarantine pest-infected plants. Other scientists not involved with the study think Buie's discovery is pretty darn cool.
"This is an excellent work in all its experimental and discussion aspects," Alexander Laskin, environmental scientist at Pacific Northwest National Laboratory told Gizmodo in an email. "The significance of this mechanism at regional and global scales need to be addressed in future studies."
"The shear number of cells estimated to be transferred by this rain-induced bubble-bursting mechanism is noteworthy and highlights the need to better understand both the aerosol formation and its eventual fate," James Bird, a fluid dynamics researcher at Boston University added.
Perhaps the most salient takeaway for the average person, however, is the simple recognition that a rainstorm doesn't just nourish the Earth -- it seeds the air with life, too.