Atmospheric Sciences News

Microbes Rain Down from Above, to the Tune of the Seasons

Every time snow or rain falls, it brings with it microbes from high in the atmosphere. Could those microbes have a seasonal signal, just like the plants on the land below?

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High in the atmosphere, at this very moment, microbes are clinging to tiny specks of dust floating in the air. These aeroplankton exist all over the globe, tossed into the atmosphere from storms and other weather patterns.

Early experiments with atmospheric microbes date back to the time of Louis Pasteur in the 19th century. But many open questions about these organisms remain. Do they exist in a random mix of species spread throughout the atmosphere, or do they travel in packs? Answering this question could help researchers track harmful microbes sailing on high winds, including pathogens and antibiotic-resistant species. A study published late last year in the Proceedings of the National Academy of Sciences of the United States of America offers new insight into this question.

“We found unexpected seasonal microbial patterns were observed in different years with highly divergent summer and winter communities,” Emilio Casamayor, the director of the Center for Advanced Studies of Blanes at the Spanish National Research Council and coauthor on the paper, told Eos. The team’s findings refute the idea that microbial communities in the air are “random mixtures and without structure,” he said.

Seven Years’ Worth of Microbes

Casamayor and his coauthors came to this discovery through a practice in perseverance. The team collected samples of rain and snow from a research station in the Pyrenees mountains in Spain twice a month for 7 years. With each sample, the team sequenced the ribosomal genes of the microbes and matched their findings with existing genetic databases. The study is the longest temporal evaluation of atmospheric microbes to date.

The samples contained a motley bunch of fungi, bacteria, archaea, and protists, summing to more than 1,200 distinct bacterial genera. In the winter, soil-loving bacteria like Acidobacteriales were common. But in the summer, the desert-dwelling bacteria taxon from North Africa Ramlibacter tataouinensis fell in abundance.

Seasonal deliveries of microbes could affect the ecosystem, said Casamayor, “similar to the case of bird migrations or flowering vegetation.” He added that microbe fallout is a “natural process that has occurred along the Earth history,” but because of recent advances in DNA and satellite technologies, researchers are uncovering aspects they’d never seen before.

The study found that seasonal changes in atmospheric circulation drove the cycle and speculated whether changing wind patterns due to climate change may alter microbe delivery.

“A change in atmospheric circulation as a consequence of climate change will have unpredictable consequences on these intercontinental microbial circulation highways,” Casamayor noted.

The rain and snow samples also included species that dominated year-round, such as infection-causing Pseudomonas and the pink-pigmented Methylobacterium that lives on leaf surfaces.

An “Unclaimed Prize”

“Seasonal patterns were long suspected in aerobiology but never measured across a full year in one fixed location, let alone 7 years,” David J. Smith, the director of the Aerobiology Laboratory at NASA Ames who was not involved in the study, told Eos. “The scope of this effort was admirable.”

Smith believes that the study could bring researchers closer to an “unclaimed prize” in aerobiology, creating models that forecast disease dispersal. “This long-term study will produce a solid foundation for atmospheric modelers to get to work,” he noted.

Casamayor and his team have two more papers in the works using the same data set. In the first, his team is scouring the samples for evidence of plant or animal pathogens, including those that could be harmful for humans. In the second, they’re combing for antibiotic-resistant genes that could be floating on the wind. Although the papers are still in progress, he said that early results indicated that experiments found both pathogens and antibiotic resistance in the rain and snow samples.

—Jenessa Duncombe (@jenessaduncombe), News Writing and Production Intern

Citation: Duncombe, J. (2019), Microbes rain down from above, to the tune of the seasons, Eos, 100, https://doi.org/10.1029/2019EO113385. Published on 08 January 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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