Clouds are our lifelong companions. Sometimes they drift overhead as wispy filigrees. On other days, they darken the sky and dump rain on us. But for all our familiarity with these veils of water vapour, they have been keeping a secret from us. Clouds are actually floating islands of life, home to trillions of organisms from thousands of species.
Along with birds and dragonflies and dandelion seeds, a vast ocean of microscopic organisms travels through the air. The French chemist Louis Pasteur was among the first scientists to recognise what scientists now call the aerobiome in 1860. He held up sterile flasks of broth and allowed floating germs to settle into them, turning the clear broth cloudy. Pasteur captured germs on the streets of Paris, in the French countryside and even on top of a glacier in the Alps. But his contemporaries balked at the idea. "The world into which you wish to take us is really too fantastic," one journalist told Pasteur at the time.
It took decades for people to accept the reality of the aerobiome. In the 1930s, a few scientists took to the sky in airplanes, holding out slides and Petri dishes to catch fungal spores and bacteria in the wind. Balloon expeditions to the stratosphere captured cells there as well. Today, 21st-Century aerobiologists deploy sophisticated air-samplers on drones and use DNA-sequencing technology to identify airborne life by its genes. The aerobiome, researchers now recognise, is an enormous habitat filled only with visitors.
Those visitors come from much of the planet's surface. Each time an ocean wave crashes, it hurls fine droplets of sea water into the air, some of which carry viruses, bacteria, algae and other single-celled organisms. While some of the droplets fall quickly back to the ocean, some get picked up by winds and rise up into the sky, where they can be carried for thousands of miles.
On land, winds can scour the ground, lofting bacteria and fungi and other organisms. Each morning when the sun rises and water evaporates into the air, it can draw up microscopic organisms as well. Forest fires create violent updrafts that can suck microbes out of the ground and strip them off the trunks and leaves of trees, carrying them upwards with the rising smoke.
Many species do not simply wait for physical forces to launch them into the air. Mosses, for example, grow a stalk with a pouch of spores at the tip, which they release like puffs of smoke into the air. As many as six million moss spores may fall on a single square metre of bog over the course of one summer. Many species of pollinating plants have sex by releasing billions of airbourne pollen grains each spring.
Fungi are particularly adept at flight. They have evolved biological cannons and other means for blasting their spores into the air, and their spores are equipped with tough shells and other adaptations to endure the harsh conditions they encounter as they travel as high as the stratosphere. Fungi have been found up to 12 miles (20km) up, high above the open ocean of the Pacific, carried there on the wind.
By one estimate about a trillion trillion bacterial cells rise each year from the land and sea into the sky. By another estimate, 50 million tonnes of fungal spores become airborne in that same time. Untold numbers of viruses, lichen, algae and other microscopic life forms also rise into the air. It's common for them to travel for days before landing, in which time they can soar for hundreds or thousands of miles.
A Guest Editorial