Researchers have understood since the 1970s that bacteria promote freezing water, but it took years to understand that these same bacteria populate the clouds and make snow.

Posted today at 19h00 Time to Reading 2 min.

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Snowflake.
Snowflake. Flora Press / Daniela Kunze / Biosphoto

Carte blanche. Can we live in the clouds? The mental images generated by this interrogation are rooted in the illustrations of the stories of our childhood, or in the imaginary representations of God. Yet no census of the atmosphere has so far found other inhabitants than birds, flying mammals, insects, and many micro-organisms.

Bacteria, fungi or microscopic algae can indeed follow the water cycle. On the surface of lakes, seas and plants, the water evaporates continuously, then condenses into tiny droplets, which the air carries: imagine a fog very little dense, charged with fine dust. These invisible droplets rise into the atmosphere and swell the clouds.

Just like dust, small microorganisms (from one to ten micrometers) are shipped, then cross oceans and continents by air. And then ? Some of these microbes seem to return to the mainland by triggering precipitation; they thus participate actively in the water cycle, rather than undergoing it.

Additive for artificial snow

One of the best-described examples of this phenomenon is that of Pseudomonas syringae, a bacterium of which some varieties infect fruit trees and vegetables, and therefore constitute a recurring agronomic concern. By the 1970s, American researchers had observed that this bacterium facilitated the transformation of water into ice when temperatures dropped below 0 ° C.

Indeed, the gel of pure water is not a spontaneous process, even at -10 ° C; it must be initiated by a first crystal of ice, or by an impurity which will serve as a germ for the rest of the crystallization. In attempting to identify bacteria that favored freezing of water on the surface of plant leaves, researchers had discovered that P. syringae accelerated the setting in ice.

Their ambition was to use these bacteria as germs to artificially trigger precipitation on demand. P. syringae being phytopathogenic, the risk to the crops made the project obsolete, until the purification of P. syringae with the same icing properties as live bacteria. This innovation was used from 1992 to 2005 as an additive for the production of artificial snow in some French ski resorts. This same additive makes it possible to considerably simplify the cryogenization procedures of mouse embryos before their reimplantation.

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