Trillions of microscopic organisms - bacteria,
viruses, fungi and single-celled organisms - move around the planet, rising kilometers into the atmosphere. Research shows that these invisible wanderers have a significant impact not only on weather patterns, but also on human health.
Clouds that appear to be a commonplace part of the sky - from light, pinnate streaks to heavy thunderstorm masses - actually hide an unexpected truth. Behind their outward simplicity, they hide an amazing biospheric phenomenon. Scientists are increasingly viewing clouds not as mere clusters of water droplets, but as peculiar aerial ecosystems - "floating islands" inhabited by a multitude of living organisms.
These airborne micro-communities include thousands of different species that can survive, move, and even interact with each other in the extreme conditions of the upper layers of the
atmosphere. Many are able to affect condensation of moisture by participating in precipitation formation, and some are able to transport pathogens or allergens over long distances.
Aerobiome
Along with birds, dragonflies, and dandelion seeds, there is a vast ocean of microscopic organisms traveling through the air. French chemist Louis Pasteur was one of the first to recognize what scientists now call the aerobiome in 1860. He kept sterile flasks of broth and allowed floating microbes to settle in it, causing the clear broth to become cloudy.
Pasteur caught germs on the streets of Paris, in the French countryside, and even on top of a glacier in the Alps. But his contemporaries rejected the idea. "The world you want to transport us to is really too fantastic," one journalist told Pasteur at the time.
It took decades for people to recognize the reality of the aerobiome. In the 1930s, several scientists took to the skies in airplanes holding slides and petri dishes to catch fungal spores and bacteria in the wind. Balloon expeditions into the stratosphere also collected cells.
Today, 21st century aerobiology researchers are mounting sophisticated samplers on drones and using DNA sequencing technology to identify life in the air by its genes. The aerobiome, as researchers now recognize, is a vast habitat filled with guests. These guests come from most of the planet's surface.
Movement of microorganisms
Each time an ocean wave breaks, it throws small droplets of seawater into the air, some of which carry viruses, bacteria, algae, and other single-celled organisms. Some droplets fall quickly back into the ocean, while others are picked up by the wind and lifted into the sky, where they travel thousands of kilometers.
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On land, winds can erode the ground, spreading bacteria, fungi, and other organisms. Each morning when the sun rises and water evaporates into the air, it can also attract microscopic organisms. Forest fires create strong upward currents that can suck microbes out of the ground and pluck them from tree trunks and leaves, carrying them upward with the rising smoke.
Many species don't just wait for physical forces to lift them into the air. Mosses, for example, grow a stem with a pouch of spores at the tip, which they release into the air like trickles of smoke. In one summer, up to six million moss spores can fall on one square meter of marsh. Many species of pollinator plants reproduce by releasing billions of pollen grains into the air each spring. Fungi are particularly adept at flight.
Fungi have developed "biological cannons" and other means to launch their spores into the air, and their spores are equipped with tough shells and other adaptations to withstand the harsh conditions they face as they rise into the stratosphere.
The fungi have been found up to 20 kilometers high, high above the Pacific Ocean, where they are carried by the wind. According to one estimate, about a trillion trillion bacterial cells rise from land and sea into the sky each year. Others estimate that 50 million tons of fungal spores rise into the air during the same time.
Untold numbers of viruses, lichens, algae and other microscopic life forms also take to the air. They usually travel for several days before landing, and can travel hundreds or thousands of kilometers in that time.
During this journey, the organism may enter an area of air where water vapor condenses into droplets. It soon finds itself in one of these droplets, and upward currents may carry it deep into the water mass. He's caught in the
heart of the cloud.
Life in the clouds
Much of what scientists have learned about life in the clouds came from the top of a mountain in France called Puy de Dome. It was formed about 11,000 years ago when a torrent of magma rushed into the hills of central France, creating a volcano that spewed lava and then subsided after a few hundred years.
For the past twenty years, the weather station atop Puy de Dome has been equipped with air samplers. The mountain is so high that clouds regularly cover its summit, allowing scientists to capture some of the life they endure.
Research led by Pierre Amato, an aerobiologist at the nearby University of Clermont-Overgne, has shown that every millimeter of cloudy water floating over Puy de Dome contains up to 100,000 cells. Their DNA showed that some belonged to familiar species, but many were new to science.
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Scientists who use DNA to identify species are in constant fear of contamination, and Amato is no exception. For example, a hawk soaring over the Puy de Dome might fly over Amato's test tubes and shake germs off its feathers. In Amato's lab, a graduate student can exhale germs into a test tube.
Over the years, Amato has culled thousands of potential species, suspecting that he or his students had accidentally applied microbes from their skin to the equipment. But they definitely found more than 28,000 species of bacteria and more than 2,600 species of fungi in the clouds.
Amato and other scientists studying clouds suspect they may be a particularly favorable place for bacteria to survive - at least for some species. "Clouds are an environment open to all, but in which only some can thrive," Amato and his colleagues wrote in 2017.
For bacteria, a cloud is like a foreign world, dramatically different from the habitat where they normally live on land or in the sea. Bacteria tend to cluster together. In rivers, they can become microbial clusters. In the human intestine, they form a dense film.
But in a cloud, each microbe exists all alone, trapped in its own droplet. This isolation means that cloud bacteria don't have to compete with each other for limited resources. But the droplet doesn't have much room to carry the nutrients microbes need to grow.
