Ubiquity of ice nucleation in lichen — possible atmospheric implications

Ice nucleation has previously been described in only a few lichens from a single location. Here we greatly extend this work and suggest that in lichens ice nucleation is a water harvesting adaption. Fifty-seven lichen samples from a variety of widespread locations were tested for ice nucleation by differential scanning calorimetry (DSC). Samples initiated freezing in the range -5.1° to -20°C and the median freezing temperature was -7.2°C. The vapour pressure difference between ice and water is significant at this temperature, and so ice grows at the expense of water (Bergeron—Findeisen process). Therefore, the ability to form ice at these temperatures provides a useful water-harvesting mechanism for lichens. Ice nucleation appears to be ubiquitous in lichens and is more likely to be associated with the mycobiont and may influence atmospheric processes.