On a lazy summer day, you might look for animal shapes in the clouds, but you know they’re not real. There’s nothing alive up there but the occasional flock of airline passengers, right? Well, if you’ve ever had a microbiologist friend remark that a cloud reminded them of a bacterium, they were more right than you might have realized.
In turns out that microbes commonly hitch a breezy ride into the clouds. And while some organisms are simply biding their time, awaiting a return to familiar terra firma, others are actively going about their business despite their unorthodox surroundings. That’s no small feat considering the conditions—it’s extremely cold, UV radiation is intense, and the tiny droplets of water they often find themselves inside are acidic and chemically caustic.
Yet experiments have shown that bacteria appear to be active. And that might be important for more than just the microbes themselves. Lots of chemistry goes on in that cloud water that modifies the way clouds form and behave.
Very small particles in the atmosphere serve as “cloud condensation nuclei”—the seeds that can facilitate the formation of the droplets that make up visible clouds. Within these tiny spheres of water, dissolved substances can interact and affect the physical properties of the droplet—from the way it reflects light to the likelihood that the droplets will grow large enough to cause precipitation. Reactions can also form new particles with the ability to act as cloud condensation nuclei if the droplet evaporates instead of falling to Earth.
Past research has found that bacteria (which can serve as condensation nuclei themselves) in cloud water are capable of taking part in that chemistry, despite the cold temperatures and ultraviolet radiation. However, the experiments done so far have left out an important ingredient in the harsh cloud water cocktail—the hydrogen peroxide and hydroxyl radicals produced by UV light. These chemicals are toxic to cells.
To take the next step, a group of French researchers collected cloud water samples from high atop Puy de Dôme, a volcanic peak in central France. Half the samples were filtered to remove organisms, to serve as a baseline for comparison. Some of the samples were exposed to ultraviolet light (to drive the creation of free radicals) and the others were kept in the dark.
The chemistry of the water was analyzed every 12 hours for a week. Measurements of ATP and ADP, components of the metabolic energy cycle of bacteria, showed that the organisms were indeed alive and well throughout the week despite the caustic chemistry.
In all the samples, hydrogen peroxide declined over time. (Even without ultraviolet light to split it, hydrogen peroxide will react with other compounds.) However, it was clear that the bacteria were breaking down a significant amount of the hydrogen peroxide themselves. This is likely a kind of coping mechanism—cells chemically stressed in this way produce an enzyme that helps neutralize oxidants like hydrogen peroxide and hydroxyl radicals.
… So then, it’s likely that bacteria really are active in clouds. And they’re not just bit players—they have a significant effect on the chemistry of cloud water. They’re controlling the concentration of hydroxyl radicals and forming particles that could be future condensation nuclei (making clouds that would be more reflective and less likely to generate precipitation). In order to fully understand the behavior of clouds, researchers are going to have to pay attention to the vagabond microbes eking out a living at 5,000 feet.
On this site I use the word “aliens” interchangeably with “bacteria” due to my belief that bacteria came to Earth from space.