Cloud research
However, Amato and his colleagues have found evidence that some microbes can indeed grow in clouds. In one study, the scientists compared samples they collected from clouds on the Puy de Dome with others collected on the mountain on clear days. The researchers looked for clues about their activity by comparing the amount of DNA in the samples to the amount of RNA. Active, growing cells create many copies of RNA from their DNA to make proteins.
The researchers found that the ratio of RNA to DNA was several times higher in clouds than in clean air, providing strong evidence that cells thrive in clouds. They also found that bacteria in clouds turn on genes needed for food metabolism and growth.
To understand how these bacteria can thrive in clouds, the researchers grew some of the species they captured in their lab and then sprayed them in chambers to simulate the atmosphere. One species of microbes, known as Methylobacterium, uses the energy of sunlight to break down organic carbon inside cloud droplets. In other words, these bacteria feed on clouds.
According to one estimate, cloud microbes break down a million tons of organic carbon worldwide each year, such findings suggest that the aerobiome is a force with a powerful influence on atmospheric chemistry and even weather changes, and a force to be reckoned with.
When a cloud forms, it creates upward currents that lift water-saturated air to high altitudes where it is cold enough for the water to turn to ice. The ice then falls back down. If the air near the ground is cold, it can fall as snow. If the air is warm, it turns into rain.
In a cold cloud, ice forms with surprising difficulty. Even at temperatures well below freezing, water molecules can remain liquid. However, one way to induce ice formation is to introduce an impurity.
When water molecules stick to the surface of a particle, they attach to each other - this process is called nucleation. Then other water molecules attach to them and assemble into a crystalline structure that, once heavy enough, falls from the sky.
Making it rain
It turns out that biological molecules and cell walls are exceptionally well suited for causing rain. Fungi, algae, pollen, lichens, bacteria and even viruses can create ice in clouds. It is even possible that clouds and life are linked in a tight cycle, and not only live and feed on clouds, but also help them form.
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One of the best rain makers is a species of bacteria called Pseudomonas. Scientists don't know exactly why these bacteria are so good at forming ice in clouds, but it may have something to do with the way they grow on leaves.
When cold rain hits the leaves, Pseudomonas can help the liquid water turn to ice at a higher temperature than normal. When the ice breaks down the leaves, the bacteria can eat the nutrients inside.
Some researchers have even suggested that plants welcome bacteria like Pseudomonas despite the damage they cause. When the wind blows the bacteria off the plants and up into the air, they rise into the clouds overhead. The clouds in which the Pseudomonas are seeded are rained on the plants below.
Plants use the water to grow more leaves, and the leaves support more bacteria that rise into the sky and spur clouds to shed even more water to nourish the life below. If this turns out to be true, it will be a majestic symbiosis connecting forests to the sky.
Clouds in space
The study of life in clouds also suggests that airborne organisms could exist on other planets - even those that might seem like the worst places for life to survive. For example, the surface temperature of
Venus is high enough to melt lead. But the clouds covering Venus are much cooler and may be capable of supporting life.
Sarah Seeger, an astrobiologist at the Massachusetts Institute of Technology, has suggested that life may have originated on the surface of Venus early in its history, when it was cooler and wetter. As the planet warmed up, some microbes may have found refuge in clouds. Instead of sinking to the surface, she said, they may have traveled in the atmosphere for millions of years, driving currents.
Reflecting on Seager's alien aerobiome may make cloud watching even more enjoyable. But as Amato's research has shown, clouds reflect on human influence on the world. When Amato and his colleagues study the genes of the microbes they capture, they find a startling number of genes that confer antibiotic resistance to bacteria.
Drug resistance
On earth, humans have influenced the widespread
evolution of resistance genes. By taking excessive amounts of penicillin and other drugs to fight infections, humans are contributing to the emergence of mutants that can resist them. Even worse, farmers feed antibiotics to chickens, pigs, and other livestock to make them grow to larger sizes.
In 2014 alone, 700,000 people worldwide died from infections caused by antibiotic-resistant bacteria; five years later, that number had risen to 1.27 million.
The evolution of antibiotic resistance occurs in humans and the animals they eat. The bacteria endowed with this resistance then leave their nurseries and enter the environment - the soil, streams and, as it turns out, even the air. Researchers have found high levels of resistance genes in bacteria floating around hospitals and pig farms.
But airborne resistance genes can spread even further. An international team of scientists tested car air conditioner filters in nineteen cities around the world. They found a rich diversity of resistant bacteria on the filters. In other words, it turns out that resistance genes are flying around cities.
In recent years, Amato and his colleagues have captured even farther journeys. In a 2023 study of clouds, they reported that they found bacteria carrying 29 different types of resistance genes. A single bacterium in the air can carry as many as nine resistance genes, each of which protects against drugs in a different way. Each cubic meter of cloud, they estimate, contains up to 10,000 resistance genes. In a typical cloud floating overhead, there could be more than a trillion of them.
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Amato and his colleagues hypothesize that the clouds contain such a large number of resistance genes because they help bacteria survive in them. Some genes provide resistance to antibiotics, allowing bacteria to quickly siphon the drugs from their insides, getting rid of them before they can cause harm. The stress of living in a cloud can cause bacteria to produce toxic waste, which they also need to pump out quickly.
Clouds can spread these resistance genes much farther than contaminated meat and water. Once in a cloud, bacteria can travel hundreds of kilometers in a matter of days before being caught in raindrops and falling back to Earth. Once on the ground, microbes can pass on their resistance genes to other microbes they encounter.
Amato and his colleagues estimate that 2.2 trillion trillion trillion resistance genes fall out of the clouds each year. It's a sobering thought to keep in mind while walking in the rain. People are standing in downpours of DNA of their own making.