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Biological ice nucleation plays a key role in the survival of cold-adapted organisms. Several species of bacteria, fungi, and insects produce ice nucleators (INs) that enable ice formation at temperatures above −10 °C. Bacteria and fungi produce particularly potent INs that can promote water crystallization above −5 °C. Bacterial INs consist of extended protein units that aggregate to achieve superior functionality. Despite decades of research, the nature and identity of fungal INs remain elusive. Here, we combine ice nucleation measurements, physicochemical characterization, numerical modeling, and nucleation theory to shed light on the size and nature of the INs from the fungusFusarium acuminatum. We find ice-binding and ice-shaping activity ofFusariumIN, suggesting a potential connection between ice growth promotion and inhibition. We demonstrate that fungal INs are composed of small 5.3 kDa protein subunits that assemble into ice-nucleating complexes that can contain more than 100 subunits.FusariumINs retain high ice-nucleation activity even when only the ~12 kDa fraction of size-excluded proteins are initially present, suggesting robust pathways for their functional aggregation in cell-free aqueous environments. We conclude that the use of small proteins to build large assemblies is a common strategy among organisms to create potent biological INs. 

Abstract. Forty years ago, lichens were identified as extraordinary biological icenucleators (INs) that enable ice formation at temperatures close to0 ∘C. By employing INs, lichens thrive in freezing environmentsthat surpass the physiological limits of other vegetation, thus making themthe majority of vegetative biomass in northern ecosystems. Aerosolizedlichen INs might further impact cloud glaciation and have the potential toalter atmospheric processes in a warming Arctic. Despite the ecologicalimportance and formidable ice nucleation activities, the abundance,diversity, sources, and role of ice nucleation in lichens remain poorlyunderstood. Here, we investigate the ice nucleation capabilities of lichenscollected from various ecosystems across Alaska. We find ice nucleatingactivity in lichen to be widespread, particularly in the coastal rainforestof southeast Alaska. Across 29 investigated lichen, all species show icenucleation temperatures above −15 ∘C, and ∼30 %initiate freezing at temperatures above −6 ∘C. Concentrationseries of lichen ice nucleation assays in combination with statisticalanalysis reveal that the lichens contain two subpopulations of INs, similarto previous observations in bacteria. However, unlike the bacterial INs, thelichen INs appear as independent subpopulations resistant to freeze–thawcycles and against temperature treatment. The ubiquity and high stability ofthe lichen INs suggest that they can impact local atmospheric processes andthat ice nucleation activity is an essential trait for their survival incold environments.