By Carl Zimmer
New York: 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. Now researchers report that on flights over Japan, at altitudes as high as 10,000 feet (3 kilometres) they have caught hundreds of different types of bacteria and fungi. The team estimated that the microbes had flown about 2km upwards 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.
In 10 flights over Japan, researchers found a host of bacteria and fungi. These high-altitude germs could help spread disease around the world. Credit: Xavier Rodo via The New York Times
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.
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.
Cases of Kawasaki disease come in surges. Rodó and his colleagues found that surges in Japan often occurred when winds blew in from north-east 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 travelling in the winds – something possibly alive. “So I thought, let’s go and chase whatever is there,” Rodó said.
His team flew a Cessna aeroplane 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 spiralled upward, taking samples as high as 3km. Then, the researchers took the samples to their lab, where they put on biosafety suits and respirators to avoid contamination.
Rodó and his colleagues found high levels of a rare mineral called hafnium, which probably came from mines in China.
A colony of bacteria and fungi caught in air samples during research flights.Credit: Pozniakova via The New York Times
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 north-western 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 fertiliser 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.
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 analyse 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 that high up 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.”
This article originally appeared in The New York Times.