Fungi that degrade recalcitrant (resistant)
polymersFungi that degrade the more resistant polymers such
as lignocellulose (cellulose complexed with lignin)
often predominate in the later stages of decomposition.
Several of them are Basidiomycota, including Mycena
galopus, a common small toadstool in the leaf litter
of woodlands. Other common examples are the fairy
ring fungi, frequently seen in old grasslands. These
fungi grow on the thick mat of accumulated dead
leaf sheaths of grasslands and spread progressively
outwards year by year. Several fungi produce these
rings, including the common edible field mushroom
(Agaricus campestris). They usually cause no damage
because they grow on dead organic matter, and often
the only sign of their presence is a ring of more vig-
orous, lush growth of the grass, resulting from mineral
nutrients released from breakdown of the organic
matter. But the “fairy ring champignon” Marasmius
oreadesis an interesting exception because it can kill
the grass (Fig. 11.10) as the rings spread progressively
outwards. The reason is that this fungus produces a mass
of fungal hyphae just beneath the grass surface,
and these hyphae dry in mid summer, becoming
hydrophobic and preventing water from penetrating
the soil. Even fungicides are ineffective in preventing
death of the grass, but surfactants (including dilute
washing-up liquids) can reduce much of this damage.
Keratin-degrading fungi, related to the dermato-
phytic pathogens of humans and other animals
(Chapter 16), are found in the later stages of decom-
position of hair or animals’ hooves. They might occur
earlier, but they become conspicuous only after an
initial phase of exploitation by Zygomycota and
Penicilliumspp. which utilize the more readily available
proteins and lipids.
The ecological success of fungi that develop later
in the decomposition sequence is related to their spe-
cialized ability to degrade polymers that most other
fungi cannot utilize. However, it does not necessarily
follow that they use these complex polymers as their
main energy source. In fact, Mycena galopus, men-
tioned above, cannot degrade lignin in culture unless
it is also supplied with cellulose or hemicelluloses as
more readily utilizable substrates. The chief attribute
of these fungi seems to be that they degrade or modify
the recalcitrant polymers and so gain access to other
substrates that are chemically or physically complexed
with the resistant polymers. Much of the cellulose
in plant cell walls is intimately associated with lignin,
either covalently bonded to it or encrusted by it,
and this “lignocellulose” is largely unavailable to
fungi that cannot modify the lignin. In one of theFUNGAL ECOLOGY: SAPROTROPHS 221
Fig. 11.9A small piece of transparent cellulose film
(about 3×2 cm) retrieved from soil and stained with
trypan blue to show fungi growing at different
levels in the film. Rhizophlyctis rosea produces finely
branched rhizoidal systems. At a different plane of
focus, Pythium graminicolaproduces finger-like branching
systems.
by low water potentials (drought stress) because
they can grow at almost undiminished rates when
the water potential is lowered to −5 MPa in culture
(Chapter 8). Trichoderma spp. often are favored by soil
acidity (pH stress) because they can grow at about
pH 3.0 in culture. A spectrum of salt-tolerant cellulolytic
fungi is found on plant remains in estuarine waters (e.g.
Lulworthiaspp., Halosphaeria hamata, and Zalerion var-
ium) and a different spectrum is found in freshwater
streams (some of the fungi with tetraradiate spores, such
as Tetracladium, Lemonniera, and Alatospora– Chapter
10). Chitinous materials are colonized by yet another
group of fungi, such as Mortierellaspp. (Zygomycota)
in soil or Chytridium confervae(Chytridiomycota) in
freshwater habitats.
Polymer-degrading fungi can exploit different
microhabitats, and thereby coexist in a single sub-
strate resource. Evidence for this has come from
microscopical examination of transparent cellulose
film (“Cellophane”) buried in soil (Tribe 1960). In wet
soils the film is colonized by cellulolytic chytrids (e.g.
Rhizophlyctis rosea; Fig. 11.9) and chytrid-like organisms
(e.g. Hyphochytrium catenoides). These degrade the
cellulose locally by forming finely branched rhizoids
between the layers of the film. Other parts of the film
can be colonized by mycelial fungi such as Humicola
grisea, Fusariumspp., and Rhizoctonia. The Rhizoctonia
and Fusarium colonies form loose networks over the sur-
face of the film, whereas Humicolais seen as localized,
compact colonies that “root” into the film and produce
fans of hyphae within it, like the fans of Pythium
graminicolashown in Fig. 11.9.