Introduction to Fungi, Third Edition

16
Lichenized fungi (chiefly Hymenoascomycetes:
Lecanorales)
16.1 Introduction
The dual nature of lichens was first hinted at
by de Bary (1866) and clearly recognized by
Schwendener (1867). A lichen is now defined as
a ‘self-supporting association of a fungus (mycobiont)
and a green alga or cyanobacterium
(photobiont)’ (Kirk et al., 2001), ‘resulting in a
stable thallus of specific structure’ (Ahmadjian,
1993). The fungal partner usually contributes
most of the biomass to this symbiosis, including
the external surface. It is thus termed the
exhabitant, whereas the unicellular or filamentous
photobiont cells are collectively called the
inhabitant because they are located inside
the lichen thallus (see Ahmadjian, 1993). Most
lichens have a characteristic appearance which
permits their identification if suitable keys are
available (e.g. Purvis et al., 1992; Wirth, 1995a,b;
Brodo et al., 2001). Since the structure of lichens
is almost entirely due to the fungal partner,
lichen taxonomy is synonymous with the taxonomy
of the mycobiont.
It is possible to grow the algal and fungal
partners of many lichens separately in pure culture
(Ahmadjian, 1993; Crittenden et al., 1995).
Whereas most photobionts multiply readily in
pure culture, the fungal partner, if it grows at
all, typically shows slow growth as a sterile
leathery mycelium but does not produce the
characteristic lichen thallus. This is in marked
contrast to the natural thallus where the
mycobiont displays its full sexual and asexual
cycle, whereas the photobiont cells often appear
swollen and are arrested in their cell cycle, i.e.
their cell division is controlled by the mycobiont.
The nature of the morphogenetic signals
exchanged between the symbiotic partners is as
yet unknown (Honegger, 2001).
Some 13 500 species of lichenized fungi have
been described to date. Since lichens are often
conspicuous and have been relatively well
researched over the past 200 years, this number
is not far below the estimated worldwide total of
some 18 000 species (Sipman & Aptroot, 2001).
In contrast, only about 100 species (40 genera) of
photobionts are known, although this number
may rise because photobionts are rarely formally
and fully identified by lichenologists. The most
common photobiont genera are the green algae
Trebouxia (found in about 50% of all lichens)
and Trentepohlia, and the cyanobacterium Nostoc.
About 85% of lichenized fungi have a green algal
photobiont, and 10% are associated with a cyanobacterium.
The remaining lichens contain both
a green alga and a cyanobacterium (Honegger,
2001). Most lichenized fungi (498%) belong to
the Euascomycetes, with only a few imperfect
fungi and some 20 species of Basidiomycota also
entering this type of symbiosis.
Because of their ability to tolerate repeated
cycles of drying and rehydration and to survive
extreme temperatures, high solar irradiation
and other adverse conditions, lichens can colonize
a range of terrestrial habitats not accessible