What it takes...

There are endless answers to the question "What does it take?"
  • It takes a village to raise a child, (ok, it's mostly proverbial)
  • it takes 475 water molecules to make a snowflake (yes really, here's the study!)
  • it takes temperatures just below 0°C to freeze water. But does it really?!
Let's have a closer look at that (and since this is still "Mic'd up"), let's look at the impact bacteria can have on this.

Water indeed freezes below 0°C but not under all conditions. For pure water, temperatures have to drop to around -40°C for it to freeze. So how come you can go ice-skating at temperatures well above -40°C? In the environment, water is rarely pure and is often found in conditions that promote freezing. But even when water is below 0°C, it's not automatically frozen: The reason for that is that the freezing process has to start somewhere and that's not as trivial as it sounds.


When liquids freeze, bonds form between the (e.g. water) molecules. Somewhere within the water, the molecules have to arrange in just the right way to form ice crystals. This spot is called a nucleation site - after that spot has formed, the liquid will freeze. Impurities in the water (e.g. salt or dirt) raise the freezing point because they serve as a nucleation site. Similarly, disturbances of water that can occur when it is in contact with a surface can also trigger crystal formation.
But not all nucleation sites are the same - some are more efficient than others! A bacterium called Pseudomonas syringae is a very efficient nucleator of water. P. syringae has a protein on its surface that rearranges water molecules in its proximity and in this way facilitates the formation of ice crystals (1).
Because P. syringae is such an efficient nucleator, it can be used for a very impressive little trick.
The bacteria can freeze water in an instant!


The trick is that the water in these bottles is already below 0°C but there's no nucleation site! This state is called supercooled. When the P. syringae suspension is added to the water, the bacteria's surface serves as a nucleation site and triggers freezing, which spreads like a chin-reaction through the whole bottle.

The ice-nucleating properties of P. syringae do not only play a role in entertaining lab stunts. The bacteria is a plant pathogen and causes frost damages in them because it raises the freezing temperature of water. The ice crystals can damage the plant cells. P. syringae is also an additive to snowmaking systems because it allows them to operate at higher temperatures and is thought to contribute to the formation of clouds with its nucleation properties.

With all the snow that we got in southern Germany in recent weeks, one can only wonder how much of it we owe to bacteria... Because sometimes, that's all it takes for a scenic winter landscape: one snow&ice-making bacterium!

Did bacteria contribute to this winter wonderland? (Photo: C. Wermser)
Sources:
(1) Study on the ice-nucleating surface proteins of Pseudomonas syringae:
Pandey et al. Sci. Adv. 2016; 2 : e1501630
http://advances.sciencemag.org/content/2/4/e1501630


Keine Kommentare:

Kommentar veröffentlichen