Fog Is Full of Microbes

“A fog droplet isn’t a bad environment to live in"

Fog rolling into the dunes of the Namib desert
Fog is one of the few sources of water in the Namib desert (Juliane Zeidler)

The Namib desert is one of the driest places on Earth. Its coast, however, is one of the foggiest. Some days, the fog—the result of a bone-chillingly cold ocean current that runs along the coast and condenses any water in the air above it—rolls inland to shroud the Namib’s famous sand dunes in white. “It’s such a dramatic landscape already,” says Sarah Evans. “Then you have this fog that’s there that looks otherworldly.”

The animals who live here have adapted to fog. The darkling beetle climbs to the top of sand dunes and sticks its butt in the air, letting fog condense on its back and drip into its mouth. Another beetle builds trenches that catch fog condensation. Microbes too rely on the moisture from fog to survive. Evans, a microbiologist at Michigan State University, was in the Namib studying some of these microbes when it occurred to her to wonder about the fog. If the fog could bring water and nutrients from the ocean to the desert, could it also bring microbes?

She decided to collect some fog. The improvised setup was pretty low tech. Her team sterilized a big sheet of metal and set it at a 45-degree angle, so that fog could condense on the surface and drip down into a trough. They took this water back to Michigan and sequenced the DNA of microbes inside it. They also set out petri dishes to pick up and grow microbes floating in the fog.

A foggy day in the Namib desert (Sarah Fitzpatrick)

When Evans later mentioned her experiment to Kathleen Weathers, a long-time fog researcher, she told Evans she had to meet Eli Dueker.

It turns out that Dueker, now a microbiologist at Bard College, had the same idea years ago while spending time on an island off the coast of Maine. On foggy days, Dueker would set up bioaerosol samplers, devices that suck air in, on a little spit that jutted into the ocean. Summer vacationers—curious and otherwise island-bound because of the thick fog—would come by to chat. Luckily for him, that summer was one of the foggiest in memory.

Evans and Dueker decided to compare notes. The sandy Namib desert and the rocky Maine shore are about as different as two coasts can be—yet the microbiology of their fogs had strikingly similar patterns.

For one, the fog at both sites was indeed bringing microbes that usually live in the ocean to land. The effect was more pronounced in Maine, where Dueker was collecting next to the coast, than in the Namid, where Evans was collecting 30 miles inland. The fog in both places also had a heavy proportion of soil microbes. The exact composition could also change day to day, which Evans chalked up to different weather conditions such as the direction of the wind.

DNA doesn’t tell you if the microbes in fog are actually alive though, which is why Evans and Dueker also set out petri dishes to catch microbes in the fog. The dishes eventually bloomed into colorful colonies of microbes. This wasn’t necessarily surprising—microbes can hitch rides in cloud droplets and particles in the air, but it confirmed that fog is a viable connection between marine and terrestrial ecosystems. “A fog droplet isn’t a bad environment to live in,” says Evans, especially compared to riding on a particle exposed to open air. In fact, microbes in fog droplets may be more viable than those in air. Evans and Dueker noticed fog seems to deposit more living microbes compared to ordinary air.

The microbial composition of the fog also seemed to change over a few hours of collecting. It could be, says Dueker, that certain microbes are living and multiplying in the fog during that time. It could also be that fog preferentially deposits certain microbes onto surfaces. The size of a microbial cell, its shape, and the structure of its outer membrane could all affect how likely a cell is deposited. In Maine, for example, Pseudomonas bacteria in fog decreased over time, and Pseudomonas are known to have membrane structures that encourage the formation of water droplets. Different Pseudomonas can inhabit a wide range of niches, with some infecting animals, plants, and even humans.

Dueker first got interested in microbes from a public-health perspective. He had studied aerosols from contaminated sewage. If fog picks up certain microbes from water and deposits them on land, it too could play a role in spreading disease. The vast majority of microbes the team found in fog is harmless to humans. But, he says, “there may be something to the old wive’s tale of fog being something you want to avoid.”