It’s Alive? Surprising Discovery Changes What We Know About Fog

Fog might be more active than it seems. New research shows that tiny fog droplets can become temporary homes for living bacteria. These bacteria actively grow and consume pollutants.

Scientists have known for a long time that bacteria float in clouds and air. But new studies suggest some microbes don't just survive in fog; they thrive there.

Researchers at Arizona State University found that fog droplets act like tiny floating habitats. In these habitats, bacteria grow, multiply, and even remove harmful pollutants from the air. This discovery changes how we view fog. It suggests fog is not just passive moisture but an active microbial environment.

Thi Thuong Thuong Cao, a cloud researcher, started this discovery by asking if fog could support life. For her PhD, Cao worked with experts in microbiology, chemistry, and atmospheric science. She collected fog samples in Pennsylvania before sunrise and studied them in the lab.

What she found surprised the team: some bacteria in fog droplets were not just dormant. They were actively growing.

Fog as a Tiny Water World

Fog forms when water vapor turns into small liquid droplets in cool air. Each droplet is tiny, but together they create a unique environment. This environment can briefly act like a miniature water ecosystem.

Scientists have found bacteria in the atmosphere before, including in rain clouds and dust. Some microbes can even survive harsh conditions like UV radiation and freezing temperatures. But it was unclear if they stayed active inside fog.

Cao, the lead author, noted that there's little known about bacteria in fog, which is like clouds at ground level.

The researchers focused on two main questions: what bacteria are in fog, and do they actually grow there?

Ferran Garcia-Pichel, a co-author, said that if bacteria are growing, then the droplets are a habitat. This changes how scientists think about fog.

The team found that bacteria appear in less than 1% of fog droplets. However, because fog has so many droplets, the total number of bacteria is surprisingly large.

Garcia-Pichel explained that the concentration of bacteria in all the droplets combined is similar to that in the ocean. A small amount of fog water, about the size of a thimble, can hold around 10 million bacterial cells.

Pollution-Eating Bacteria in Fog

One type of microbe, methylobacteria, caught their attention.

These bacteria became more common during foggy conditions compared to dry air. This is important because methylobacteria eat simple carbon compounds, like formaldehyde. Formaldehyde is a toxic pollutant linked to smog and breathing problems.

Formaldehyde enters the air from car exhaust, factories, wildfires, and some household products. In cities, it helps create ground-level ozone pollution.

The study suggests that fog droplets might become small reaction chambers. Here, bacteria help remove some of these pollutants from the air.

Cao observed that the bacteria were growing and dividing. They were also using formaldehyde as food to support their growth.

The bacteria processed formaldehyde so well that researchers first thought other chemical reactions were involved. But it seemed the microbes were breaking down the pollutant both for food and to protect themselves. High levels of formaldehyde can be toxic even to bacteria.

By changing formaldehyde into carbon dioxide, the microbes reduce harmful buildup inside the droplets.

Why Scientists Studied a Special Kind of Fog

Studying airborne bacteria is hard because wind constantly moves the air. To see how microbe populations changed over time, researchers needed fog that formed in stable air.

They focused on radiation fog. This type of fog forms when the ground cools overnight and chills the air above it. Water vapor then condenses into fog close to the surface, especially in humid, calm valleys.

Unlike fast-moving storms, radiation fog can stay stable for hours. This made it easier to observe changes in microbes before, during, and after the fog formed.

This work shows how much scientists still don't know about the biological side of the atmosphere. Most atmospheric models focus on chemistry and physics. Living microorganisms are often seen as passive particles.

But microbes might be doing much more than scientists realized.

Tiny Microbes Could Affect Air Quality and Climate

These findings could impact several research areas, including pollution studies, climate science, and even how we collect drinking water.

In some dry areas, special mesh systems collect fog water for drinking. Fog is often thought to be naturally clean. However, this study suggests fog droplets can carry active microbial communities, so the water should likely be treated before drinking.

The research also raises bigger questions about the atmosphere. If bacteria are active in fog and clouds, they might affect chemical reactions that shape air quality and possibly even weather patterns.

Cao noted that it's important to consider that bacteria grow inside these droplets, not just drive chemical reactions. This could change how scientists model everything.

Scientists are just starting to explore these atmospheric ecosystems.

Pierre Herckes, a co-author, said that studying biological activities in clouds is relatively new. There's still a lot to learn. For example, not much atmospheric chemistry happens at night, as it's driven by the sun. But if bacteria are active at night, they could be important.

Many questions remain. Researchers don't yet know how microbial communities differ between coastal fog, mountain fog, urban smog, and high-altitude clouds. Scientists also want to understand if these airborne microbes affect rainfall, pollution cycles, or human health in unknown ways.

Deep Dive & References

Growth and formaldehyde degradation of photoheterotrophic Methylobacterium within radiation fogs - mBio, 2026