Aquatic fungi, in the broadest sense, include fungi present transiently in water, terrestrial fungi that release spores which are dispersed in water, and species that function entirely within water. Water is found in transient pools after rain, swamps, estuaries, permanent rivers and oceans. Within a single environment, the water can have predictable characteristics. At the edge however, water bodies may contain soil and plant materials, and will be a changing enironment.
Fungi living in these environments require a wide array of characteristics to cope with the constraints. Conditions in fresh water are markedly different to those in the ocean. The open sea has quite different salinity to salterns where the water is slowly evaporating. Sometimes the water is extremely acidic or toxic. Fungi are found in all these environments. The mechanisms that enable successful occupation of these environments are largely unexplored. Indeed, fungi have only recently been acknowledged as important inhabitants of extreme aquatic habitats. This section provides some details of the simple responses widely studied in the fungi.
The major input of plant materials in streams comes from leaf fall. These materials contain a range of endophytes of terrestrial origin. LINK These endophytes are soon replaced by a characteristic group of hyphomycetes with specialised spores. The fungi are known as Ingoldian fungi. A typical succession takes place, analogous to succession found in compost and terrestrial leaf litter LINK. Like the common terrestrial fungi, the Ingoldian fungi include many species that appear to lack specificity to the host plant; they are also found around the world.
Aero-Aquatic Hyphomycetes: Ingoldian fungi are not the only fungal inhabitants of litter. Another group form large coiled or rounded conidia which trap air, and these are known as aero-aquatic hyphomycetes. The advantage of their spores is that when the water dries, spores can lodge in the mud. On wetting again, the conidia float away on the water, being available to colonise leaf material when they make contact. Colonisation is rapid, and sporulation may take place within 7 days. The hydrophobic conidia are formed on aerial conidiophores, which on release, float on the surface of the water.
Chytrids: Zoosporic fungi can only disperse in water LINK. The zoospore requires at least a film of water to move away from the sporangium. The fungi are readily trapped from bodies of water, soil, or from surfaces where a water film forms transiently.
The importance of chytrids in aquatic habitats is only now becoming clear. Chytrid DNA is closely associated with processes of removal of picoplankton in lakes and large water bodies. Some chytrids are known to parasitise cyanobacteria and single celled algae. Thus these poorly studied fungi appear to be potential biological control agents of important aquatic toxigenic microbes, and indicators of eutrophication.
Ingoldian fungi have a number of interesting features. While the spores are characterised by a set of similar morphological features, it turns out that the teleomorphs are in a diverse range of both Ascomycota and Basidiomycota. The fungi can enzymically degrade the litter and are probably far more common on the leaves than bacteria, at least in the early stages of decomposition. The fungi form conidia while ever leaf litter is available. In Australian streams, seasonal peaks of spores in late summer will be found in the south, and in the late winter in the north, coinciding with the main season of leaf fall. However, the spore season is much longer and less pronounced in Australia where the leaf fall is less seasonal than in northern Europe and North America where most leaves of deciduous trees fall in autumn.
Many of the fungi also colonise roots of trees on the stream banks. The endophytic fungi form sexual spores that lack any features enabling dispersal in water. LINK In fact, the sexual stages may enable dispersal from stream to stream, with the asexual stages increasing colonisation within a stream. After all, release of conidia into a stream inevitably leads to dispersal downstream, not upstream. Further, as the source of leaf litter is highly variable, a source of propagules unrelated to the litter would enable faster build-up of inoculum in the stream.
The size of the population of Ingoldian fungi is also controlled by invertebrates. These animals graze on leaves, preferring colonised leaves, presumably because they have higher protein, lipid and mineral phosphate content due to the fungi, and because the leaf has been markedly predigested. Invertebrate competition markedly increases the rate of litter breakdown, and litter usually disappears rapidly from streams. However, it is argued that the presence of fungi is essential to the invertebrates because the fungi increase the nutritive value, providing essential nutrients not available in leaf material.
Fungi found in aquatic habitats have to gain access to organic materials to maintain themselves. Ingoldian and other aquatic fungi appear to have evolved unique life histories. The Ingoldian fungi are essentially biphasic, with one life component dispersing the fungi over land, and the second taking advantage of the transient flushes of nutrients following leaf fall to water. Fungi have important roles in water. Apart from diminution of leaf litter, the aquatic chytrids appear to be important regulators of the picoplankton in water.
Dix NJ & Webster J. 1995 Fungal Ecology. Chapman Hall. Ch 9.
Gunde-Cimerman N. et al 2009. Halotolerant and halophilic fungi. Mycological Research 113: 1231-1241.
Ingold CT & Hudson HJ 1993. The Biology of Fungi. Chapman Hall. Ch 6.