A new study has found that fog isn’t a sterile suspension of water droplets. It’s a habitat.

The mist hanging over the street on a grey morning contains living, growing, dividing bacteria – and some of them are quietly eating the pollution in the air around you.

The research was led by Thi Thuong Thuong Cao, who started the project as a PhD student in the School of Molecular Sciences and has since moved to Virginia Tech as a postdoctoral researcher.

Bacteria inside of fog

Scientists have known for a while that bacteria travel through the atmosphere, hitching rides in air currents, turning up in clouds, drifting across continents.

What’s been much less clear is what they actually do once they get there. 

Are they dormant, slowly dying, or carrying on as normal, eating and growing and dividing?

Fog has been particularly understudied. “There’s very limited knowledge about what kinds of bacteria are present in fogs, which are like clouds at the ground level,” Cao said.

She set out to answer two questions: which bacteria are present in fog, and are they actually alive and growing inside the droplets? The second question, it turned out, was the one that changed everything.

“If they are growing, then the droplets are a habitat. That’s a mindset change,” Garcia-Pichel said.

An ocean of life in the air

Fewer than one percent of individual fog droplets contain any bacteria. That sounds like almost nothing. 

But fog contains an almost unimaginable number of droplets, and when you add them all up, the picture shifts completely.

“When you take all of the droplets together, the concentration of bacteria is the same as in the ocean,” said study co-author Ferran Garcia-Pichel.

A thimble of fog water holds around ten million bacteria. The soft grey mist you walked through this morning was, in microbial terms, about as dense with life as the sea.

One group stood out in the samples: methylobacteria. Air collected before fog events contained fewer of them than air collected after, which suggests fog actively encourages them to multiply.

Methylobacteria have a diet that turns out to be very convenient for the rest of us.

They eat simple carbon compounds, including formaldehyde, a common pollutant that contributes to ozone smog and causes genuine harm to human health.

How bacteria detoxify the air

To see what bacteria were actually doing inside fog droplets, Cao collected samples in the field, waking before dawn in Pennsylvania to catch the fog as it formed.

Back in the lab, she placed the samples under a microscope and watched the microbes grow and divide.

“We observed them under the microscope to see that, yes, the bacteria are getting bigger and they’re dividing, so there is growth,” Cao said. “We also found that they’re using the formaldehyde as food to support their growth.”

The bacteria were clearing the formaldehyde so fast, though, that simple eating didn’t fully explain it.

The team found that at high concentrations, formaldehyde becomes toxic to the bacteria themselves, so they break it down into carbon dioxide to keep their environment safe. 

The bacteria are not just consuming a pollutant. They’re detoxifying the air as an act of self-preservation and in doing so, making it cleaner for everyone else too.

A practical problem, cleverly solved

Studying fog turns out to be surprisingly difficult. To track how bacterial populations change during a fog event, you need to sample the same parcel of air before, during, and after. 

Wind makes that nearly impossible – the air you sampled five minutes ago has usually moved on.

The team’s solution was to focus on radiation fog, which forms on still, calm nights when the ground cools and the air just above it cools with it, until moisture condenses close to the surface. 

This tends to happen in quiet valleys where the air barely moves. The same air stays put long enough to sample properly across the full arc of a fog event.

Implications for fog harvesting

Fog harvesting – collecting fog as drinking water – is being explored by communities in water-scarce regions. It’s often presented as a clean, natural source. 

This research suggests it should be treated like any other water: tested and purified before drinking. 

Fog water isn’t sterile. It contains living bacteria that have been breaking down chemical pollutants, and nobody yet knows exactly what’s in it or in what concentrations.

“If we harvest fog, we are getting rid of our little friends in the air,” Garcia-Pichel said. “We don’t know if that’s going to make a big impact or not, but we should be considering that.”

What happens to bacteria at night?

The models that scientists use to understand atmospheric chemistry are largely built around reactions driven by sunlight. Bacteria, though, don’t stop working when it gets dark.

“At nighttime, there isn’t that much atmospheric chemistry going on. Chemistry is largely driven by the sun and by light,” said study co-author Pierre Herckes.

“But if the bacteria are still doing their thing even during the nighttime, they can be important.” 

“It may be important to consider that besides driving chemical reactions, bacteria also grow inside these droplets,” Cao added. 

“It can change the story – rather than just a catalyst, they have other activity there. It can change the way we model everything so far,” Cao said.

Many questions remain

This study opens more questions than it closes. Do fogs in different places harbor different microbial communities? 

What else might those bacteria be consuming? How much does fog actually contribute to cleaning urban air overall? 

How does that weigh against the potential of harvesting it for water? Nobody knows. The field of biological activity in clouds and fog is still relatively new, noted Herckes. 

What the study does is establish something that changes the baseline. Fog is a living system, doing quiet, useful work – and we’ve been walking through it our whole lives without knowing.

The study was published in the journal mBio.

NOTE – This article was originally published in Earth and can be viewed here