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Study from Arizona State University shows that fog harbors living bacteria, capable of growing inside droplets and decomposing formaldehyde, a pollutant linked to ozone formation, revealing that this common phenomenon may have a silent role in air cleaning
Fog is a living system that harbors growing bacteria and can help clean air pollutants, shows a study from Arizona State University. The discovery changes the view on droplets near the ground, water, atmosphere, and hydroelectric plants.
The research was led by Thi Thuong Thuong Cao, who started the project as a doctoral student at ASU’s School of Molecular Sciences and later moved to work at Virginia Tech. The study was also published in the journal mBio.
The central point is that fog is not just a sterile suspension of water. It can function as a habitat, with living bacteria, capable of growing, dividing, and using compounds present in the air.
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Fog, bacteria, and hydroelectric plants
Scientists already knew that bacteria travel through the atmosphere, carried by air currents, appear in clouds, and cross continents. What was still unclear was what these organisms do when they reach these environments.
The question was whether they remained dormant, died slowly, or continued to be active, feeding, growing, and multiplying. Fog, described as clouds at ground level, was a little-studied part of this process.
Cao sought to identify which bacteria were present in the fog and if they were truly alive and growing inside the droplets. The second question became decisive for the team’s interpretation.
A microscopic ocean in the air
Less than 1% of individual fog droplets contain any bacteria. The proportion seems small, but the total number of droplets in a fog event changes this perception.
By summing all the droplets, the concentration of bacteria reaches the same level as the ocean, according to Garcia-Pichel. A thimble of fog water can contain about 10 million bacteria.
One group caught attention in the samples: methylobacteria. The air collected before fog events contained fewer of these microorganisms than the air collected afterward, indicating stimulation of multiplication.
These bacteria consume simple carbon compounds, including formaldehyde. This common pollutant contributes to atmospheric ozone pollution and is associated with human health damage.
How microbes reduce pollutants
To understand what was happening inside the droplets, Cao collected samples in the field, in Pennsylvania, waking up before dawn to capture the fog as it formed.
In the laboratory, the samples were placed under a microscope. The researcher observed bacteria getting larger and dividing, a sign of growth. The team also identified that they used formaldehyde as food.
The removal of formaldehyde occurred so quickly that simple feeding did not explain everything. At high concentrations, this compound becomes toxic to the bacteria, which start breaking it down into carbon dioxide.
Thus, the microorganisms not only consume a pollutant. They detoxify the air as a form of self-preservation and, during this process, make the environment cleaner for other living beings.
The challenge of following the same air mass
Investigating fog presents a practical difficulty. To track changes in bacterial populations during an event, it is necessary to sample the same parcel of air before, during, and after the fog formation.
The wind makes this task almost impossible because the air collected minutes before tends to move. The solution was to focus on radiation fog, formed on calm and still nights.
This type of fog occurs when the ground cools, the air just above also cools, and the moisture condenses near the surface. The phenomenon tends to arise in quiet valleys.
Under these conditions, the same air mass remains in place long enough for collections throughout the event. The strategy allowed for comparison before, during, and after the fog.
Fog water requires caution
The collection of fog as drinking water has been explored by communities in regions with water scarcity. The study suggests that this source should be treated like any other water: tested and purified before consumption.
Fog water is not sterile. It contains live bacteria that decompose chemical pollutants, and it is not yet known exactly which elements are present or in what concentrations.
Nighttime activity and new questions
The models used to understand atmospheric chemistry are largely based on reactions driven by sunlight. However, bacteria do not stop their activity when night falls.
Cao added that it is necessary to consider not only chemical reactions but also bacterial growth within the droplets. For her, this may change the way atmospheric models are constructed.
It is still unknown whether fogs from different places harbor distinct microbial communities, what else these bacteria consume, or how much fog contributes to cleaning urban air.
The main conclusion is that fog should be understood as a living, active system capable of performing silent work in the air. Like rivers used by hydroelectric plants, fog shows invisible, useful processes that are still poorly understood by current science.
The study was conducted by Arizona State University (ASU).
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