Clouds are not just weather phenomena but dynamic ecosystems teeming with life. Trillions of microorganisms—including bacteria, fungi, viruses, and single-celled organisms—circulate through the Earth’s atmosphere. These airborne travelers, collectively known as the aerobiome, have increasingly been recognized as essential players in both atmospheric chemistry and biological processes on Earth.
The concept of the aerobiome dates back to Louis Pasteur, who in the 1860s demonstrated the presence of airborne life. Despite early skepticism, the idea gained scientific traction in the 20th century through airborne sampling from planes, balloons, and later drones equipped with DNA-sequencing tools. Modern studies reveal that clouds are dense microbial habitats, harboring up to 100,000 cells per millimeter of cloud water. These cells originate from land and sea, lofted into the air by waves, winds, evaporation, and even forest fires. Fungi, for example, have evolved mechanisms to propel spores into the stratosphere, while mosses and plants release billions of pollen grains and spores seasonally.
One of the most important discoveries is that microbes not only survive in clouds but can thrive. Research from France’s Puy de Dôme mountain, led by aerobiologist Pierre Amato, found that bacteria trapped in cloud droplets often show higher RNA activity than those in clear air, indicating active metabolism. Certain bacteria like Methylobacterium can even consume organic carbon in clouds, essentially “eating” clouds and altering their chemical makeup. These biological processes influence the atmosphere, including the breakdown of carbon compounds and the formation of precipitation.
Biological particles also play a key role in cloud physics. Ice formation often requires a nucleus, and biological molecules like fungal spores and bacteria such as Pseudomonas are especially effective at initiating crystallization. This not only helps clouds form but also affects where and when rain or snow falls. Intriguingly, some scientists theorize a feedback loop where plants host ice-nucleating bacteria that rise into clouds, trigger rain, and in turn nourish the plants—an elegant example of biosphere-atmosphere interaction driven by the aerobiome.
Beyond weather, the aerobiome has serious health implications. Amato’s research uncovered bacteria in clouds carrying dozens of antibiotic resistance genes. These genes likely emerge from human and agricultural antibiotic overuse and are then lifted into the sky. A single cloud can hold trillions of such genes, which may later fall with rain, potentially spreading resistance across ecosystems. Studies even found resistant bacteria in car air filters in urban centers, confirming the global reach of this genetic material.
Some scientists extend these findings to astrobiology. Venus, despite its hostile surface, has cloud layers with temperate conditions. Researchers like Sara Seager speculate that life may exist in Venusian clouds, similar to Earth’s aerobiome.
The growing body of research shows that the sky is not empty but alive, and that clouds are both carriers and creators of life. As we continue to alter the biosphere, the aerobiome reminds us that what we do on Earth doesn’t just rise—it circles the globe, affects the weather, and returns to us in the rain.
https://www.bbc.com/future/article/20250610-the-microbes-that-thrive-in-the-clouds