Viruses and Bacteria Travel Along a High-Altitude Superhighway

A new theory explains how the very same microbes exist on every continent, even when suitable hosts are nowhere to be found.

This NASA satellite image shows a low-pressure weather system over the Middle East on the morning of March 26, 2003. | NASA/Getty Images
This NASA satellite image shows a low-pressure weather system over the Middle East on the morning of March 26, 2003. | NASA/Getty Images

If you asked three people living in wildly different locations — on a beach in the Gulf of Mexico, on a wheat farm in the Russian steppes, or a high-rise apartment in Shanghai — to leave a slice of bread outside for a few days, something remarkable would happen. The very same army of microbes would slowly colonize the bread, never mind that nothing else about the three environments is remotely the same.

This observation is especially puzzling for viruses, which need to infect a specific host organism to replicate. What are the odds that the very same host organisms would exist in Arctic melt ponds and the Gobi Desert? Yet the exact same viruses are there, and they number in the billions.

New microbial data from high in the Sierra Nevada Mountains of Spain has spawned an intriguing theory for the widespread dispersal of viruses and bacteria. The microbes hitch a ride on tiny dust and soil particles that are whisked around the world on high-altitude air currents, according to a paper in the journal of the International Society for Microbial Ecology.

Spanish researcher Isabel Reche from the University of Granada placed microbe-collecting devices on two mountain peaks standing about three kilometers (9,800 feet) above sea level, which is beyond what’s known as the boundary layer. Below the boundary layer, winds and air currents are strongly influenced by interactions with Earth’s surface. Above the boundary layer, swift and steady winds can carry objects across oceans and continents.

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That might explain why Reche’s high-altitude microbe collectors found billions of viruses per square meter settling on the mountains each day along with tens of millions of bacteria. The majority of the microbes had piggybacked on specks of soil-dust from both water and land environments that got swept up by surface winds into the upper atmosphere, where they cruised tens of thousands of kilometers before settling on the Spanish peaks.

Curtis Suttle, a virologist at the University of British Columbia and co-author of the study, told Seeker that while scientists have long known that viruses and bacteria can become aerosolized — when tiny particles go airborne — this study was the first to prove that “incredible numbers” of viruses and bacteria are floating around above the boundary layer.

“It’s always been a bit puzzling how come these things are everywhere,” said Suttle. “It’s now pretty clear that they’re being swept around the planet.”

But Suttle was quick to quash fears that infectious viruses were constantly raining down from the sky.

Viruses are the most abundant life form on the planet — 10 to the 30th power in the oceans alone — and each of us breathes in and swallows millions and millions of them every day with absolutely no ill effects. The handful of virus strains that are infectious to humans, including the common cold, influenza, chicken pox, herpes, Ebola, and AIDS, make up such a tiny fraction of all viruses that the odds of finding one in the upper atmosphere is “statistically absent,” said Suttle.

“If it were human diseases that were blowing around across the planet, we’d all be sick all the time,” Suttle explained. “The chances of even getting two viruses that were human in origin is infinitesimally small. It’s really important so that people don’t get freaked out.”

Instead of infecting humans, the vast majority of viruses go after bacteria, the second most abundant life form on Earth.

Even though we often equate viruses and bacteria with illness and infection, the widespread dispersal of viruses and bacteria worldwide isn’t a bad thing, the researchers concluded. With increasing understanding of the microbiome, we know that bacteria play critical roles in human processes like immunity and digestion. But bacteria are also the world’s biogeochemical engines, said Suttle, cycling organic material back to its chemical constituents.

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Viruses, for their part, keep bacterial colonies in check. Viruses are also natural genetic engineers, slipping their DNA into infected cells. During the exchange, some of the host’s genetic material may get carried away by the virus and transported to another bacteria. This process, called transduction, may serve as a driver of evolutionary change at the microbial level.

The diffusion of viruses and bacteria across the globe amounts to a microbial “seed bank,” said Suttle. Like seeds, which can remain dormant in the soil until the conditions are right to sprout, viruses and bacteria can lay inactive until environmental changes in the local ecosystem spring into action.