The remarkable bacterium Pseudomonas syringae forms ice from water that would otherwise be too warm to freeze. This capacity has many effects, both beneficial and harmful, on our world, yet we are only just starting to understand how it performs this alchemy. A paper in Science has shed some light on the question.
P. syringae is a plant pathogen, causing disease in many valuable crops. What sets it apart from most other bacteria, however, is that it produces ice crystals on chilly nights when the temperature drops into the -4 to -2 °C (25-28 °F) range. Although this is below the melting point of water, ice will not form at these temperatures without a point of nucleation, something to start the process of crystallization.
Frost damage from P. syringae destroys plant cells that might otherwise have made it through a chilly night intact. The ski industry, however, has put a commercial version of the bacteria to use by making snow. P. syringae carried into the atmosphere contributes to the formation of ice crystals in clouds, a poorly understood function that may turn out to be more important than any effects closer to ground.
While at the Max Plank Institute for Polymer Research, Dr. Ravindra Pandey took steps to explain the process. It appears that P. syringae has more than one trick to turn water to ice.
Without something to start the crystals forming, ice like this needs to be way below freezing point to form. The bacterium Pseudomonas syringae does this better than anything else. AAAS
“Hydrogen bonding at the water-bacteria contact imposes structural ordering on the adjacent water network,” Pandey and his co-authors write. Ice crystals are highly ordered. Water, as a liquid is not, so ordering is a step towards freezing.
The paper explains that the proteins P. syringae uses are so large that they have defeated past attempts to study them through X-ray diffraction or nuclear magnetic resonance. The authors instead used sum frequency generation (SFG) spectroscopy, which combines infrared and optical laser pulses to study vibrations at the interface where bacterium and water meet. The results supported the theory that the proteins were ordering the water molecules, assisting crystallization.
“As the temperature of the water decreased, we observed a progressive increase of the SFG intensity, which implies a significant increase of interfacial water order and alignment,” the paper reports. Things looked very different when they measured the interaction of water with surfaces that do not have the same ice-producing effect.
The inaZ protein, which the bacterium uses to produce the icing effect, was examined in detail. It has hydrophilic and hydrophobic spots patterned in such a way as to produce nucleators far more potent than inanimate equivalents such as specs of dust.
Finally, the molecular alignment the bacterium produces “can promote long-range energetic coupling,” the paper reports. This efficiently transmits latent heat away from the nucleation site, promoting the formation of small ice crystals that then expand to bring in the water around them.
It remains to be seen if this knowledge is useful to farmers fighting damage to crops, or meteorologists hoping to understand cloud formation.