Introduction to Fungi, Third Edition

AERO-AQUATIC FUNGI
699
Fig 25.22 Propagules of aero-aquatic fungi which have
developed at the surface of moist leaves incubated in air.
(a) Helicodendron giganteum. Note that secondary conidia
can develop as branches from the first-formed conidia.
(b) Clathrosphaerinazalewskii.(c)Aegerita candida. All images
to same scale.Reprinted from Dix and Webster (1995); original
micrographs kindly provided by P. J. Fisher.
shed leaves which fall onto the water are rapidly
colonized (Premdas & Kendrick, 1991). The leaves
eventually sink to the bottom of the pond and
growth of the fungi continues so long as
dissolved oxygen is available in the water.
Underwater colonization may also take place
by leaf-to-leaf contact, shown by the fact that
sterilized beech leaf discs submerged among
detritus develop conidia of aero-aquatic fungi
when later incubated under suitable conditions
out of water. If there is a substantial accumulation
of fallen leaves, the metabolic activity of
decomposer organisms will lead to anaerobic
conditions and the evolution of hydrogen
sulphide (H 2 S) which is toxic at low concentrations
to eukaryotic organisms. Anaerobic conditions
are evidenced by the sulphurous smell of
disturbed mud and by the black colour of the silt
caused by the accumulation of metallic
sulphides, especially iron sulphide. Although
aero-aquatic hyphomycetes grow best at atmospheric
oxygen levels, their growth in anoxic
conditions is still superior to that of other fungi
(Fisher & Webster, 1979). Under strictly anaerobic
conditions, five species of Helicodendron
showed almost 100% survival for 6 months, and
substantial survival even after 12 months (Field &
Webster, 1983). Survival in most cases appears to
be by thick-walled hyphae because chlamydospores
and sclerotia are rarely found. Similar
comparative studies of the survival of aeroaquatic
fungi and aquatic hyphomycetes under
anaerobic conditions in the presence of low
concentrations of H 2 S showed better survival of
aero-aquatic fungi than aquatic hyphomycetes
(Field & Webster, 1985).
Following a prolonged period of submersion
under the anaerobic or near-anaerobic conditions
of the bottom silt of a pond, it takes several
days for sporulation of aero-aquatic fungi to
commence in air. Incubation in well-aerated
water before exposure to air improves the
recovery, suggesting that a period of aerobic
growth is a stimulus to sporulation. For most
species studied, exposure to light also enhances
sporulation (Fisher & Webster, 1978).
Aero-aquatic fungi (Helicodendron spp.) grown
on a homogenized beech leaf mash can survive
on the soil surface for several months if airdried
(Fisher, 1978), suggesting a capacity for
vegetative existence out of water, a conclusion
confirmed by reports of some species from soil
(Abdullah & Webster, 1980). Dispersal of colonized
wind-blown leaves from one body of water
to another is clearly a possibility. Other possible