More than just a weather phenomenon: the fine droplets of the fog contain a world of their own. Biologists have discovered that millions of bacteria grow and thrive in just one teaspoonful of fog water. Overall, just as many microbes could live and reproduce in the fog as in the ocean. According to the team, nebula should therefore be viewed as our planet’s own habitat – especially since some nebula inhabitants could be useful to us.
It has long been known that there are bacteria in the air and in the clouds: They are whirled up with desert dust or water droplets and blown away by the wind. Some bacteria and fungal spores can be transported over thousands of kilometers. It is estimated that air can therefore contain several thousand to several million bacteria per cubic meter, depending on the dust and aerosol content. However, it is unclear whether these microbes are active or are only passively transported in a persistence state.
Search in the fog
The same applied to fog – dense, ground-level veils of countless fine water droplets that arise on cool autumn mornings, for example. “So far we know very little about which bacteria exist in nebulae,” says lead author Thi Thuong Thuong Cao from Arizona State University. To find out more, she and her colleagues collected fog samples in various windless valleys in Pennsylvania over two years.
The researchers used DNA analyzes to investigate whether and which microbes are present in these fog samples. They also observed the microbes under the microscope and created cultures on culture media to find out more about their metabolism and activity. For comparison, Cao and her colleagues also examined air samples and aerosols from the same sampling sites on fog-free days.
“Just as many bacteria as in the ocean”
The analyzes revealed: The researchers detected millions of copies of bacterial DNA in every milliliter of fog water. Although not every single droplet of fog contained such bacterial gene signatures, overall the fog turned out to be surprisingly lively: a thimbleful of fog water contained tens of millions of bacteria. “If you take all the fog droplets together, they contain as many bacteria as the ocean,” says Cao.
The newly discovered fog bacteria also turn out to be relatively species-rich: the researchers detected an average of 188 different types of bacteria in the fog samples. “At the phylum level, Proteobacteria were dominant with around 69 percent, followed by Firmicutes with twelve percent and Bacteroidota with around seven percent,” report Cao and her team. The microbial fog inhabitants differed significantly in their species composition from those in dry air or aerosols outside the fog periods.
Mist bacteria are active – and useful
What is surprising, however, is the condition of the fog microbes: Unlike many bacteria carried by the wind, the fog dwellers are active: they eat, grow and reproduce. “We observed them under the microscope and saw that the bacteria were growing larger and dividing – a clear sign of active growth,” explains Cao. Laboratory experiments and DNA analyzes suggest that many of these fog bacteria feed on organic compounds contained in the fog droplets – including air pollutants such as formaldehyde.
“The metabolically active bacteria in the fog can break down formaldehyde at an unprecedentedly high rate,” report the researchers. If the fog bacteria were to just use all the formaldehyde as food, they would have to double their biomass every few minutes at this rate. “That would be extremely unrealistic,” said the team. They therefore suspect that the fog bacteria decompose the formaldehyde primarily to generate energy and get rid of this compound, which is toxic to them.
“This changes our entire perspective”
Taken together, these results prove that the mist droplets comprise a diverse living environment of their own. Unlike dry air or dust carried by the atmosphere, the nebula’s tiny water droplets contain large numbers of active, growing and multiplying microbes. “The atmospheric water in nebulae should therefore be considered a real aquatic habitat, albeit a temporary one,” write Cao and her colleagues.
“This changes our entire view of the fog phenomenon,” says senior author Ferran Garcia-Pichel from Arizona State University. The microbial fog inhabitants influence the composition of the air through their decomposition activity; in the case of formaldehyde, they even improve air purity. Conversely, their presence also means that drinking water obtained from fog is less sterile than previously assumed.
The researchers have only just begun to examine the living environment in the fog. The bacterial species found in the mist droplets and their nutritional strategies have only been rudimentarily investigated so far. How the nebula’s microbiome differs in different regions is still completely unknown. “It’s still pretty new to even look at what lives in such ground-level clouds, so there’s a lot we don’t understand,” says Cao’s colleague Pierre Herckes.
Source: Thi Thuong Thuong Cao (Arizona State University, Tempe) et al., mBio, 2026; doi: 10.1128/mbio.00463-26