Introduction to Fungi, Third Edition

454 LICHENIZED FUNGI (CHIEFLY HYMENOASCOMYCETES: LECANORALES)
be too hostile for free-living forms. The lichen
symbiosis can thus be considered an alternative
adaptation to terrestrial life as compared to
higher plants.
16.2.6 Lichens and pollution
Lichens are particularly sensitive to aerial pollutants,
and especially to sulphur dioxide, SO 2
(Seaward, 1993; Gries, 1996; Nash & Gries, 2002).
The photobiont appears to be generally more
sensitive than the mycobiont. The disappearance
of lichens from the centres of urban and
industrial areas was first recognized by Nylander
(1866), who had already correlated this phenomenon
with aerial pollution. Because different
lichens show a differential sensitivity to SO 2 ,
the presence or absence of key species can be
used as an index of the level of air pollution
(Hawksworth & Rose, 1970). The most SO 2 -
tolerant lichen, Lecanora conizaeoides, may have
evolved in SO 2 -polluted areas and went on to
become Northern Europe’s most abundant lichen
by the 1950s (Richardson, 1975). This lichen may
actually require elevated SO 2 levels for good
growth (Nash & Gries, 2002), as shown by its
disappearance from some areas after the implementation
of legislation to curb SO 2 emissions.
At the same time, formerly polluted areas are
being re-colonized by many SO 2 -sensitive species
(Rose & Hawksworth, 1981; Seaward, 1993). An
example of this trend has been given by Masuch
(1993) for the city of Munich. Between 1891 and
1956, the ‘lichen desert’ (i.e. lichen-free zone) in
the city centre increased from 8 to 56 km 2 , and
then it decreased again, disappearing altogether
by 1983. The size of the ‘lichen desert’ has been
correlated with the degree of SO 2 pollution in
the air. Careful studies of lichen population
dynamics have revealed that lichen species recolonizing
a lichen desert may be different from
those initially present. This phenomenon has
been explained by the eutrophication of urban
habitats, i.e. their enrichment especially with
nitrogen (Seaward, 1997; Seaward & Coppins,
2004). One of these newcomers in urban lichen
deserts is Dirina massiliensis f. sorediata, which is
the cause of a rapid decay of limestone monuments
(Seaward & Edwards, 1997).
Lichens obtain most of their mineral nutrients
from the air and rainwater in which these
are present only in very low concentrations.
Not surprisingly, therefore, lichens can accumulate
dissolved substances from very dilute
solutions. For instance, lichens concentrate
radioactive nuclides which enter the food chain
lichen!reindeer!man, leading to their accumulation
in human tissues (Richardson, 1991).
Lichens are also being used to monitor the
radioactive contamination resulting, for example,
from the explosion of the Chernobyl nuclear
reactor in 1986 (Seaward, 2004).
16.2.7 Taxonomy of lichens
The discovery of a fossilized cyanolichen in the
Rhynie chert sediments (Taylor et al., 1997)
indicates that lichens were present some
400 million years ago when the terrestrial habitat
was first colonized. Indeed, there is evidence of
even older lichen-like associations (Yuan et al.,
2005). A huge diversity of lichens exists today,
and there is good phylogenetic evidence that the
lichenized habit has been developed and lost
independently on several occasions in the course
of evolution (Gargas et al., 1995; Lutzoni et al.,
2001). This is also evident from the scattered
placement of lichenized fungi in a wider ascomycete
context. Lutzoni et al. (2001) even suggested
that some of today’s groups consisting
entirely of non-lichenized species, notably the
Plectomycete lineage (Chapter 11), originated
from lichenized ancestors. Part of the proposed
argument is a chemotaxonomic one, i.e. the
presence of numerous secondary metabolites
(especially polyketides) in the lichens and
Plectomycetes, but their absence or less-frequent
occurrence in certain other groups of fungi. As
with many DNA-based analyses, the phylogenetic
arrangement of taxa may vary with the kinds
of sequences used, and other schemes showing a
less scattered distribution of lichenized fungi
within the Ascomycota have been put forward
(Fig. 8.17; Liu & Hall, 2004).
Whereas much work remains to be done on
the taxonomy of ascomycetes in general and
lichenized ascomycetes in particular, some
orders are beginning to take shape. These are