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How the world’s chillest bacteria make ice

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They may be responsible for creating clouds and rain

Cultures of Pseudomonas syringae
Cultures of Pseudomonas syringae
Howard F. Schwartz, Colorado State University

Today, scientists have uncovered how tiny bacteria — nature’s ice machines — create ice crystals. By bringing the cold, these incredible organisms are believed to create clouds and cause snow and rain. Though the new study, published today in the journal Science Advances, doesn’t confirm whether these are rain-making bacteria, it points to how exactly they turn water into ice.

The bacteria, Pseudomonas syringae, have equipped themselves to cause the cold with proteins that create ice crystals at temperatures that don't normally freeze water. P. syringae live on agricultural crops, plants, and trees and use their ice-making abilities to cause frost damage. The ice crystals they produce basically shatter plants’ tissues so the bacteria can access the plants’ nutrients. We’ve even harnessed these organisms for our own purposes: P. syringae are routinely used to make artificial snow in ski resorts around the world.

"It’s pretty spectacular."

In past decades, P. syringae have been found in the atmosphere, as well as in (real) snow from all over the world, from the US to Europe and even Antarctica. Their ice-making powers are widely believed to create clouds and cause rain. Because of this potentially important role in atmospheric events, P. syringae have been studied extensively in past years. But up until now, scientists could only guess how the bacteria made water freeze. Today’s study is the first one to show in an experiment how the ice-making mechanism actually works.

"They nucleate ice to attack plant cells. They’re also used in artificial snowmaking. Then, they’re also involved in climate processes," says Tobias Weidner, a bioengineer at the Max Planck Institute for Polymer Research who co-authored the study. "It’s pretty spectacular."

In the study, an international team of researchers used a commercial product called Snowmax, a powder used to make artificial snow that contains dead P. syringae bacteria. Though the bacteria aren’t alive, they still have the ice-making protein anchored to their outer cell walls, so they still form ice. The researchers found that, when P. syringae interact with water, the protein shifts molecules around — ordering them into a template for forming ice crystals. In addition, the bacteria acted almost like a refrigerator, by removing heat from the water. These observations, researchers say, explain why P. syringae is such an effective ice-making bacteria.

(Mark Martin/University of Puget Sound)

"People have tried to understand how [the bacteria] control ice nucleation and they’ve done theoretical and computational studies," says Weidner. "And now, this is the first time we have experimental data that shows this actually happens."

The study, of course, has limitations. In fact, it didn’t only use experimental data; it also used computer modeling. The researchers observed how the water and the bacteria interacted; based on those observations, they made an informed guess about what the bacteria’s ice-making proteins were actually doing. Boris Vinatzer, a plant pathologist at Virginia Tech’s College of Agriculture and Life Sciences who’s studied P. syringae for years, says that’s one of the study’s major flaws. "It is still a hypothesis, an interpretation of some measurements and some modeling, but there is no direct observation of what happens," he says. "In my interpretation, this is not a firm conclusion based on data. It’s a likely explanation of the observation." It’s nonetheless interesting, Vinatzer says — just not the final word.

"To make rain, your clouds have to form first an ice crystal, even in the Sahara desert."

Cindy Morris, a biologist and ecologist at the French National Institute for Agricultural Research who’s been working on P. syringae for 30 years, criticized the researchers’ use of Snowmax. The commercial product used to make artificial snow doesn’t only contain the dead bacteria, she says. Also in the mix are a variety of nutrients used to grow the bacteria before they're "killed" by being freeze-dried and treated with radiation. Morris says she would have preferred that the researchers used fresh cultures of P. syringae, just to make sure that no other substances were getting in the way of the results. "It doesn’t mean that their study is not valid," she says. "It just would have been, from a microbiologist point of view, cleaner." However, using Snowmax could have its advantages. Other researchers have easy access to the product and could replicate the study.

Both Morris and Russ Schnell, an atmospheric scientist with the National Oceanic and Atmospheric Administration said that though the study is very interesting and thorough, it doesn’t explain whether the bacteria’s ice-making process has a role in creating ice crystals in the atmosphere and contributing to rain. And that’s what scientists are really dying to know. To understand why P. syringae are thought to be the rainmaker bacteria, it’s helpful to remember how water actually works.

Pure water doesn’t freeze at 32 degrees Fahrenheit (0 degrees Celsius). It actually stays liquid until about - 40 degrees Fahrenheit (- 40 degrees Celsius). To freeze at higher temperatures, water needs a fleck of dust, soot, or sea salt — something to serve as a center that water molecules can latch onto. Most scientists believe that P. syringae are swept by wind from the ground to the sky. In the atmosphere, these high-flying, ice-making bacteria lower the freezing temperature to around 25 to 18 degrees Fahrenheit (- 4 to - 8 degrees Celsius) and form ice crystals. That creates clouds, which are basically agglomerations of water droplets and ice crystals.

Pure water doesn’t freeze at 32 degrees Fahrenheit

"To make rain, your clouds have to form first an ice crystal, even in the Sahara desert," says Schnell. As the ice crystals fall down, they turn into rain if it’s warm and snow if it’s cold. The theory that bacteria like P. syringae have a role in causing precipitation, however, has never been proved. "Intuitively it feels right, circumstantial evidence says yes, but that final link has not been done yet," says Schnell. "But everyone is confident that it will."

Today’s study, unfortunately, isn’t the paper Morris and Schell hoped to see. "This paper tells us about the physics of what’s going on," says Morris. "It doesn’t tell us any more than we knew before about the role of these bacteria in precipitation." Schnell added that this is a paper the scientific community was expecting to see at some point. "There’ll be more papers like this," he says.