Fog can feel empty. This one isn’t.

Researchers at Arizona State University and Susquehanna University found bacteria are living and growing inside water droplets in fog, at concentrations comparable to seawater, according to a study published in mBio.

The team collected air samples before, during and after fog events on 32 occasions over a two-year period. To limit the effect of wind on the readings, they focused on radiation fog, which forms in calm, still air overnight.

“There’s very limited knowledge about what kinds of bacteria are present in fogs, which are like clouds at the ground level,” said Thi Thuong Thuong Cao, a microbiologist at Arizona State University.

The researchers detected a sizable microbiome in the fog. Bacteria were present in less than one percent of fog droplets, but that still averaged about 1 million 16S rRNA gene copies per millilitre of water, a common marker used to estimate bacterial abundance.

“When you take all of the droplets together, the concentration of bacteria is the same as in the ocean,” said Ferran Garcia-Pichel, a microbiologist at Arizona State University.

Genetic analysis showed bacteria in the Methylobacterium genus dominated the samples. The researchers also found signs the microbes were active, not simply drifting through.

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

In a subsample of six fog events, the team found that after the fog cleared, the air still contained about 45 percent more bacteria than at the same location before the fog settled in.

“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.

Methylobacteria are known to consume volatile carbon compounds such as formaldehyde. The researchers tested fog water samples and measured how levels of those compounds changed over time.

“Existing formaldehyde at the start of the incubation was swiftly consumed to undetectable levels,” the researchers wrote, “roughly 200-fold faster than rates measured elsewhere in cloud water.”

The team said that rate was too fast for the compound to act only as a food source, and was probably also used for “detoxification purposes” because high levels of formaldehyde can be toxic to the bacteria.

Those compounds are also pollutants for people, raising the possibility that the fog microbiome may help clean the air. The researchers said more work is needed to determine how beneficial that effect is outside the lab.

“The sky’s the limit,” Garcia-Pichel said.

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