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Ice nucleation. A water harvesting mechanism in lichens?- Part II


by Bruce Moffett

Although some species of fungi and lichen catalyse the freezing of water, the extent and basis of eukaryotic ice nucleating abilities have been less fully explored than those of bacteria.  Of fifteen lichens tested previously for ice nucleation activity, thirteen were active at temperatures warmer than -8°C(Kieft 1988).  Following the estimation that there are 1014 tonnes of lichen on earth, we have greatly extended this work to include a much greater variety of lichens, and have demonstrated that this is a very common, perhaps ubiquitous feature.   We use Differential Scanning Calorimetry (DSC) to determine ice nucleation. Approximately 0.1 mg of sample immersed in 10 µL of molecular grade water was put in a sealed aluminium crucible. This was then cooled progressively from 0ºC to -30ºC (1ºC min-1).  At the point of freezing, the release of latent heat was detected with DSC.   The overall distribution of ice nucleation temperatures are illustrated in Fig 1.  There is no obvious correlation with either location or genus. The samples from more northerly regions appear to have ice nucleation activities that cluster at the higher temperatures.   This may be due to the low number of samples analysed from these regions.   Most biological ice nucleation has been studied in bacteria in which ice nucleation can occur at temperatures up to -1.5ºC. This higher temperature of activity might be useful for bacteria because it is thought to play a role in helping bacteria obtain nutrients by damaging plant material. We suggest that lichens freeze water at lower temperatures as a water gathering mechanism. However further work is required to test this hypothesis.

Why do we suspect that ice nucleation helps lichens acquire water?  The reason is based simply on  the laws of physics. The vapour pressure over ice is lower than that of water at the same temperature. Therefore once ice has formed, by the nucleation of dew for example, additional water will be drawn onto the ice crystal and freezes on contact (Fig 2). When the temperature increases, the ice melts and the water is available for metabolism. By initiating freezing at temperatures at which the vapour pressure difference between water and ice is at its maximum (-5 to -15ºC (Pruppacher and Klett 1997)), lichens could effectively gather water in addition to what has accumulated via condensation.

Kieft, T.(1988). Ice Nucleation Activity in Lichens Applied And Environmental Microbiology 54(7):1678-168

Pruppacher H. and Klett J. (1997). Microphysics of Clouds and Precipitation. 2nd edition, Kluwer Academic Publishers.  



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