Introduction to Fungi, Third Edition

696 ANAMORPHIC FUNGI
& Klug, 1976, 1980). Streams bordered by trees
receive the bulk of their fixed carbon input not
from attached macrophytes or algae, but from
leaves, twigs and other debris shed by trees and
other plants. Such material is relatively poor in
nitrogen or is otherwise unpalatable to the
invertebrate animal population. The colonization
of leaf litter by aquatic fungi and bacteria is
an important part of the ‘processing’ which
makes it a more attractive substrate to animals
(Berrie, 1975). There are two main reasons for
this, namely pre-digestion and enrichment in
organic nitrogen.
First, the activities of the fungi soften the leaf
tissues. Aquatic hyphomycetes possess a range of
pectolytic, cellulolytic, proteolytic and ligninolytic
enzymes capable of degrading leaf tissues
(Suberkropp & Klug, 1980; Chamier, 1985; Zemek
et al., 1985; Gessner et al., 1997). Colonized
softened leaves are more easily grazed and
shredded than uncolonized leaves by aquatic
invertebrates such as Gammarus and Asellus and
by the larvae of aquatic insects feeding directly
on leaf tissue or on particulate organic matter
released into streams as leaves decay. In general,
aquatic animals do not possess enzymes capable
of degrading the cell walls of leaf tissue, but the
enzymes present within ingested leaf fragments
may continue to be active within the animals’
guts. When presented with a choice of uncolonized
or colonized leaf tissue, either as separate
discs or as patches on the same leaf, caddis fly
larvae feed preferentially on colonized tissue
(Arsuffi & Suberkropp, 1985).
Second, the protein content of leaf material is
enhanced by microbial colonization. Aquatic
fungi can concentrate inorganic nitrogen
present in solution in the water at low concentrations
but in large total amounts and, making
use of the organic matter in the leaves, manufacture
microbial protein. Fungi can make up
over 90% of the microbial biomass which develops
on decomposing leaves in streams. Aquatic
animals may feed directly on the fungal mycelium
and on fungal spores. Detritivorous animals
such as Gammarus pulex and Asellus aquaticus
fed on a fungus diet make a much greater
weight increase than those fed solely on a diet
of uncolonized leaves. They are also more
fecund, i.e. produce more eggs per brood
(Graca et al., 1993). In order to sustain their
restricted growth rate on leaf diets, the animals
consume about 10 times more leaf material
(by dry weight) than individuals fed on fungus
diets. Thus aquatic hyphomycetes play a very
important role as intermediaries in the diet
of aquatic invertebrates and their major food
source, the leaves of riparian trees. Since aquatic
invertebrates in turn provide the food source
of other animals, including fish, the activities
of aquatic hyphomycetes are vital to the food
chain in maintaining stream productivity.
25.3 Aero-aquatic fungi
If leaves and twigs from the mud surface of
stagnant pools or slow-running ditches are
rinsed and incubated at room temperature in
a humid environment (e.g. a Petri dish or plastic
box lined with wet blotting paper), fungi with
very characteristic large conidia usually develop
within a few days. The common feature of the
conidia of these fungi is that they trap air as they
develop, which assists in floating off the conidia
if the substratum is submerged in water. Conidia
of this type have been termed bubble-trap
propagules (Michaelides & Kendrick, 1982).
Such fungi grow vegetatively on leaves and
twigs, often in water with quite low amounts
of dissolved oxygen. Under submerged conditions
these fungi do not sporulate, but do so
only after incubation under aerial conditions
in which a moist interface between air and water
is provided, as might happen at a pond margin
as the water dries up and previously submerged
twigs or leaves become exposed to air. They have
therefore been termed aero-aquatic fungi.
Aero-aquatic fungi are an ecological group
of organisms without phylogenetic coherence,
as shown in Table 25.3. Although the taxonomy
of the group is fairly well known (see e.g. Linder,
1929; Moore, 1955; Webster & Descals, 1981;
Goos, 1987; Voglmayr, 2000), it is likely that
many more species remain to be discovered.
Careful studies should also reveal new ascomycetous
teleomorphs because several species