Recent Study Uncovers How Fungi Influence Ice Formation

 When liquid water cools down to a low enough temperature, it solidifies and freezes into ice – a seemingly simple process. Yet, what many don’t realize is that fungi can play an important role in this affair. One notable fungus, Fusarium acuminatum, has the remarkable ability to transform water into ice.

A recent study backed by the U.S. National Science Foundation has uncovered how the biological agents from F. acuminatum initiate ice formation, offering valuable insights into fungi’s role in icy environments.

Living ice catalysts 

Most people are taught that water freezes below 0 degrees Celsius (32 degrees Fahrenheit), but this isn’t always the case, as the freezing process can be delayed due to energy barriers during the transition phase. Even in low temperatures, pure water can remain a liquid down to -46°C.

To get past these energy barriers, water molecules need some sort of push to trigger the crystallization process (1). Particles or impurities, known as ice nucleators (INs), allow water molecules to build ice crystals. INs can come from natural non-living sources like dust, which makes water freeze between -15  to -30°C, or from microorganisms, like bacteria and fungi, which are more effective, with freezing temperatures between 2 and -15°C. These organisms are thought to have evolved this ability as a means to survive in cold environments.

Many fungi in the Fusarium genus are excellent INs and are widely found in soil, on plants, and even in the clouds and atmosphere (2). Since these fungi are located in such places, it’s crucial for scientists to understand how they affect the atmosphere, as the production of ice can cause frost damage to plants and potentially change weather patterns.

Scientists still don’t fully understand the role of ice formation in nature or the chemical makeup and structure of ice-making molecules in fungi compared to bacteria. Although previous research has attempted to determine how large these fungal INs are, it’s difficult to measure them accurately.

Fungal INs are believed to be partially made of protein since they stop working once exposed to heat and UV light at a certain wavelength. The researchers of the study also believe this claim and think that Fusarium has ice nucleation proteins surrounding their cell membranes that are stable in various conditions. To test this claim and find why F. acuminatum’s INs work so well for ice growth, the study looked into their size, chemical makeup, and capabilities.

Groundbreaking discoveries 

The researchers conducted multiple tests on an F. acuminatum liquid extract, which included all of the ice-forming parts of the fungus in varying concentrations. The extract was diluted in different portions and then frozen in tiny drops to figure out at what temperature they froze. These diluted drops were slowly frozen at 1 °C per minute to form a detailed profile of the fungus’s IN capabilities under different conditions.

The results indicated that the INs worked best at freezing at just a little below -4°C, demonstrating the F. acuminatum efficiency at triggering ice formation even at temperatures where water does not typically freeze. Furthermore, the study found that the fungus’ INs were particularly good at sticking to ice. Using a specialized technique called ice-affinity purification, the ice-bindings INs were integrated into growing ice formations while filtering out the other materials that don’t bind to ice. This process allowed the researchers to successfully isolate the IN molecules from F. acuminatum for a more precise examination.

When these purified INs were heated, researchers found that their structures were changed irreversibly, making them lose their ability to form ice. The researchers also determined the size and makeup of the INs, finding that they are, in fact, made of incredibly small fungal proteins that can group together to form larger structures capable of making ice.

Research indicates that F. acuminatum is unique compared to bacteria since it can regain its more stable INs from smaller units. Its protein pieces can bind together in water without any other cells around. When this is done, they down groups that can create ice despite how cold it is (3).

Scientific impact and future research directions 

 F. acuminatum’s ability to form ice at relatively higher temperatures suggests that fungi might play a more significant role in ecological and atmospheric processes than previously understood. The findings also provide substantial insights into the specific functions of the biological agents of the fungus, which contributes to a broader scientific comprehension of the nature of ice nucleation. Additionally, this research can contribute to improvements in various practical applications such as food freezing, snow production, and cloud seeding.

More research is still required to entirely understand ice nucleation, as scientists are still unsure how and why the proteins in the fungus combine. The process could be for some biological purpose, or the protein could just happen to have that property.

Nonetheless, the findings are a promising contribution to the overall knowledge surrounding fungi’s role in ice formation.

“This is the positive message. Solving the puzzle of biological control of ice formation drives scientists to collaborate,” said Valeria Molinero, study co-lead and director of the University of Utah’s Henry Eyring Center for Theoretical Chemistry. “Each of us has a piece of the knowledge, but altogether we can do so much. It’s been fun.”

References

1. Moore, Emily B., and Valeria Molinero. 2011. “Structural Transformation in Supercooled Water Controls the Crystallization Rate of Ice.” Nature 479 (7374): 506–8. https://doi.org/10.1038/nature10586**.
2. Kunert, Anna T., Mira L. Pöhlker, Kai Tang, Carola S. Krevert, Carsten Wieder, Kai R. Speth, Linda E. Hanson, et al. 2019. “Macromolecular Fungal Ice Nuclei in Fusarium: Effects of Physical and Chemical Processing.” Biogeosciences 16 (23): 4647–59. https://doi.org/10.5194/bg-16-4647-2019**.
3. Schwidetzky, Ralph, Ingrid, Nadine Bothen, Anna T Backes, Arthur L DeVries, Mischa Bonn, Janine Fröhlich-Nowoisky, Valeria Molinero, and Konrad Meister. 2023. “Functional Aggregation of Cell-Free Proteins Enables Fungal Ice Nucleation.” Proceedings of the National Academy of Sciences of the United States of America 120 (46). https://doi.org/10.1073/pnas.2303243120**.