At 10,000 meters high, the atmosphere is full of microorganisms. Millions of bacteria and viruses fall from the sky every day on every square meter of our planet
The biosphere is the system formed by the set of living beings on the planet. Most of us will immediately think of terrestrial and marine ecosystems where life of all kinds flourishes: fish, algae or invertebrates in the sea; plants, fungi and animals on earth. Compared to the other layers of rock or gas on the planet, the biosphere can seem rather thin, a very tenuous living varnish, which on Earth should be little more than the height of trees.
However, as a group of researchers from the University of Georgia discovered, the biosphere is not so fine: the air at 10,000 meters is teeming with living bacteria, up to 20% of what was previously thought to be atmospheric dust are actually living beings in suspension.
Something similar happens in the sea. Not only is the volume of water in the oceans full of viruses and bacteria, but the deepest ocean sediments, several hundred meters below the seafloor, are also full of living microorganisms.
Let's get back on the air. In addition to the soil, on the surface of plants or in our digestive tract, there is a permanent cloud of microorganisms around us, which begins at ground level and ends well above the summit of Everest.
The bacteria that make it rain
There is a little known fact about these bacteria that float in the atmosphere and that is that they play a fundamental role in the formation of rain, since they act as tiny nuclei of ice crystallization at high altitude. These ice crystals turn into snowflakes, hail or rain. Many of these "rain-making" bacteria are actually plant pathogens, that is, they cause disease in plants, and are only transiently found in the atmosphere.
The main species of these bacteria is Pseudomonas syringae. This microorganism has a protein on its surface with a great affinity for water, which facilitates the formation of ice crystals at temperatures that are not too low. This peculiarity allows it to team up with the cold to damage the plant by freezing its leaves and then infecting it.
Wind and rising air currents carry many of these bacteria from plants to relatively high areas in the atmosphere, where their ability to generate tiny ice crystals allows them to return to the ground as rain or snow.
It is fascinating to think that this ability to form ice crystals by plant pathogenic bacteria is adaptive, that is, fixed by natural selection. It is a kind of security system that allows bacteria, when they are dragged by the wind to practically the stratosphere, to return to the surface, where they can infect plants again, thus closing an amazing life cycle, which passes –literally– through the clouds.
A round trip 15 km high
Rain, but also sedimentation (i.e. "settling" by gravity) is responsible for the return to Earth of millions of bacteria and trillions of viruses that fall from the sky every day, on every square meter of our planet.
The vast majority of microorganisms that make up this "microbial shower" are harmless to humans, but it is virtually certain that at least some of the pathogens that affect us can be transported over great distances through a great leap of up to 15 km high and several days long.
The presence of microorganisms in the atmosphere, their involvement in the climate or in the transmission of diseases over long distances is a fascinating field of study that is just beginning.
It should be borne in mind that this type of process, and others yet to be discovered, have undoubtedly taken place for millions of years, and probably with a generally very positive role, as is evident in the case of rain.
The (bacterial) scent of rain
Airborne bacteria are not only behind the infection of plants. One of his contributions is the scent of rain, that pleasant perfume that emanates from the earth with the first drops of a storm and has an evocative name: petrichor.
Petrichor is a complex mixture of volatile compounds, the main one of which is geosmin, a terpene molecule produced by bacteria. Specifically, geosmin is produced by cyanobacteria and actinomycetes, especially those belonging to the Streptomyces genus.
Streptomyces produce this molecule to attract insects, which feed on these microorganisms but also spread their spores along the way. The smell of geosmin produced by stagnant water bacteria attracts not only insects, but also camels who identify it – like us – as the “smell of water”.
So, before the beautiful spectacle of a summer storm, it is worth remembering that, with the rain, millions of microorganisms that come from far away come back to the surface of our planet, and that, when they hit the dry ground , catapult the delicious aromas of other less traveling bacteria to our pituitary.
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