The biosphere is the system formed by all the 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 rest of the rock or gas layers on the planet, the biosphere can seem rather thin, a very thin living varnish, which on Earth should reach little more than the height of the trees.
However, as a group of researchers from the University of Georgia discovered, the biosphere is not so thin: the air at ten thousand meters above sea level is full of living bacteria, up to 20% of what was previously thought to be. Atmospheric dust is actually living beings in suspension.
Something similar happens in the sea. Not only is the volume of ocean water full of viruses and bacteria, but the deepest ocean sediments, several hundred meters below the seabed, are also full of living microorganisms.
Let’s go back to the air. In addition to the soil, the surface of plants or our digestive tract, there is a permanent cloud of microorganisms around us, which begins at ground level and ends far 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 crystallization nuclei of ice at high altitude. These ice crystals transform into snowflakes, hail or rain. Many of these “rain-making” bacteria are actually plant pathogens, that is, they cause diseases in plants, and are only found in the atmosphere transiently.
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 ally itself with the cold to damage the plant by freezing the leaves and then infecting it.
Wind and rising air currents carry many of these bacteria from plants to relatively high areas of the atmosphere, where their ability to generate tiny ice crystals allows them to return to the ground in the form of rain or snow.
It is fascinating to think that this ability to form ice crystals by plant pathogenic bacteria is adaptive, that is, it has been fixed by natural selection. It is a kind of security system that allows bacteria, when they are carried by the wind to practically the stratosphere, to return to the surface, where they can infect plants again, thus closing a surprising life cycle, which literally passes through the clouds.
A round trip at 15 km altitude
Rain, but also sedimentation (i.e. “setting” by gravity) is responsible for the return to Earth of millions of bacteria and billions of viruses that fall from the sky every day, on every square meter of our planet.
The vast majority of the microorganisms that make up this “microbial shower” are harmless to humans, but it is practically certain that at least some of the pathogens that affect us can be transported over long distances through a great jump 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 is worth keeping in mind that these types of processes, 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) aroma of rain
Airborne bacteria are not the only ones behind the infection of plants. One of his contributions is the aroma of rain, that pleasant perfume that emanates from the earth with the first drops of a storm and that has an evocative name: petrichor.
Petrichor is a complex mixture of volatile compounds, the main of which is geosmin, a terpene molecule produced by bacteria. Specifically, geosmin is produced by cyanobacteria and actinomycetes, especially those belonging to the genus Streptomyces.
Streptomyces produce this molecule to attract insects, which feed on these microorganisms but also, in the process, spread their spores. The smell of geosmin produced by bacteria in stagnant water attracts not only insects, but also camels that 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 come back to the surface of our planet that come from very far away, and that, upon impact with the dry soil , catapult the delicious aromas of other less traveling bacteria to our pituitary.
This article has been published with ‘The Conversation‘.