Water-repellent lichens have the solution ... to
the problem of SO2 pollution
It is
widely stated that lichens are very susceptible to SO2 damage. It is certainly true that many
lichen species disappeared from areas polluted by smoke and acid rain, but this
only gives half the story. A number of lichen species are actually tolerant of
SO2 and have been seen
to increase in frequency in polluted areas. According to Hauck et al. at Göttingen, Germany (pp. 531–539), the
basis of this tolerance lies in the hydrophobicity of
the thallus surface. Their initial observation was
that the very SO2-tolerant
species Lecanora conzaeoides has a very hydrophobic
(‘super-hydrophobic’) surface. The observation was followed by relatively
simple but very informative experiments: lichens ranging from susceptible to
tolerant were air-dried. Water droplets (500 mm in
diameter, the size of an average rain drop) were placed on the thallus surface and the droplet contact angles were
measured to within ±1°. High contact angles (90° and
above) indicate hydrophobicity. At the other end of
the scale, contact angles of less than 50° could
not be measured because, as expected, water spread over the thallus
surface more readily on very hydrophilic thalli. When
contact angles were compared with known SO2 tolerances a very clear correlation
emerged. All but one of the super-hydrophobic species (contact angles ³120°) were
highly tolerant while, with one exception, the very hydrophilic species all
exhibited low tolerance. It seems therefore that it is the inability of SO2 to enter the thallus
in solution that confers tolerance on the hydrophobic species (which, for the
same reason, are also tolerant of heavy metals). Interestingly, thallus hydrophobicity almost
certainly originally evolved to prevent waterlogging
of the thallus (which in turn inhibits photosynthesis
of the algal symbiont) in lichens that grow in wetter
habitats. Thus, tolerance of SO2 and of heavy metals is a beneficial
side-effect of the adaptation to wet places that adds further selective
advantage in polluted areas.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
