Ice nucleation activity in the widespread soil fungus Mortierella alpina

Abstract. Biological residues in soil dust are a potentially strong source of atmospheric ice nuclei (IN). So far, however, the abundance, diversity, sources, seasonality, and role of biological – in particular, fungal – IN in soil dust have not been characterized. By analysis of the culturable fungi in topsoils, from a range of different land use and ecosystem types in southeast Wyoming, we found ice-nucleation-active (INA) fungi to be both widespread and abundant, particularly in soils with recent inputs of decomposable organic matter. Across all investigated soils, 8% of fungal isolates were INA. All INA isolates initiated freezing at −5 to −6 °C, and belonged to a single zygomycotic species, Mortierella alpina (Mortierellales, Mortierellomycotina). To our knowledge this is the first report of ice nucleation activity in a zygomycotic fungi because the few known INA fungi all belong to the phyla Ascomycota and Basidiomycota. M. alpina is known to be saprobic and widespread in soil, and Mortierella spores are present in air and rain. Sequencing of the ITS region and the gene for γ-linolenic elongase revealed four distinct clades, affiliated to different soil types. The IN produced by M. alpina seem to be proteinaceous, < 300 kDa in size, and can be easily washed off the mycelium. Ice nucleating fungal mycelium will ramify topsoils and probably also release cell-free IN into it. If these IN survive decomposition or are adsorbed onto mineral surfaces, their contribution might accumulate over time, perhaps to be transported with soil dust and influencing its ice nucleating properties.