They kill crops, coax snowfall to fall out of the atmosphere, and help ski resorts stay open during unseasonably warm weather. These tiny life-forms are called ice-minus bacteria, and their talent is forcing water vapor to form ice—creating frost at temperatures where the water wouldn't otherwise freeze. Today scientists may finally understand how these bacteria work their molecular magic.

A team of scientists led by Tobias Weidner, a physicist at the Max Planck Institute for Polymer Research in Mainz, Germany, has just completed a fascinating new study on how ice-minus bacteria manipulate water into freezing. Weidner's team tracked how they use two strange tricks to stack water into ice crystals, molecule by molecule. The research appears in Science Advances.

Whether on the ground or airborne, ice-minus bacteria can have an enormous effect on making sure water becomes ice as close to the freezing point (0 degrees Celsius, or 32 Fahrenheit) as possible. "For example, if you have pure water suspended in the atmosphere, sometimes the water droplets won't freeze until they're at -40 degrees centigrade. But if these bacteria are around, that freezing point can drop to just -5 degrees," Weidner says.

Ski resorts put these bacteria in with the water and pressured air they blast from snow cannons to make fake powder

To figure out what exactly these bacteria are doing, the scientists studied a commercial brand of inactive ice-minus bacteria. The species is Pseudomonas syringae, but is available under the product name Snomax. Ski resorts put these bacteria in with the water and pressured air they blast from snow cannons to make fake powder, ensuring the water will freeze before it hits the ground.

Weidner's team analyzed the bacteria with an imaging tool called a sum frequency generation spectrometer. As Weidner explains it, this tool uses infrared and visible light to gently "wiggle and jiggle the molecules on the surface" of the bacteria while it's creating ice. These vibrations paint an unprecedented view of how these surface-level molecules forge and steer the growth of ice crystals.

The ice-minus bacteria are doing two very interesting things. The first is happening the surface of the bacteria, which is covered in a layer of particular proteins. "It looks to us like these proteins can form patches that are really hydrophobic, meaning they repel water, and other patches that are really hydrophilic, meaning they attract water," says Weidner.

How these patches are organized is part of the magic. With specific patterns "these different patches create nano-sized pockets of water that have different densities, and it's those different densities that create the tension" that coaxes water molecules to arrange in just the right way to form ice crystals—a process called ice nucleation. In other words, when water vapor lands onto an ice-minus bacteria, it becomes molecularly squished in a specific way that results in crystallization.

The second thing the bacteria are doing—and Weidner's team still doesn't fully understand how they're doing it—is shuffling around heat while the bacteria's patchy coatings start forming those ice crystals. The process of ice nucleation—forcing water into a crystal state— actually creates a small amount of energy, which can interrupt the growth of additional frost. "In our study, we observed for the first time that these bacteria basically can shuffle away that extra energy really efficiently," he says.

The team hopes to confirm this discovery by seeing if they can copy the bacteria's tactics onto man-made materials. "These patches are something we can make in our nano-fabrication lab, and it would be exciting to copy the patterns we found onto various surfaces and see if our hypothesis add up," he says. "If we can figure out ways to mimic this bacteria and learn either how to initiate surface icing, or do the exact opposite and avoid it, well that could have a lot of applications..." for everything from "advances in cryobiology and learning how to keep tissues cool, to developing electric land lines that could better weather snowstorms and lots of ice."

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