Investigation of Physical and Chemical Characteristics of Ice-Nucleating Macromolecules from Birch Trees

Several biological particles are able to trigger heterogeneous ice nucleation at subzero temperatures above -38°C. The influence of these biological ice-nucleating particles (BINPs) on atmospheric processes including cloud glaciation and precipitation formation, as well as transport mechanisms of BINPs from the land surface into the atmosphere remain uncertain. Plants are often not considered as sources of atmospheric BINPs. However, many species of plants like winter rye [1], certain berries [2, 3], pine and birch trees [4, 5] have shown to contain BINPs or rather ice-nucleating macromolecules (INMs). These plants use INMs in their extracellular spaces to survive cold stress. If those INMs are easily available to their direct surroundings, they add an important new source for BINPs in the atmosphere, which has not received enough attention in the past.

The chemical structure of INMs from birch trees is still not elucidated. It is hypothesized that polysaccharides may be responsible for ice nucleation activity of birch pollen washing water [6, 7]. In this study, we analyzed aqueous extracts of different birch tree tissues including pollen, leaves and woods. Freezing experiments were carried out in immersion freezing mode using the cryo-microscopy setup VODCA (Vienna Optical Droplet Crystallization Analyzer). Purification techniques were applied to not only isolate INMs but also to investigate their physical and chemical characteristics. Centrifugal filtration experiments show that the size of INMs from birch tree samples varies between 10 and 100 kDa. Solid phase extraction measurements indicate amphiphilic properties. Furthermore, results from fluorescence measurements and enzyme assays underline the importance of proteins in the freezing process.

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