An investigation into radiation fog occurrences in Pennsylvania has unveiled that bacteria residing within fog droplets are actively engaged in metabolic processes and consuming hazardous compounds such as formaldehyde, thereby exposing an unforeseen biological influence operative within the atmospheric environment.
A foggy field in Pennsylvania has a little secret: its suspended water droplets form a habitat for helpful bacteria that eat air toxins. Image credit: Thi Thuong Thuong Cao.
“The atmosphere is host to an extensive population of bacterial cells, numbering in the thousands to millions per cubic meter,” stated Thi Thuong Thuong Cao, a doctoral candidate at Arizona State University, alongside her research associates.
“Nevertheless, it remains an open question whether these microorganisms are metabolically active or propagating in their immediate surroundings, or if they are simply being transported in a dormant state.”
“Our comprehension of the bacterial species present in fogs, which are essentially ground-level clouds, is markedly limited.”
To elucidate the role of atmospheric water droplets as microscopic habitats for microorganisms, Cao and her team conducted an extensive observational study encompassing 32 instances of radiation fog over central Pennsylvania, spanning a two-year period.
Their findings indicated that less than one percent of fog droplets harbor bacteria. However, when aggregated, these represent a substantial quantity of microbial life.
“When all the droplets are considered collectively, the density of bacteria is comparable to that found in oceanic environments,” remarked Professor Ferran Garcia-Pichel, also affiliated with Arizona State University.
“A mere thimbleful of fog water contains approximately 10 million bacteria.”
A particular group of bacteria, known as methylobacteria, garnered significant attention. Samples of dry air collected prior to fog events exhibited a lower prevalence of these bacteria compared to samples gathered immediately following the fog. This observation suggests that fog events transiently augment their population numbers.
Methylobacteria subsist on simple carbon compounds, which encompass noxious substances like formaldehyde. Formaldehyde constitutes a prevalent atmospheric pollutant that exacerbates ozone smog and poses risks to human well-being.
“Through microscopic examination, we observed that the bacteria were indeed increasing in size and undergoing cell division, indicating active growth,” Cao explained.
“Furthermore, we determined that they were utilizing formaldehyde as a nutrient source to sustain their growth.”
The rate at which these bacteria eliminated formaldehyde was so substantial that the researchers hypothesized their interaction was more than mere consumption.
At elevated concentrations, formaldehyde proves detrimental to these bacteria; consequently, they metabolize it into carbon dioxide as a mechanism to mitigate its toxicity. This process presents a mutually beneficial outcome for both the microbial community and human populations.
“The scientific community’s exploration of biological activities within clouds is a relatively recent development, and consequently, much remains to be understood,” commented Professor Pierre Herckes of Arizona State University.
“For instance, during nocturnal periods, atmospheric chemical processes are considerably attenuated. Chemical reactions are primarily driven by solar radiation and light. However, if bacteria continue their metabolic functions irrespective of daylight, their significance could be substantial.”
“The extent of our ignorance regarding these minuscule ecosystems within fog and clouds is truly astounding. Does the composition of bacteria in fog vary based on its geographical origin? What other nutrients might these diverse bacterial populations consume? And what is their influence on atmospheric quality?”
“The possibilities are virtually limitless, with no intended pun,” Professor Garcia-Pichel remarked.
The findings were disseminated this week in the peer-reviewed journal mBio.
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Thi Thuong Thuong Cao et al. 2026. Growth and formaldehyde degradation of photoheterotrophic Methylobacterium within radiation fogs. mBio 0:e00463-26; doi: 10.1128/mbio.00463-26
