Water freezes at zero degrees Celsius - that's what we learn in school. However, pure water without impurities only freezes at much lower temperatures around -40 degrees Celsius. Only so-called condensation nuclei - such as dust or dirt particles - cause higher freezing temperatures. Certain biological substances found in pollen, bacteria or even fungi are particularly efficient in supporting such ice formation. However, the molecular basis and precise mechanisms of such "biological ice nuclei" were poorly understood until now.
"We looked at a fungus of the genus Fusarium that is particularly efficient at driving ice formation," said Konrad Meister, a professor at Boise State University (USA) and group leader at the Max Planck Institute for Polymer Research in Mainz in Mischa Bonn's department. The fungus is a well-known plant pathogen worldwide. Spores can, for example, rise up into the atmosphere, where they boost ice formation with special ice-nucleating substances.
The international research team led by Meister has now been able to show what exactly enables ice formation and how the ice-making substances are structured. They were able to show that small proteins are involved, which can aggregate outside the fungus to form larger protein aggregates. Thanks to their size, these aggregates enable very efficient ice-making. This could be confirmed by Meister and colleagues experimentally and not least also with theoretical calculations of American colleagues around Prof. Valeria Molinero of the University of Utah.
According to Meister, the mechanism of forming larger aggregates from smaller building blocks is found not only in fungi but also in other organisms. "Nevertheless, we were surprised by the small size of the fungal protein building blocks compared to their efficiency," Meister points out. "Other known and similarly efficient ice-making proteins from other organisms, for example, are 25 times larger."
An even more detailed understanding of the molecular mechanisms could help to produce such efficient proteins artificially as well. Applications would include energy-efficient freezing of food, artificial generation of snow, or biological cloud seeding.
The researchers have published their results in the journal Proceedings of the National Academy of Sciences (PNAS).
