Fog harbors bacteria – some of which can degrade formaldehyde

A new study by U.S. researchers at Arizona State University raises an important question: can the atmosphere, and more specifically fog droplets, be not only a means of transport for microorganisms, but also an active habitat for bacterial life and growth? The surprising results point to the potential of the aerobiomes in the context of formaldehyde degradation.

The study, the results of which were published on May 11 in the journal Environmental Microbiology, was conducted during 32 episodes of radiation fogs in central Pennsylvania (USA), observed over a two-year period. The researchers analyzed not only the composition of the microorganisms present in the fogs, but also their metabolic activity, i.e. their ability to carry out vital processes such as respiration, growth and the breakdown of chemical compounds.

Microorganisms in the air – what exactly is an aerobiom?

The authors use the concept of the aerobiom, which is a community of microorganisms present in the atmosphere – bacteria, viruses and other airborne microorganisms. Until now, they were thought to be merely passengers transported by the wind. With the occurrence of fog, however, the situation may be different, as the presence of water activates dormant organisms.

In the analyzed fogs, the concentration of biological material was very high. On average, about 1 million copies of the 16S rRNA gene were detected in 1 ml of fog water. This gene is a standard marker used to identify bacteria, so its quantity reflects the number of microorganisms. Significantly, bacteria in the mist droplets were up to 6-7 orders of magnitude more concentrated than in dry aerosol, which is fine airborne dust and water particles.

The researchers point out that fog droplets make up only about 0.000013 percent of the air volume, which means that almost all bacterial life is concentrated in a minimal volume of water. In practice, this makes fog something like a very dense but temporary water environment in the atmosphere.

What determines the number of bacteria in the fog?

The study showed that the number of bacteria in fog evidently increases with temperature. The duration of the fog, on the other hand, did not have a simple correlation with the number of bacteria. This means that microorganisms not only accumulate over time, but there are parallel processes of their disappearance, such as falling to the ground surface along with water droplets.

Among all identified bacteria, the Proteobacteria group was by far the dominant one, accounting for about 65 percent of all detected organisms. Within it, the genus Methylobacterium played a particularly important role, accounting for an average of 29 percent of the total bacterial community of fogeys.

Methylobacterium are bacteria known for their ability to utilize very simple organic compounds that contain only one carbon atom, so-called C1 compounds. Formaldehyde, for example, belongs to this group. Some of them are also able to use light as an additional source of energy – this kind of nutrition is called photoheterotrophy, a combination of using organic matter and light energy.

Fog changes the composition of the air

Analysis of six episodes of fog showed that after the fog cleared, the number of bacteria in the air increased by an average of 45 percent, and in some cases by as much as 90 percent. This means that the fog not only contains bacteria, but can also affect their subsequent distribution in the atmosphere. Despite this increase, the taxonomic composition of the bacterial community remained relatively stable.

In-depth studies have shown that during periods of haze, the spatial distribution of bacteria is not random. Some groups end up in water droplets, while others remain in dry aerosol particles. Of the more than 1,300 bacterial types detected, only seven showed a clear preference for living in droplets. The most important of these were just Methylobacterium, as well as bacteria from the Caulobacteraceae and Beijerinckiaceae groups.

One of the key indicators of biological activity is the presence of dividing cells, indicating the proliferation of bacteria. In the mists, their percentage averaged 2.4 percent, while in the dry aerosol it was only 1 percent. On this basis, it can be estimated that the doubling time of the bacterial population in the droplets is about 60 hours. This rate – compared to laboratory conditions – is very slow, but significant because it indicates the possibility of a real increase in their numbers in the atmosphere.

Formaldehyde – a fuel and a toxin at the same time

One of the study’s most important discoveries was the ability of microorganisms in the mists to break down formaldehyde, a toxic volatile organic compound present in the atmosphere, very quickly. In the samples studied, its concentration ranged from 6 to 25 micromoles per liter, but it was quickly reduced – at a rate of about 25 micromoles per day, or about 200 times faster than in other cloud studies.

Formaldehyde is both a carbon source and a toxic substance for bacteria. Therefore, its decomposition can serve not only growth, but especially detoxification, that is, neutralization of the harmful compound.

Two strains of Methylobacterium bacteria were isolated from the mist. Both had genes that allow formaldehyde to be used as a building material. This process occurs as a result of the so-called serine cycle, a metabolic pathway that allows simple compounds to be converted into more complex ones. The strains grew using formaldehyde as their only carbon source, although the rate of this process was very slow – it took 184 to 237 hours to fully double the population. Importantly, they differed in their sensitivity to its concentration.

Fog as a temporary atmospheric ecosystem

The study’s authors suggest that fog is a short-lived but active aquatic ecosystem in the atmosphere, where selection, metabolism and probably bacterial growth processes take place.

Particularly important is the role of Methylobacterium, which can participate in naturally cleansing the atmosphere of toxic compounds such as formaldehyde. At the same time, the study suggests that the previous perception of fog as “pure water” is probably an oversimplification – since it contains significant amounts of active microorganisms that can affect both the chemistry of the atmosphere and the life cycle of bacteria in the environment.

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Source:

Cao TTT, Herckes P, Straub D, Sarkar S, Garcia-Pichel F. 0. Growth and formaldehyde degradation of photoheterotrophic Methylobacterium within radiation fogs. mBio 0:e00463-26., https://doi.org/10.1128/mbio.00463-26