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All told, from fires and dust storms, from crashing ocean waves and hurricanes, a trillion trillion bacteria cells are emitted into the air each year—a mass of more than 100,000 tons. Close to the same number of fungal spores are released as well, and with their bigger size, they weigh in at about 50 million tons. What comes up sooner or later comes down. The aerobiome—the Earth’s entire habitat of airborne creatures—is a peculiar realm: an ecosystem of visitors. A flea may hop into the air for a second, a diatom may be carried by the wind for days before falling back into an ocean, and a common swift can fly for ten months before landing to build a nest. But sooner or later, they all return to Earth. By one estimate, a single square meter of ground may be pelted with 100 million bacteria during every hour of a rainstorm.
While the aerobiome is transient, scientists now recognize it as a distinct zone of life, one that follows its own ecological rules and that exerts a powerful influence on the planet below. One of the best places to appreciate its power is a mountain in France called Puy de Dôme. Since 2003, a scientist named Pierre Amato and his colleagues have regularly climbed to its peak to study the life that floats through the air.
Puy de Dôme formed about 11,000 years ago when a fist of magma punched up into the rolling hills of central France. It created a volcano that spilled out lava before going dormant. As a graduate student at the nearby University of Clermont Auvergne, Amato would visit the mountain when clouds hid its peak. He would reach the summit and then climb onto the roof of a weather station. Surrounded by a blank white glare, Amato would open sterile metal tubes to capture the mist. He would then ferry the cloud water back down the mountain to the university.
Air-Borne: The Hidden History of the Life We Breathe
Award-winning New York Times columnist and author Carl Zimmer leads us on an odyssey through the living atmosphere and the history of its discovery.
When Amato analyzed the droplets, he discovered that they contained microbes. After finishing his degree, Amato became a staff scientist at the university and began bringing students of his own to the mountaintop. Over the years, Amato got better at counting microbes. He learned how to spot the proteins each cell was making in the air. He began to sequence the DNA of the organisms to determine which species they belonged to.
Clouds, Amato demonstrated, are alive. Every teaspoon of mist floating over Puy de Dôme contains several thousand microbes. While many are dead airborne husks, some are still alive. They make new proteins and destroy old ones. They grow in the clouds and even divide in two. Their DNA has revealed that some belong to familiar species, but many are new to science. Scientists who use DNA to identify species have to stay perpetually anxious about contamination, and Amato is no exception. A hawk soaring over Puy de Dôme might fly over Amato’s tubes and shake microbes off its feathers. In Amato’s lab, a graduate student may exhale germs into a test tube. Over the years, Amato has rejected thousands of potential species, suspicious that he or his students smeared skin microbes onto the equipment. Yet a single cloud, by his estimation, can contain thousands of species.
Amato and other scientists who study clouds suspect that they may be particularly good places for bacteria to survive—at least for some species. “Clouds are environments open to all, but where only some can thrive,” Amato and a team of colleagues wrote in 2017. The successful species are the ones that can thrive on the food in the atmosphere. Every organism on Earth has to make the same kind of fuel: a molecule called adenosine triphosphate, or ATP for short. As organisms ascend thousands of feet, they use up the fuel they produced before the flight when they were sitting happily on a pine needle or immersed in a bog. But Amato has found that bacteria in clouds have a healthy supply of ATP—one that they must be making as they float. Cloud droplets contain many different molecules, and some microbes are able to use them to grow. In other words, bacteria eat clouds. Worldwide, by one estimate, cloud microbes break down a million tons of organic carbon every year.
Findings such as these suggest that the aerobiome is a force to be reckoned with—one that exerts a powerful influence on the chemistry of the atmosphere. The aerobiome even alters the weather.
As a cloud forms, it creates updrafts that lift water‑laden air to high altitudes that are cold enough to turn the water to ice. The ice then falls back down. If the air near the ground is cold, it may land as snow. If it is warm, it turns to rain. But it can be surprisingly hard for ice to form in a frigid cloud. Even at temperatures far below the freezing point, water molecules can remain liquid. A seed of impurity is required. As water molecules stick to its surface, they bond to one another. Other water molecules then lock onto them and assemble into a crystal structure. Scientists have found that fungi, algae, pollen, lichens, insects, bacteria and viruses are especially good at encouraging water to freeze. The life that floats in clouds seeds much of the rain and snow that falls back to Earth.
