Investigating the Atmospheric and Biological Significance of Ice-Nucleating Macromolecules Produced by Antarctic Mosses

Ice-nucleating particle (INP) concentration in the atmosphere over the Southern Ocean represents a significant source of uncertainty in the representation of clouds in global climate models. A better understanding of the sources, properties and abundance of INPs in this region is essential to reduce this uncertainty (Murray, 2021). There is increasing evidence to suggest that ice nucleators are produced ubiquitously by land plants. Yet, the molecular basis and evolutionary origins of these ice nucleators remain obscure. Adapted for cold- and drought-tolerance, mosses dominate the Antarctic flora, with over one hundred species colonising the ice-free coastal regions of Antarctica and their land cover increasing (Roland 2024). Ice nucleation by moss spores and leaves was recorded separately by Weber (2016) and by Moffett (2015), who suggested that this activity may afford benefit as a means of gathering essential water in dry climates. In the years since, the atmospheric and biological significance of these ice nucleators has remained unexplored. Here we present evidence that mosses produce water-soluble ice-nucleating macromolecules that are present in the gametophyte and sporophyte generations. We hypothesise that the same class of ice-nucleating macromolecules are produced by the spore-producing mosses and ferns and the pollen-producing seed plants, tracing back to an ancient common ancestor. The widespread and variable nature of ice-nucleating activity in plants suggests that this activity is secondary or ‘incidental’ in function (Kinney, 2024). Nevertheless, it is conceivable that such activity may constitute an exaptation, having been selected for in the evolution of taxa adapted to specific environmental conditions. Notably, we find Antarctic moss species exhibit high ice-nucleating activity, reaching temperatures of -7.4 °C, within the range where secondary ice-production may further enhance ice crystal numbers in clouds. As such, we suggest that mosses may represent a previously unknown source of ice nucleators in the atmosphere over the Southern Ocean.

References:

N. L. H. Kinney, C. A. Hepburn, M. I. Gibson, D. Ballesteros, and T. F. Whale. High interspecific variability in ice nucleation activity suggests pollen ice nucleators are incidental. Biogeosciences, 21(13):3201–3214, 2024.

B. F. Moffett. Ice nucleation in mosses and liverworts. Lindbergia, 38(1):14–16, 2015.

B. J. Murray, K. S. Carslaw, and P. R. Field. Opinion: Cloud-phase climate feedback and the importance of ice-nucleating particles. Atmospheric Chemistry and Physics, 21(2):665–679, 2021.

T. P. Roland, O. T. Bartlett, D. J. Charman, et al. Sustained greening of the Antarctic peninsula observed from satellites. Nature Geoscience, 17:1121–1126, 2024.

C. F. Weber. Polytrichum commune spores nucleate ice and associated microorganisms increase the temperature of ice nucleation activity onset. Aerobiologia, 32(2):353–361, 2016.