Abstract
Abstract. Fungal spores are ubiquitous biological aerosols, which are considered to act as ice nuclei. In this study the ice nucleation (IN) activity of spores harvested from 29 fungal strains belonging to 21 different species was tested in the immersion freezing mode by microscopic observation of water-in-oil emulsions. Spores of 8 of these strains were also investigated in a microdroplet freezing array instrument. The focus was laid on species of economical, ecological or sanitary significance. Besides common molds (Ascomycota), some representatives of the widespread group of mushrooms (Basidiomycota) were also investigated. Fusarium avenaceum was the only sample showing IN activity at relatively high temperatures (about 264 K), while the other investigated fungal spores showed no freezing above 248 K. Many of the samples indeed froze at homogeneous ice nucleation temperatures (about 237 K). In combination with other studies, this suggests that only a limited number of species may act as atmospheric ice nuclei. This would be analogous to what is already known for the bacterial ice nuclei. Apart from that, we selected a set of fungal strains from different sites and exposed them to occasional freezing stress during their cultivation. This was in order to test if the exposure to a cold environment encourages the expression of ice nuclei during growth as a way of adaptation. Although the total protein expression was altered by this treatment, it had no significant impact on the IN activity.
Highlights
Ice nucleation (IN) induced by primary biological aerosol particles, which include pollen, bacteria, and spores of fungi, is a current topic of intense debate
We chose the set of samples depending on four criteria: (1) species which had not been analysed concerning their IN activity before, (2) two well-studied Fusarium species in order to be able to compare our results with other published studies, (3) species which are abundant in the atmosphere, such as Agaricomycetes (Fröhlich-Nowoisky et al, 2009; Pashley et al, 2012), Cladosporium (Schüller, 2008; Ariya et al, 2009) and typical food molds (Aspergillus and Penicillium), and (4) species that have been found beyond the tropopause, like Engyodontium album, A. niger and P. chrysogenum (Imshenetsky et al, 1978; Pearce et al, 2009)
Cultures of A. fumigatus, A. niger, A. oryzae, C. cladosporioides, C. herbarum, E. album, F. avenaceum, F. oxysporum, P. chrysogenum, P. digitatum, P. glabrum, T. atroviride, T. virens, T. longibrachiatum and T. reesei were obtained from the University of Natural Resources and Life Sciences in Vienna (Austria) and the TU Collection of Industrial Microorganisms (TUCIM; Vienna University of Technology, Vienna, Austria)
Summary
Ice nucleation (IN) induced by primary biological aerosol particles, which include pollen, bacteria, and spores of fungi, is a current topic of intense debate. Several bacterial species (Pseudomonas syringae, Pseudomonas putida, Erwinia herbicola) were already found to catalyse ice formation at temperatures as warm as about 271 K, which is much higher than for most organic or inorganic substances (e.g. GurianSherman and Lindow, 1993; Morris et al, 2004; Vali et al, 1976; Warren and Wolber, 1991). Microorganisms, which inhabit plant surfaces, are able to initiate the ice formation and cause frost injury to frost-sensitive plants (Gurian-Sherman and Lindow, 1993) – what resolves in a negative impact on agricultural crops. Ice nuclei are useful in ice nucleation-limited processes such as artificial snow production, the freezing of some food products, and possibly in future weather modification schemes (Gurian-Sherman and Lindow, 1993). It was proposed that these ice nuclei are proteins, but have
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