published on 24 February 2017 in air
Bacteria in the clouds
Who has never studied the water cycle, from the sea to the clouds and from the clouds to the sea? And who has never looked up to the sky in an attempt to make weather forecasts? We have probably all done it. And yet, few people know that, at least in part, it is living organisms that decide whether it must rain or whether the clouds can fly by without precipitation. More specifically, bacteria.
Condensation nuclei and bacteria
Before getting into the life of bacteria in the clouds, let’s quickly refresh our memory on the formation of rain. For a cloud to produce rain, or snow, the formation of droplets, or ice particles, is necessary. This in turn require the presence of aerosols, i.e. of very fine particles that can perform the function of condensation nuclei. Traditionally, it was thought that these particles were of mineral origin, but for some years now scientists have discovered that, not infrequently, condensation, and therefore precipitation, is on the contrary caused by living organisms.
These are mostly bacteria, fungi or very thin algae. All microbial organisms, which, contrary to aerosols of mineral origin, have the ability to catalyse the formation of ice, even at temperatures approaching zero degrees Celsius. A detail of some significance, since approx. 50% of precipitations originate from the solid phase of ice.
One of the most important studies on this phenomenon was conducted in 2008 and showed that “rain producer” bacteria have spread all over the world, from Antarctica to France, with some differences in terms of numbers. This result confirms the hypothesis that microbes can easily cover rather long distances staying in the clouds and suggests that “bioprecipitation” occurs at any point on Earth.
Normally, when they are on the Earth’s surface, many of these bacteria live as plant parasites. Exploiting their ability to speed up freezing at relatively low temperatures, they are in fact able to break the walls of plant cells and therefore feed on their contents. Most likely, the fact of being able to cause rain – or snow – is also an advantageous factor for their survival: through rain, in fact, they can return more quickly to the Earth’s surface, in a more suitable environment to life. In this way, they ensure a trip long enough to “conquer” new areas of the planet, but short enough to allow their survival. It is no coincidence, in fact, that some of these bacteria are among the most common species on the planet, from the ground to the clouds in the troposphere.
In short, in the popular English expression “it’s raining cats and dogs”, the cats and dogs should be replaced by “bacteria”. But which bacteria are we talking about? The undisputed star of bioprecipitation is Pseudomonas syringae, an elongated micro-organism equipped with flagella, capable of producing a protein that performs the nucleus condensation function. The water droplets or ice crystals which then precipitate in the form of rain or snow are formed around this molecule, but that’s not all. This unicellular organism, in fact, is able to “confuse” the water molecules and make it freeze at much higher temperatures than under normal conditions.
The characteristics of Pseudomonas syringae, its habit of fluctuating in the clouds and its influence on water solidification temperature, has attracted the curiosity not only of scientists (especially fascinated by the relationship between the biosphere, atmosphere, hydrosphere and geosphere) and entrepreneurs (who have used it for certain activities, such as the production of artificial snow), but also of certain artists. The fact that microorganisms are able to cause precipitation, in fact, compels us to investigate the relationship between the animate and the inanimate world and question ourselves in a visionary and responsible way on the future.
By Anna Pellizzone
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