Bacteria use cool trick to make ice

Scientists have discovered how one microbe plays it cool.

Until now, it was a mystery how Pseudomonas syringae bacteria turn water into ice at temperatures above a normal freezing point. P. syringae pulls off its cool trick by rearranging nearby water molecules, researchers in the United States and Germany report online April 22 in Science Advances. This chill ability makes the microbes useful in making artificial snow at ski resorts.

Researchers knew that a particular protein on the microbes’ membranes was somehow responsible for making ice form. The team found that this ice nucleation protein, inaZ, acts as a mold for ice crystals. Alternating water-repelling and water-attracting parts of the protein tug nearby water molecules into an orderly, icelike arrangement. Once arranged into an ice-promoting formation, water molecules can quickly disperse heat energy.

This alignment process becomes more prominent as water temperatures drops toward 5˚ Celsius, a degree above the freezing point of the water the team used in their experiment (which contained a heavy form of hydrogen). Outside the lab, P. syringae can crystallize water at around –2˚ C, several degrees above the temperature at which ice crystals commonly form.       

Understanding how P. syringae freezes water could inform science beyond the slopes. In gardens, the bacteria can wreak havoc on frost-sensitive plants. And ice-forming bacteria play an important role in climate by affecting patterns of cloud formation and precipitation, the researchers say.