It’s possible that clouds and life are linked in an intimate cycle. It turns out that one of the best rainmakers is a type of bacteria called Pseudomonas. Scientists are not sure why those bacteria in particular are so good at forming ice in clouds, but it could have to do with the way they grow on leaves. When cold rain falls on a leaf, Pseudomonas may help the liquid water turn to ice. As the ice cracks open the leaves, the bacteria can feast on the nutrients inside.
Some scientists have even speculated that plants welcome bacteria like Pseudomonas, despite the damage they cause. As the wind blows the bacteria off the plants and lofts them into the air, they rise into clouds overhead. Clouds seeded with Pseudomonas pour down more rain on the plants below. The plants use the water to grow more leaves, and the leaves support more bacteria, which rise into the sky and spur clouds to rain down even more water to nurture life below.
Research on the life in clouds also raises the possibility that airborne organisms might exist on other planets—even ones that might seem the worst places for life to survive. Venus, for example, has a surface temperature hot enough to melt lead. But the clouds that blanket Venus are much cooler, and at an altitude of 30 miles, they have the same temperature and pressure as clouds on Earth. Sara Seager, an astrobiologist at the Massachusetts Institute of Technology, has speculated that life might have arisen on the surface of Venus early in its history, when it was cooler and wetter. As the planet heated up, some microbes found refuge in the clouds. Instead of sinking back to the surface, they might have bobbed up and down in the atmosphere, riding currents for millions of years.
Above the Earth’s clouds, the aerobiome ebbs into the unknown. In 1935, American explorers piloted a balloon called Explorer II to the stratosphere. They captured bacteria and fungal spores from the black sky 12 miles above Earth’s surface. In the decades since, a few teams of scientists have searched at even higher altitudes. In 1974, Soviet scientists fired rockets from the steppes of Kazakhstan into the upper atmosphere. The rockets broke apart to release sterile microbe traps. The traps parachuted down to Earth, and inside them the scientists found bacteria and fungi. One of their four rockets found life 48 miles above the planet, three times higher than the record setting high-altitude balloon Explorer II reached in 1935. Aerobiologists today are leery about embracing that record. The Soviet rockets actually went beyond the stratosphere to another layer, called the mesosphere, where meteors falling toward Earth burn into shooting stars. It hardly seems like a place where life could endure. But more recent studies have confirmed findings that the stratosphere is alive. Over the years, NASA has launched a number of balloons that have found life as high as 25 miles.
Even if microbes don’t ascend to the mesosphere, getting 25 miles into the air is an impressive feat. For one thing, the physics of the atmosphere makes it hard for a microbe to reach that height. The wind that stirs microbes up from land and sea is almost entirely confined to the troposphere, the lowest layer of the atmosphere.
It’s possible that massive thunderheads can ram their way above the troposphere and spray microbe‑loaded droplets into the stratosphere above. And some microbes may get catapulted there by volcanoes. A NASA research jet once captured microbes as it screamed across the lower stratosphere. Among the organisms the jet caught, microbiologists identified a species called Bacillus luciferensis, which gets its diabolical name not from the devil but from Lucifer Hill, a very active South Atlantic volcano on Candlemas Island. The bacteria that NASA trapped in the stratosphere were 99 percent genetically identical to ones growing on top of Lucifer Hill, which were first found by a team of researchers with the British Antarctic Survey in 1996.
However microbes get to the stratosphere, they end up in what may be the most ruthless environment on Earth. Gases become wispy, water practically nonexistent. In the stratosphere, microbes can be ravaged by ultraviolet light, fast‑moving subatomic particles blasted out from the sun and cosmic rays streaming in from other parts of the galaxy. The collisions can destroy genes and proteins alike. It’s possible that the microbes that manage to reach the stratosphere are equipped with proteins that repair radiation damage. They may also survive by hiding on the shady side of dust grains. And they may then return to Earth, to the land or the sea, to continue to multiply and create more life that may have a chance to rise back up into the air. Whatever their secret, those stratospheric voyages mark the outer limits of the aerobiome—and thus of life as we know it.
Excerpted from Air-Borne: The Hidden History of the Life We Breathe by Carl Zimmer. Published on February 25, 2025, by Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright (c) 2025 by Carl Zimmer.
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