Bio-precipitation or self-cryopreservation: Why does pollen nucleate ice?

Nucleation of crystalline ice from supercooled water plays a critical role in the glaciation of mixed-phase clouds and thereby impacts both weather and climate (Murray et al., 2012, Vergara-Temprado et al., 2018). Ice-active polysaccharides (IAPs) associated with pollen grains of some plant species are known to nucleate ice effectively and are present in the atmosphere in potentially significant quantities (Dreischmeier et al., 2017, Pummer et al., 2012, O'Sullivan et al., 2015, Diehl et al., 2002, von Blohn et al., 2005).
Ice nucleation caused by birch pollen IAPs is remarkably specific in nature, occurring sharply at around -16C (Fig. 1(a)), indicating that IAPs have evolved to interact with ice. It is not known why birch pollen produces IAPs or how well the vast majority of pollens nucleate ice as less than 10 species have been studied (von Blohn et al., 2005). There are two obvious reasons for pollens to evolve IAPs.
1) To control precipitation and aid return to the ground having been lofted to altitude by wind. This is an example of 'bio-precipitation' where life interacts with clouds to propagate itself.
2) To improve tolerance of cold temperatures by controlling ice formation, be it in clouds or at ground level. Different species pollens are known to be desiccation tolerant to varying extents, depending on the specific dispersal methods they employ. This may be related to their ability to nucleate ice.
The student will collect pollen from plant species held in the collections at Royal Botanic Gardens (RGB) Kew and measure their ice nucleation ability and freeze tolerance to generate a comprehensive multi-species dataset. Armed with this dataset the student will conduct an analysis of the phylogenetic background and ecological role of pollen ice nucleation, allowing an assessment of why pollen nucleates ice. This information will be of ecological interest and improve understanding of biosphere/atmosphere interaction.

Methodology: Pollen grains from a wide range of plant species will be obtained from the RGB Kew collections and their ice nucleation ability assessed using droplet freezing techniques (Whale et al., 2015). The polysaccharides responsible for ice nucleation will be isolated by ice chromatography and characterized. A test for the presence of pollen IAPs and a method for selectively denaturing them will then be developed. The relationship between ice nucleation and pollen desiccation tolerance will be obtained from the literature and empirical determination (Nebot et al., 2018). Pollen grains will be frozen in liquid water after Williams (2013), and post-thaw viability assessed. Pollen viability tests will be optimized at the species level. This will show whether the IAPs help pollen grains to survive freezing. Results will be compared with existing phylogenetic and ecological datasets to assess which plant families nucleate ice and to trace back the genetic origin of plant ice nucleation (e.g. Colville and Pritchard, 2019)