Introduction to Fungi, Third Edition

AQUATIC HYPHOMYCETES (INGOLDIAN FUNGI)
695
essential role in rendering leaves which they
colonize in streams more palatable and nutritious
to aquatic invertebrates feeding on them
(Bärlocher, 1992; Dix & Webster, 1995; see below).
Ecological studies have been facilitated by the
fact that these fungi can, in general, be recognized
to a high degree of certainty from their
spores, which is not the case for most other
groups of fungi.
Distribution
Ingoldian hyphomycetes have a worldwide distribution
from the equator to the Arctic. They are
found most frequently in babbling brooks overhung
by deciduous trees and are less abundant
in wider rivers or where streams flow through
afforested regions in which trees have been clearfelled
on both sides (Metwalli & Shearer, 1989).
They are relatively infrequent in streams in
mountainous or moorland areas devoid of
riparian trees, but can grow there on plants
such as rushes (Juncus spp.). As rivers flow
towards the sea and the brackish condition is
encountered in the estuaries, Ingoldian hyphomycetes
decline in frequency. A few species grow
in lakes, but the lotic (flowing) habitat is
preferred to the lentic (smooth).
Substrates
The main substrates for aquatic hyphomycetes
are leaves of deciduous trees. In general these
fungi show little host specificity but certain tree
leaves support a richer fungal population with
more abundant sporulation than others (Gulis,
2001). A particularly rich mycota is associated
with the leaves of alder (Alnus glutinosa), a
common riparian tree, and this is possibly correlated
with their relatively high nitrogen content
due to the fact that Alnus can fix gaseous N 2 .
In contrast, the leaves of beech (Fagus sylvatica)
are a poor substrate. Needles of conifers are
resistant to colonization by aquatic hyphomycetes,
related partly to their thick cuticles and
also to the presence of compounds inhibitory to
mycelial growth (Bärlocher & Oertli, 1978a,b).
Wood which has fallen into streams is an
important substratum because it is more enduring
than leaves, which decompose more rapidly
and are consumed by animals or scoured by
water currents, so that they may not survive in
great quantity to provide inoculum for the next
season’s autumnal input. Additionally, colonized
wood is a substratum for the development of
teleomorphs (Shearer, 1992). Living riparian tree
roots also support a population of aquatic
hyphomycetes (Fisher et al., 1991; Sridhar &
Bärlocher, 1992a,b), with roots of alder and
willow (Salix) particularly important because
they extend into streams. Aquatic hyphomycetes
have even been reported from beech roots
growing in woodland soil (Waid, 1954). In
lowland streams and rivers a few species may
colonize living and decaying leaves of in-stream
macrophytes (Kirby et al., 1990). Treeholes,
cavities formed within tree trunks where the
stumps of fallen branches have decayed, intermittently
fill with rain water. Samples of the
water and the leaf and other debris which
accumulates in treeholes reveal the frequent
presence of Ingoldian fungi such as Alatospora
acuminata (Gönczöl & Révay, 2003).
Spore concentrations in streams
Concentrations of aquatic hyphomycete spores
in streams can be readily estimated by the
filtration of water samples through a Millipore
filter (preferably with 8 mm pore size, to facilitate
rapid filtration) followed by treatment which
stains the spores and renders the filter transparent.
Concentrations reach a peak soon after the
main period of autumnal deciduous leaf fall
in temperate countries. Spore counts as high as
2 3 l0 4 l 1 have been made in October and
November in Britain and elsewhere (Iqbal &
Webster, 1973b). As the leaves are decomposed,
consumed or swept downstream, the concentration
of spores in suspension may fall to undetectable
levels. This points to the significance of
colonization of more enduring woody substrata
and also growth in the roots of riparian trees
which extend into the water.
Feeding of invertebrates on aquatic
hyphomycetes
It is now becoming clear that aquatic hyphomycetes
play an important role in the cycling of
nutrients in streams (e.g. Kaushik & Hynes, 1968,
1971; Bärlocher & Kendrick, 1973a,b; Suberkropp