Ice Nucleation Activity in Lichens

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 × 10 6 to more than 1 × 10 8 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 × 10 6 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.