March 2008 - Mycological Society of America

egin. This kind of phenomenon occurs in many
aquatic ecosystems throughout the world.
The chytrid epidemic appears suddenly as long as
substrates are available (Sparrow, 1960). The life
cycle of most chytrids is completed rapidly, and many
zoospores are released from each sporangium. Therefore,
a large increase in population numbers is possible
within a very short period of time. For this reason
all chytrids are considered to be ruderal in their life
histories (sensu Dix and Webster, 1995, p.7).
Nearly fifty years ago Frederick Sparrow (1960,
pp. 104-114) defined the chytrid epidemic to include
host-parasite interactions involving chytrids (Phyla
Blastocladiomycota and Chytridiomycota). Sparrow’s
examples considered only phytoplankton and fungi as
hosts but zooplankton and larger aquatic animals need
to be included as well. According to Sparrow, beginning
in early spring, population densities of parasitic
chytrids rapidly increase for a short period, then decline
due to the loss of available substrates and finally produce
resting spores. We would expect to find the same
pattern of growth in both parasitic and saprophytic
chytrids in aquatic ecosystems.
Donald Barr, Hilda Canter, John Couch, John Karling,
Joyce Longcore, Frederick Sparrow, Howard
Whisler, Guy Willoughby and many other mycologists
have spent their lives recording the presence of parasitic
chytrids on aquatic plants and animals and saprophytic
chytrids colonizing nonliving substrates in
aquatic environments. Sparrow (1960, pp. 1073-1104)
provides a long list of substrates reported for the
growth of chytrids. Studies on chytrid parasites and
saprophytes in freshwater lakes, particularly in northern
Michigan, the Netherlands and the English Lake
District, have continued for many years. Dix and Webster
(1995, pp. 227-231) briefly summarized the roles
of chytrid parasites and saprophytes in freshwater
ecosystems. Yet, the ecological importance of this
group of fungi and its roles in food web dynamics remain
unappreciated and are not very well understood.
Furthermore, chytrids sometimes have been either totally
ignored or wrongly classified in studies on the biodiversity
of fresh water ecosystems (Kagami et al.,
2007; Lefèvre et al., 2007).
Recently, there has been renewed interest in the
ecology of chytrids in aquatic environments. Ibelings
et al. (2003) and Kagami et al. (2007) have
again examined the large number of species of phytoplankton
affected by chytrid parasites. The role of
Batrachochytrium dendrobatidis in reduction in size
of amphibian populations and in extinctions of
species is currently under investigation (Fisher &
Garner 2007). Chytridiomycosis is an emerging in-
2 Inoculum 59(2), March 2008
fectious disease of amphibians world-wide. Kagami
et al. (2007) have suggested that chytrid zoospores
are an excellent food source for zooplankton and facilitate
the transfer of energy to higher trophic levels
in food webs. Chytrid zoospores are rich in
stored lipids and glycogen. Studies on DNA extracted
from water samples from a mountain lake in
France have revealed a significant number of ribosomal
DNA sequences belonging to known and unknown
clades of chytrids (Lefèvre et al., 2007).
Chytrid biodiversity is apparently surprisingly high
in many freshwater lakes. Research on the ecology
of chytrids in freshwater lakes in France is being
continued by T. Sime-Ngando and his colleagues
and students at Laboratoire Microorganismes:
Génome et Environnement, Université Blaise Pascal,
63177 AUBIERRE cedex.
Evidence is emerging that the interactions between
the chytrid parasite and its host may involve
strong phenotypic selection pressures. A recent
study by D. J. Macarthur (unpublished data), funded
by the NSW Environmental Trust, involved the inoculation
of cultures of the bloom forming cyanobacteria
Anabaena circinalis and Microcysis aeruginosa
with diatoms infected with chytrids (Fig. 1) and with
pure cultures of chytrids in the Rhizophydiales from
our culture collection. One of these isolates, SPP,
adapted to the new host environments by parasitising
both species of cyanobacteria. SPP was isolated into
pure culture from a phytoplankton sample containing
infected diatoms (by E. Lefèvre) and was tentatively
identified as Rhizophydium sp. These results tend to
support the assertion by Kagami et al. (2007) that the
host range of chytrid parasites, often thought to be
host-specific, could be altered by environmental
stress. More interesting, however, was an apparent
enhancement of growth and survival of A. circinalis
cultures. Clearly, in this case, there are benefits to
both the chytrid parasite and the cyanobacteria host.
However, more data is required in order to unravel
the complicated interactions between the chytrid parasite
and its host.
The growth and survival of chytrids are quite
sensitive to physical factors, such as moisture, temperature,
salinity and dissolved oxygen (Gleason et
al., 2007) and possibly toxic chemicals. With global
warming and environmental deterioration on the increase,
it is high time that more mycologists begin
to undertake intensive studies on the ecology of the
chytrids in aquatic ecosystems. The survival of
many aquatic ecosystems in their present state could
even depend on the activities of chytrids along with
other groups of microorganisms.