Air samples collected at altitudes up to 2,000 miles above Japan held an astonishing variety of living microbes, such as bacteria and fungi, many of which are toxic to humans.
Not just the homegrown ones either. Some of these microbial stowaways traveled over 1,000 miles with the help of aerosols, originating from regions lush with fertilizers and pesticides.
This has launched the scientific world into a wave of questions about the journey of human, animal, and plant pathogens across vast geographical terrains, invisible to the naked eye.
Leading this microbial exploration was the Barcelona Institute for Global Health ( ISGlobal ) with support from the Daniel Bravo Andreu Private Foundation ( FPDBA ).
Microbes travel in the air
Pathogens hitching a ride on the wind isn’t unheard of. But the diversity of microbes that can survive at high altitudes, where conditions could give even an Everest mountaineer pause, is a fresh revelation.
“We know that above a certain point in the troposphere (called the planetary boundary layer), certain materials can be transported over long distances because the air in that region is isolated from the surface and there is less friction. But we did not suspect that viable microorganisms could be there also,” says Xavier Rodó, ICREA researcher at ISGlobal.
Rodó and his team carried out ten tropospheric flights to examine microbial diversity at high altitudes -- a significant jump from studies usually executed just a few meters above the ground or the ocean.
Tracing the microbial travel path
Rodó and his international team boarded a Cessna aircraft and conducted air surveys above Japan, tracing wind currents from mainland Asia.
These transcontinental airways , known as tropospheric bridges, connect air from distant parts of the world.
For context, think about air that lifts in mainland China and then descends over Tokyo, courtesy of typical winter weather conditions.
A total of 22 aerosol filter samples were gathered during two periods (in February and April 2014), methodically analyzed for their chemical and biological composition.
Sequencing DNA from the samples, the team identified over 266 fungal and 305 bacterial genera associated with the aerosols.
Types of microbes in the air
This microbial soup included potential pathogens for humans, animals, and plants -- familiar names like Escherichia coli, Serratia marcescens, Haemophillus parainfluenzae , and several Staphylococcus species were among the bacterial mix.
The fungal factions included genera like Candida and Cladosporium, to name a couple.
“Surprisingly, the Micrococcus luteus strain isolated was resistant to multiple drugs, including carbapenems, glycopeptides, ciprofloxacin, and trimethoprim-sulfamethoxazole," says Sofya Podzniakova, co-first author of the study.
"Our findings suggest that antimicrobial resistance could spread over long distances via this previously unrecognized route",
Tropospheric travel tunnel of air microbes
If you're curious about the origin of these airborne pathogens, the answer could be right underneath our noses -- or rather, our feet.
The presence of elements such as zinc sulfate and potassium, usually found in fertilizers and pesticides, implies an agricultural source. A scenario quite consistent with the extensively farmed croplands in northeast China.
The high-altitude and ground samples from the days examined showed a striking similarity in microbial diversity . A consequence of air descending from altitudes to the ground , perhaps?
The particle transport models simulated by Roger Curcoll, currently a researcher at the Universitat Politècnica de Catalunya -- BarcelonaTech ( UPC ), would certainly support this theory.
New chapter in microbe transport study
The implications of this study are profoundly impactful. “Our findings uncover a rich and unprecedented diversity of microbes that are dispersed by wind currents thousands of kilometers away from their sources by intense tunnels of wind that form high in the troposphere,” Rodó emphasized.
“They represent a paradigm shift in our understanding of how human health can be affected by pathogens thriving in the environment, particularly in the air."
Though there's no direct trace yet linking the presence of known human pathogens in aerosols and health effects, the study affirms the pressing need for further exploration of air microbes and microbial pathogens' long-distance travel.
So, the next time you feel the wind against your face, remember, it has stories to tell -- tales of microbial hitchhikers scaling the heights of the troposphere and crossing continents. Globe-trotting pathogens show us how interconnected our world truly is, even in ways we can't see.
The study is published in the journal Proceedings of the National Academy of Sciences .
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