Cloudy, With a Chance of Bacteria

Germophobes, beware—that cloud on the horizon contains more than just rain. The atmosphere is filled with airborne bacteria, and a new study finds that even in the harsh conditions inside a storm cloud, some bacteria manage to survive—and may even reproduce and grow. “Storm clouds are among the most extreme habitats on earth where microbial life exists,” says Tina Santl-Temkiv, a postdoc at Aarhus University in Denmark and one of the authors of a paper published in the journal PLOS One.

Santl-Temkiv and colleagues sampled the insides of a storm cloud by collecting hailstones, scooped from the ground in sterile bags within five minutes of a 2009 storm. The team was interested in airborne bacteria, and a serendipitous phone call set this experiment in motion. “It was by pure chance that this happened,” says Santl-Temkiv. “I was talking on the phone to a friend from Slovenia while the storm happened, so I asked him to go out and collect the hailstones for me.” The friend’s speedy work preserved the large icy spheres, each around 3-5 cm in diameter, as frozen time capsules of cloud conditions.

Only about one in every million tiny cloud droplets contains a bacterial cell, according to the researchers. When lots of those droplets coalesce into a hailstone, however, that can add up to thousands of bacterial cells per milliliter of liquid. The researchers found many species of bacteria in the hailstones, as well as big differences in bacterial communities from one stone to another. But the majority of species were varieties normally found either in soil or on plants, swept into the skies from the ground. Dissolved organic material from soil particles carried in the air can provide nutrients for the bacteria to live on while aloft.

When the researchers tried to cultivate the bacteria they recovered, they learned that strains normally associated with plants had an edge in viability. “Bacteria that grow on plant surfaces have to deal with extreme conditions such as dryness, high UV-radiation, and large variations in temperature,” says Santl-Temkiv. “Being adapted to these stressful conditions can help plant-surface bacteria to survive and remain active in the atmosphere.” While bacteria need liquid to be active, the plant-surface bacteria may even be able to persist after a cloud dissipates, floating freely in the air until they find a new liquid home, she says.

So are blue-sky days less bacteria-ridden than cloudy days? Not necessarily. “The numbers of bacteria getting from their ground environments into the air depends on the weather conditions,” says Santl-Temkiv. “It may well happen that on a sunny day the surface air gets heated and starts rising, bringing with it bacteria from the ground. As a consequence, there may be a larger number of bacteria ending up in the atmosphere.”