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H. Grothe:
"Biological Ice Nucleation in the Atmosphere and the Biosphere";
Vortrag: Seminar of the school of industrial engineering, Universidad de Castilla-La Mancha, Toledo, Spain (eingeladen); 30.10.2017.

From the thermodynamic point of view, ice and snow can form already at temperatures slightly below its melting point, i.e. below zero degrees Celsius. Actually, ultrapure, liquid water can be supercooled down to minus forty degrees Celsius without freezing. The reason is a kinetic activation barrier, which hinders the phase transition. However, water impurities, e.g. biological material or organic particles, can lower this activation barrier and can thus catalyze the phase transition. This process is called heterogeneous ice nucleation and it plays an important role in many biological, meteorological and technical processes, e.g. in the formation of atmospheric ice clouds [1].
The most effective ice nucleus is ice itself, since it provides the own hexagonal structure, on which water molecules from the liquid phase can be oriented to form further ice phase. The most effective heterogeneous ice nucleus is the bacterium pseudomonas syringae. The reason is a protein at its outer cell membrane, which exhibits a hexagonal, ice-like structure. Furthermore, many other bacteria, fungal spores, and pollens carry also very effective ice nuclei, many of which in fact are macromolecules.
Macromolecular ice nuclei have for a long time been neglected by atmospheric scientists. However, plants are known by biologists to produce macromolecular ice nuclei as a part of their low-temperature survival strategy. In the past, it has been shown by us that birch pollen exhibit ice nucleation active macromolecules at their surface [2, 3]. These molecules can be washed off from the pollen grains and nucleate ice independently. Only very recently, we found the same ice nuclei also on secondary and primary wood and on leafs of birch trees. The question remains if these biological ice nuclei can be dispersed through the atmosphere and can impact cloud glaciation processes.

[1] T. Bartels-Rausch, V. Bergeron, J. Cartwright, R. Escribano, J. Finney, H. Grothe, P. Gutierrez, J. Haapala, W. Kuhs, J. Pettersson, S. Price, C. Sainz-Dıaz, D Stokes, G. Strazzulla, E. Thomson, H. Trinks, and N. Uras-Aytemiz, Rev. Mod. Phys. 84 (2012)885.
[2] B.G. Pummer, H. Bauer, J. Bernardi, S. Bleicher, and H. Grothe, Atm. Chem. Phys., 12 (2012) 2541.
[3] B.G. Pummer, C. Budke, S. Augustin-Bauditz, D. Niedermeier, L. Felgitsch, C. Kampf, R. Huber, K. Liedl, T. Loerting, T. Moschen, M. Schauperl, M. Tollinger, C. Morris, H. Wex, H. Grothe, U. Pöschl, T. Koop, and J. Fröhlich-Nowoisky, Atm. Chem. Phys. 15 (2015) 4077.

http://publik.tuwien.ac.at/files/publik_265658.pdf