Plant pollen and spores as sources of ice nucleating particles

Soluble molecules released from plant pollen can nucleate ice from supercooled water and are an enigmatic source of atmospherically relevant biological ice nucleators. Recently, it has been highlighted that ice nucleating particles from pollen may possess greater potential to impact cloud glaciation than previously considered, as fragments generated by pollen bursting under atmospheric conditions could act as carriers of ice nucleating molecules, with significantly longer residence times than whole pollen grains1,2. Previous studies have indicated a range in ice nucleation activity across pollen samples, but still relatively little is known about the structure of the molecules responsible or the basis for this variability3,4. Our collaboration with the Royal Botanic Gardens, Kew, UK has enabled the collection of over fifty pollen samples from across taxa, from representatives with different pollination methods, pollination times and growth climates. Immersion mode ice nucleation experiments reveal that the ice nucleation ability of pollen is highly diverse; amongst our collections we identify particularly active samples (mean freezing temperature of microlitre droplets, T50 = -7.6 °C for Pinus mugo pollen solution) and others with far lower activity (T50 = -23.8 °C for Musa rubra pollen solution). Examining the relationship between this activity and selected characteristics, no dependency on various plant and pollen features could be determined, which may indicate that the ice nucleating molecules from pollen fulfil a distinct biological function and nucleate ice incidentally. Looking to earlier diverging plant lineages, we tested the activity of fern spores and find that they also release molecules in water which can nucleate ice. These ice nucleating molecules demonstrate absorbances consistent with polysaccharides from pollen. Ferns colonise diverse habitats and their spores, primarily transported by wind, are present in quantities comparable to pollen grains in the air over vegetated regions5. Better understanding these potential sources of atmospheric ice nuclei is essential for improving climate model prediction of their impacts. Our results suggest that these ice nucleating molecules evolved prior to the divergence of seed plants and are conserved in the spores and pollen of extant plants across the phylogeny. References 1. Burkart, J., Gratzl, J., Seifried, T., Bieber, P. & Grothe, H. Subpollen particles (SPP) of birch as carriers of ice nucleating macromolecules. Biogeosciences Discuss. 1–15 (2021). 2. Werchner, S. et al. When Do Subpollen Particles Become Relevant for Ice Nucleation Processes in Clouds? J. Geophys. Res. Atmos. 127, e2021JD036340 (2022). 3. Pummer, B. G., Bauer, H., Bernardi, J., Bleicher, S. & Grothe, H. Suspendable macromolecules are responsible for ice nucleation activity of birch and conifer pollen. Atmos. Chem. Phys. 12, 2541–2550 (2012). 4. Dreischmeier, K., Budke, C., Wiehemeier, L., Kottke, T. & Koop, T. Boreal pollen contain ice-nucleating as well as ice-binding ‘antifreeze’ polysaccharides. Sci. Rep. 7, 1–13 (2017). 5. Després, V. R. et al. Primary biological aerosol particles in the atmosphere: A review. Tellus, Ser. B Chem. Phys. Meteorol. 64, (2012).