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Appl Environ Microbiol. 1988 July; 54(7): 1678-1681
Copyright © 1988, American Society for Microbiology. All Rights Reserved.

Ice Nucleation Activity in Lichens

Department of Biology, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801

ABSTRACT

A newly discovered form of biological ice nucleus associated with lichens is described. Ice nucleation spectra of a variety of lichens from the southwestern United States were measured by the drop-freezing method. Several epilithic lichen samples of the genera Rhizoplaca, Xanthoparmelia, and Xanthoria had nuclei active at temperatures as warm as –2.3°C and had densities of 2.3 x 10⁶ to more than 1 x 10⁸ nuclei g^–1 at –5°C (2 to 4 orders of magnitude higher than any plants infected with ice nucleation-active bacteria). Most lichens tested had nucleation activity above –8°C. Lichen substrates (rocks, plants, and soil) showed negligible activity above –8°C. Ice nucleation-active bacteria were not isolated from the lichens, and activity was not destroyed by heat (70°C) or sonication, indicating that lichen-associated ice nuclei are nonbacterial in origin and differ chemically from previously described biological ice nuclei. An axenic culture of the lichen fungus Rhizoplaca chrysoleuca showed detectable ice nucleation activity at –1.9°C and an ice nucleation density of 4.5 x 10⁶ nuclei g^–1 at –5°C. It is hypothesized that these lichens, which are both frost tolerant and dependent on atmospheric moisture, derive benefit in the form of increased moisture deposition as a result of ice nucleation.

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