We humans pride ourselves on our adaptability, but bacteria have always been one step ahead in this billion-year-old game. Our microbiological brothers live quietly in the most unpleasant environments for existence, from deep-sea vents to Antarctic lakes. Some microbes survive even stronger in the upper atmosphere - the stratosphere - and recent evidence suggests they can affect our weather, our crops, and even our health.
Our understanding of atmospheric microbes is still evolving, and most of our research is currently focused on the troposphere, the layer in which we live and breathe. In 1979, Russell Schnell, now deputy director of the National Oceanic and Atmospheric Administration's Global Monitoring Division, wondered why tea plantations in western Kenya hold the world record for hail collection. It turned out that the pest of tea plants, Pseudomonas syringae, is to blame, which is lifted into the air by people in the process of collecting tea leaves, because ice crystals formed around the tea-loving microbes much faster.
Evidence for this process - called bioprecipitation - has since been found around the world, involving all manner of microbial criminals. Recent studies have shown that microbes can influence cloud formation and cover and even light up the sky to an incredible degree. The lower layers of the atmosphere are filled not only with rainmakers, but also with organisms that carry diseases wherever the wind blows them. As the climate changes, and it changes around the world, there is growing interest in quantifying the effects of these microbes.
But fundamental data is still lacking. “We scientists still don't have a good budget to research the biology of our atmosphere,” says Daniel Chicho, professor of atmospheric chemistry at MIT.
This is because there is a lot of air in the atmosphere and it is very difficult to collect samples without any contamination. There are not enough systems available for scientific experiments, regardless of whether you are collecting or studying biological samples.
And while we know little about the troposphere, we know even less about the stratosphere, which starts at about 10,000 meters in mid-latitudes. However, we know that even in the thin, dry air of the stratosphere, where temperatures can drop to -60 degrees, there are a small number of tenacious microbes.
Priya Dassarma, who studies stratospheric microbes and their relationship to climate, writes that these high-altitude microbes can spread allergens or even disease. "Some isolated stratospheric strains pose a threat to plants and animals, and clinical isolates survived even at this altitude," she writes. "The potential consequences for public health and medicine underscore the need for more thorough research into the movement of microbes in the atmosphere and research into their survival mechanisms."
The idea that microbes can spread in the atmosphere is far from new - studies back in the 1990s, for example, show that bacteria move from Africa to Florida every summer riding on mineral soil from the Sahara. And long-distance transport is especially effective at high altitudes, thanks to the existence of jet streams, the fastest air currents on Earth.
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But Chicho believes the risk that the new work suggests - that high-altitude atmospheric paths may be the fastest method for transporting diseases around the world - is exaggerated. He thinks we ought to learn more about the lower atmosphere before we start worrying about what is happening in the upper atmosphere.
In any case, all of this new research is gaining relevance in light of climate change: in a stormy world, more and more microbes are being sucked into the lower atmosphere by the wind. From there, some of them will enter the stratosphere, mostly through vertical mixing of the layers. As new regions dry up, more dust particles will rise into the air, which means there will be more material in the atmosphere as a whole.
Currently, any impact that stratospheric microbes may have is not factored into climate change projections. All scientists point to the need for a better understanding of what is in the air above our heads, so that we have a basis for understanding future changes.
Ilya Khel