Supercooled Water and Bacteria’s Bag of Tricks

Ice crystals have the potential to rupture cells, killing plants. Image source

Ice crystals have the potential to rupture cells, killing plants. Image source

It may seem intuitive to think water freezes at 0°C (32°F) and boils at 100°C (212°F), but unfortunately you’d be wrong. Supercooling is when water can be lowered well past its freezing point without it turning to a solid. Normally water is liquid at room temperature. This means that water molecules are passing freely through each other. Once water turns to solid ice the molecules can no longer move past one another. They are not completely still mind you, they still vibrate; however, the molecule is held in place and cannot escape.

Phase transition refers to changing from one form of matter into another (liquid, solid, gas). These changes are dependent on both temperature and pressure. Here on Earth, we are accustomed to the weight of 14.7 pounds of pressure per square inch (at sea level). Trillions of air molecules in our atmosphere provide this weight, and this affects the phase transitions of substances. By now you may be curious, if water can be supercooled then what else is needed to freeze it other than adequate temperature and pressure? The answer might shock you, it has to do with nucleation and that is where bacteria come into play!

 FREEZING WATER IS HARDER THAN YOU MIGHT THINK

Water generally won’t freeze without the aid of microscopic particles. A nuclei, nucleator, or seed, (all different ways of saying the same thing) is needed to jump start the transition of water into ice. If the sample is pure, meaning free of all particles except the water molecules themselves, it may exist as a liquid much lower than freezing. Theoretically, water can get to -55°C before solidifying, and astonishingly, scientists have discovered liquid water in clouds at -40°C (The Naked Scientists, Cambridge University).

The term heterogeneous ice nucleation is just a fancy way of saying that an impurity is used to start the seeding of ice crystals. As you may know, a lot of stuff is floating around in the atmosphere; pollen, dust, soot, aerosols, algae, sand, and spores are just to name a few, these can all be used as nuclei to freeze water. In general terms, the water molecules form a cage around these particles. This allows the substance to take the approximate shape of an ice crystal. That crystal is now the platform needed to form more crystals, changing the sample from a liquid to a solid as it freezes. It is exciting to realize that every snowflake caught on your tongue contains a seed.

 BACTERIA’S BAG OF TRICKS

Now we can get to those ever so evolutionarily clever microbes. Not every microscopic particle will nucleate water at the same rate. We can thank bacteria for their most amazing ability to freeze water at relatively high temperatures (-8°C to -2°C). Some of the most affective ice nucleators are found in the types of bacteria: Erwinia, Pseudomonas and Xanthomonas (Nature). But why would these bacteria be so efficient at freezing water?

It is thought that bacteria use this skill for mobility. If they’re able to freeze the water around them, for instance while in a cloud, they can then be carried back to Earth in the search for food as hale or snowflakes. These unique plant pathogenic microbes can now use their talented freezing ability for a second time. Once the microbe lands on a leaf it uses a special protein on its outer membrane to mimic the shape of ice crystals.

Pseudomonas syringae shown using SEM. Source: Gordon Vrdoljak, Electron Microscopy Laboratory, U.C. Berkeley

Pseudomonas syringae shown using SEM. Source: Gordon Vrdoljak, Electron Microscopy Laboratory, U.C. Berkeley

Once ice is introduced into a plant cell the membrane bursts. It is the same way for water that gets into cracks in the road, once frozen the concrete ruptures. Now that the bacterium has cracked the shell it can feast on the rich intercellular nutrients.

One of the most amazing things to consider is that scientists estimate 40% of the nuclei floating in the atmosphere are organic in composition. It was also found that cross sections of hale could contain a thousand viable bacteria per milliliter. Without organic material and other microscopic particles, ice would be harder to come by. The ability for water to be supercooled is a fascinating aspect of chemistry. And bacterial nucleators have real world applications in food science, medicine and possible weather modification. So the next time you fill your glass with ice think about the creatures contained within that made that ice possible!

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