Written by Carl Zimmer
The sky may not seem like a promising place to find life. But in the 1920s, scientists flew planes across the United States and caught floating spores.
A century later, the living atmosphere remains a fairly mysterious place. On Monday, researchers reported that on flights over Japan, at altitudes as high as 10,000 feet, they had caught hundreds of different types of bacteria and fungi. The team estimated that the microbes had flown more than 1,200 miles when they were captured. Most intriguing of all, some of the species might be able to cause diseases in people.
“A third of the bacteria and slightly more of the fungi can be considered potential human pathogens,” said Xavier Rodó, a computational ecologist at the Barcelona Institute for Global Health, who led the work.
Rodó cautioned that the new study offered no direct evidence, however, that high-altitude microbes could cause human outbreaks when they fell to the ground. But he argued that it offered hints that winds helped spread diseases. “This is something that deserves attention,” he said.
Inspiration from the Kawasaki disease
The idea for the flights originated from Rodó’s research into a mysterious condition known as Kawasaki disease, which causes fevers, rashes and, sometimes, deadly heart attacks. Researchers have long debated whether the disease is caused by a pathogen, a faulty immune system or a combination of the two.
In an undated photo from Pozniakova, a colony of bacteria and fungi caught in air samples during research flights. In 10 flights over Japan, researchers found a host of bacteria and fungi. These high-altitude germs could help spread disease around the world. (Pozniakova via The New York Times)
Cases of Kawasaki disease come in surges. Rodó and his colleagues found that surges in Japan often occurred when winds blew in from northeast China. And when those same winds reached California, cases rose there as well.
“We didn’t expect winds to be really playing a role, but it came out in a way that was really striking for me,” Rodó said.
The region of China where the winds came from has many massive farms and livestock operations, along with open-pit mines. The research suggested that something was traveling in the winds — something possibly alive. “So I thought, let’s go and chase whatever is there,” Rodó said.
His team flew a Cessna airplane into air masses that blew from China over the Sea of Japan. The plane rose above the turbulent lower atmosphere, reaching the free troposphere, a layer where air masses can quickly move long distances. An inlet opened on the side of the plane, allowing air to flow into a tube that led to a filter where airborne particles were trapped.
On 10 flights, the team spiraled upward, taking samples as high as 10,000 feet. Then the researchers took the samples to their lab, where they put on biosafety suits and respirators to avoid contamination.
Important findings from the study
Rodó and his colleagues found high levels of a rare mineral called hafnium, which probably came from mines in China.
The samples also contained fungal spores, along with bacteria sticking to tiny dust grains. Despite the organisms’ long journey, some were still alive; they managed to grow into colonies in petri dishes.
When Rodó and his colleagues extracted DNA from the microbes, they discovered at least 266 kinds of fungi and 305 types of bacteria. Many of the microbes belong to groups that grow on plants or in soil. Some are particularly good at growing in heavily polluted soil.
Others belong to groups known to live in our bodies. Some are peaceful members of the human microbiome, while others can cause dangerous infections.
Rodó speculated that northwestern China may be a potent source of pathogens thanks to the large-scale farming of crops and livestock. Erosion can produce big plumes of dust. It’s possible that some microbes from the soil — or even from fertilizer or sewage — might be able to cause diseases.
“I am surprised by the number of human pathogens that they revealed,” said David Schmale, an aerobiologist at Virginia Tech who was not involved in the study. He suggested that they might actually be harmless relatives of disease-causing germs. The best way to pin down their true nature, he said, would be to infect human cells or lab animals.
Scope for further research
Rodó said that he and his colleagues were preparing experiments to expose air samples to human lung cells.
The new study does not solve the mystery of Kawasaki disease, Rodó said. It’s possible that more than one pathogen may be able to trigger the disease; it’s also possible that children become vulnerable only if they inhale air pollution carried by the same winds. “We do not yet know what is going on,” Rodó said.
He and his colleagues are now using new technologies to analyze their samples from Japan and are finding evidence of additional organisms, including viruses.
Even if Rodó and his colleagues find clearer evidence of pathogens in the winds, it remains an open question whether those germs could reliably spread diseases from place to place. Pathogens riding 10,000 feet in the air are far sparser than the ones that build up when infected people exhale in poorly ventilated buildings.
Still, “sometimes, it takes just one,” Schmale said. “These are unexplored environmental spaces, and we need to study them more.”
n an undated photo from Rodo, a research flight over Japan. In 10 flights over Japan, researchers found a host of bacteria and fungi. These high-altitude germs could help spread disease around the world. (Rodo via The New York Times) 
In an undated photo from Pozniakova, a colony of bacteria and fungi caught in air samples during research flights. In 10 flights over Japan, researchers found a host of bacteria and fungi. These high-altitude germs could help spread disease around the world. (Pozniakova via The New York Times)
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