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Thirsty lichens

2012/02/13

Franz Conen & Cindy Morris

What is the adaptive advantage of being able to induce ice formation at relatively warm temperatures?  For ice nucleation-active bacteria it has been proposed that their capacity to induce frost damage to plants gives them access to nutrients as the plant cells begin to leak thereby favoring growth and the ability to cause disease to plants (Lindow, 1983).  Induction of freezing of cloud droplets has also been suggested to be a means for bacteria to actively promote their deposition out of the atmosphere (Morris et al, 2008).  When the ice nucleation activity of lichens was discovered, the most plausible selective advantage of this capacity was seen in the possibility to collect extra moisture in the form of ice during a cold night with no precipitation, which would become physiologically useable when temperature increased above freezing the following day (Kieft, 1988; Ashworth and Kieft, 1995).

To test this assumption, we ran some simple experiments in an attempt to simulate how lichens might capture moisture as freezing occurs.  We used 2.5-cm diameter Millipore absorbant cellulose pads to simulate lichen “bodies”, treated them with ice nucleation-active bacteria, and then exposed them to natural conditions during clear cold nights where freezing occurred during radiative heat loss under clear night skies (Figure 1, below).  As controls, some pads were not treated with any bacteria, and others were treated with bacteria that were boiled to destroy the ice-nucleation activity.  The amount of water collected during the night was determined by the change in mass of the initially air-dry pads during the period of exposure. Several series of experiments were conducted in Basel, Switzerland and in Avignon, France.  Each experiment involved 3 replicate pads for each treatment that were exposed in an open area (on a car top or building top, for example) in a randomized complete block design on a windless night.

When we recovered the pads in the morning, they were always frozen. When weighed, some pads had collected up to 80 mg (80 µL) of water (Figure 2, below).  This was only a fraction of the total 300 µL holding capacity of the pads, but it was clearly measurable with a precision balance.  The amount of water collected by the pads was variable, but in general twice as much water was collected when relative humidity at night was 90 % or more, compared to when it was 80 % or lower. Pads treated with fresh Snomax® suspensions collected as much or more water than those treated with a boiled suspension of Snomax®.  However, in only a few cases the pads treated with ice nucleation active bacteria absorbed significantly more water than pads harboring bacteria whose activity had been destroyed by boiling.  Furthermore, non-treated pads had a rather unpredictable behavior, in some cases absorbing more water than pads treated with either boiled or fresh Snomax® suspensions.

These preliminary results suggest that there might be some conditions under which ice nucleation activity is a means for lichens to quench their thirst.  This leaves us hungry for more ideas about how to go about assessing this potential adaptive advantage of biological ice nucleation activity.

(click on the figures to enlarge them)

Figure 1. Air temperature and relative humidity over the course of three days in Basel, Switzerland, January 2012. (We thank Dr. Roland Vogt, University of Basel, and his colleagues for the data!).

Figure 2. Average water gain of cellulose pads treated with ice nucleation active and in-active bacteria (Snomax®) and exposed to conditions of freezing under radiative heat loss under open winter skies.

References

Ashworth, EN. and Kieft, TL. 1995: Ice nucleation activity associated with plants and fungi, In: Lee et al. (eds.): Biological Ice Nucleation and its Applications, American Phytopathological Society, St. Paul, MN, USA, pp. 137-162

Kieft, TL. 1998. Ice nucleation activity in lichens. Appl. Environ. Microbiol. 54:1678-1681.

Lindow SE. 1983. The role of bacterial ice nucleation in frost injury to plants. Annu. Rev. Phytopathol. 21: 363-384.

Morris CE, et al. 2008. The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle. ISME Journal 2: 321-334.

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One Comment leave one →
  1. 2014/01/07 20:33

    Hi Cindy,
    I like this idea, and came to the same conclusion for plant pathogenic bacteria that survive on seeds and leaves of cereals in dry area.
    Alex Ignatov
    PS: we have met at Pseudomonas syringae Conferences at Morocco and Oxford.

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