The bacteria that turns water into ice
Meet Pseudomonas syringae, a bacterium that causes disease in plants and helps make snow machines work.
It all has to do with ice nucleation — the process that forms ice crystals in the atmosphere and, thus, snow. You probably know that raindrops and snowflakes form around something. There's always a central nucleus that serves as the backbone of the water molecule structure. Usually, when people talk about this process, they use soot or some other kind of particulate matter as the example of what a nucleus can be. But bacteria can also become the nucleus of a snowflake. In fact, P. syringae is so good at forming ice crystals around itself that, in the video above, you can watch it turn cold water into ice almost instantly.
Here's how biologist Mark Martin described the process in the video:
In my Microbiology course at the University of Puget Sound, I like to demonstrate the wild and wonderful and weird microbial world. In this video, I supercool a bottle of water to approximately -6 degrees C. I then a drop of a Pseudomonas syringae culture. This organism makes an ice nucleation protein, which allows the supercooled water to immediately change to water ice...while you watch.
P. syringae gets this skill from the proteins that cover its surface membrane. The proteins basically form a physical structure that water molecules latch onto. That structure also orients the molecules in a way that prompts the formation of ice crystals. It's these proteins that really serve as the instigator of ice nucleation and they're incredibly efficient at it — far more so than dust. That means that P. syringae can get water to freeze at higher temperatures than would happen without its help. Pure water won't crystallize until temperatures dip down to -40 degrees F. If the water in our atmosphere were pure, most of us would have never seen snow. Add in the proteins from P. syringae, though, and, suddenly, ice can form at 27 degrees F. You can even get ice formation at higher temperatures than that, depending on the specific strain of P. syringae involved, and how densely the ice-forming proteins are packed along its surface.
Commercial snow machines use the proteins (though not the bacteria itself) to help instigate the creation of snow on ski mountains. In other words, you can thank P. syringae for all the snowboarding and downhill ski action the Winter Olympics.
Why would bacteria develop this particular ability? Nobody knows for sure, but the current hypothesis is that it's part of P. syringae's life cycle and how the bacteria spreads. The idea is that the bacterium infects plants, multiplying and growing the same way that a bacterial infection multiplies and grows in your body. There's evidence, though, that instead of just spreading on the air from one plant to another, P. syringae can also go higher, pushed up into the atmosphere by wind currents. By forming ice crystals, which become rain or snow, the bacteria are able to fall back down to Earth, infecting plants far away from their original hosts.