<i>Peltigera</i> lichen thalli produce highly potent ice-nucleating agents

Abstract. From extracellular freezing to cloud glaciation, the crystallization of water is ubiquitous and shapes life as we know it. Efficient biological ice nucleators (INs) are crucial for organism survival in cold environments and, when aerosolized, serve as a significant source of atmospheric ice nuclei. Several lichen species have been identified as potent INs capable of inducing freezing at high subzero temperatures. Despite their importance, the abundance and diversity of lichen INs are still not well understood. Here, we investigate ice nucleation activity in the cyanolichen-forming genus Peltigera from across a range of ecosystems in the Arctic, the northwestern United States, and Central and South America. We find strong IN activity in all tested Peltigera species, with ice nucleation temperatures above −12 °C and 35 % of the samples initiating freezing at temperatures at or above −6.2 °C. The Peltigera INs in aqueous extract appear to be resistant to freeze–thaw cycles, suggesting that they can survive dispersal through the atmosphere and thereby potentially influence precipitation patterns. An axenic fungal culture termed L01-tf-B03, from the lichen Peltigera britannica JNU22, displays an ice nucleation temperature of −5.6 °C at 1 mg mL−1 and retains remarkably high IN activity at concentrations as low as 0.1 ng mL−1. Our analysis suggests that the INs released from this fungus in culture are 1000 times more potent than the most active bacterial INs from Pseudomonas syringae. The global distribution of Peltigera lichens, in combination with the IN activity, emphasizes their potential to act as powerful ice-nucleating agents in the atmosphere.