Just as animals depend on the microbiome in our guts to digest food, plants rely on microbial communities on their leaves. Biologists puzzled by how these organisms survive hot days in dry conditions have found the answer in microscopic droplets of water on apparently dry leaves, maintained both by the plants and the bacteria themselves.

A gardener may only think of plant-dwelling bacteria as disease-carriers. However, for hundreds of millions of years plants have been forming symbiotic relationships with leaf bacteria that provide protection against predators and increase stress tolerance. It's important we understand this relationship because agricultural practices, for example, the way we disperse pesticides, may disrupt the balance at our peril.

Few bacteria survive being dried out, so Hebrew University PhD student Moar Grinberg wondered how these bacteria – which average 10 million to a single leaf – manage when the Sun beats down. “While leaves may appear to be completely dry during the day, there is evidence that they are frequently covered by thin liquid films or micrometer-sized droplets that are invisible to the naked eye,” Grinberg said in a statement. “It wasn’t clear until now whether this microscopic wetness was enough to protect bacteria from drying out.”

In eLife, Grinberg describes creating artificial surfaces designed to replicate leaves, placing 13 bacterial species on them and exposing the bacteria to varying humidity. Although there is no shortage of real leaves to study, the authors noted their complexity, and thought artificial versions would make it easier to identify the features that matter.

Even when the objects appeared completely dry, microscopes revealed bacteria located in tiny droplets of moisture a few hundredths of a millimeter across, or huddling together in larger wet spots.

The droplets don’t evaporate because aerosol particles, including the bacteria themselves, absorb water vapor and hold onto it, a process known as deliquescence. This combines with capillary pinning, where surface roughness helps hold liquids, and the extra local humidity as the leaf transpires to do what is needed.

“We found that bacteria cells can survive inside these droplets for more than 24 hours and that survival rates were much higher in larger droplets,” said co-author Dr Tomer Orevi. By sticking together, the bacteria also help maintain the moisture for each other. Dependence on this huddling approach varied by species.

It’s a risky life, however. Many bacteria failed to make it through a moderately dry day, but those that did made up the numbers by multiplying when night brought renewed moisture, or during rain.

Plants produce a lot of aerosols, which can help bring rain to forested areas, but this research shows they may get a more localized benefit in keeping their bacterial buddies alive.