Research on freshwater fungi has concentrated on their role in plant litter decomposition in streams. Higher fungi dominate over bacteria in terms of biomass, production and enzymatic substrate degradation. Microscopy-based studies suggest the prevalence of aquatic hyphomycetes, characterized by tetraradiate or sigmoid spores. Molecular studies have consistently demonstrated the presence of other fungal groups, whose contributions to decomposition are largely unknown. Molecular methods will allow quantification of these and other microorganisms. The ability of aquatic hyphomycetes to withstand or mitigate anthropogenic stresses is becoming increasingly important. Metal avoidance and tolerance in freshwater fungi implicate a sophisticated network of mechanisms involving external and intracellular detoxification. Examining adaptive responses under metal stress will unravel the dynamics of biochemical processes and their ecological consequences. Freshwater fungi can metabolize organic xenobiotics. For many such compounds, terrestrial fungal activity is characterized by cometabolic biotransformations involving initial attack by intracellular and extracellular oxidative enzymes, further metabolization of the primary oxidation products via conjugate formation and a considerable versatility as to the range of metabolized pollutants. The same capabilities occur in freshwater fungi. This suggests a largely ignored role of these organisms in attenuating pollutant loads in freshwaters and their potential use in environmental biotechnology.

Empirical studies investigating the role of species diversity in sustaining ecosystem

This review highlights the presence and ecological roles of fungi in lakes, and aims to stimulate research in aquatic mycology. In the study of lentic systems, this field is an almost completely neglected topic and, if considered at all, has often been restricted to specific groups of fungi, such as yeasts, filamentous fungi (e.g. aquatic hyphomycetes), or phycomycetes (an obsolete taxonomic category that included various fungal and fungal-like organisms). We document that aquatic fungi are common in various lentic habitats. They play potentially crucial roles in nutrient and carbon cycling and interact with other organisms, thereby influencing food web dynamics. The development and application of molecular methods have greatly increased the potential for unraveling the biodiversity and ecological roles of fungi in lake ecosystems. We searched the literature for reports on all fungi occurring in lake ecosystems. The present study summarizes information on the highly diverse mycoflora in lake microhabitats and highlights the main processes they influence. We also point out ecological niches of fungi in lakes that have been examined only superficially or ignored completely. We demonstrate that we now have the methodology to perform systematic studies to finally fill in some of the large gaps in this field.

Dead leaves falling into streams are an important food source for many invertebrates. They are generally made more palatable and more nutritious if they are first colonized by aquatic hyphomycetes and other micro‐organisms. At least two mechanisms appear to be responsible for this conditioning effect: microbial production (addition of easily digested microbial compounds to the nutritionally poor leaf substrate), and microbial catalysis (conversion of indigestible leaf substances into digestible subunits by microbial enzymes). Different invertebrate species vary in their ability to take advantage of microbial conditioning. This appears to be influenced by their mobility, the range of their food resources and their ability to overcome defense mechanisms of leaf‐colonizing microorganisms.

Traditional microscope-based estimates of species richness of aquatic hyphomycetes depend upon the ability of the species in the community to sporulate. Molecular techniques which detect DNA from all stages of the life cycle could potentially circumvent the problems associated with traditional methods. Leaf disks from red maple, alder, linden, beech, and oak as well as birch wood sticks were submerged in a stream in southeastern Canada for 7, 14, and 28 days. Fungal biomass, estimated by the amount of ergosterol present, increased with time on all substrates. Alder, linden, and maple leaves were colonized earlier and accumulated the highest fungal biomass. Counts and identifications of released conidia suggested that fungal species richness increased, while community evenness decreased, with time (up to 11 species on day 28). Conidia of Articulospora tetracladia dominated. Modifications of two molecular methods-denaturing gradient gel electrophoresis (DGGE) and terminal restriction fragment length polymorphism (T-RFLP) analysis-suggested that both species richness and community evenness decreased with time. The dominant ribotype matched that of A. tetracladia. Species richness estimates based on DGGE were consistently higher than those based on T-RFLP analysis and exceeded those based on spore identification on days 7 and 14. Since traditional and molecular techniques assess different aspects of the fungal organism, both are essential for a balanced view of fungal succession on leaves decaying in streams.Aquatic hyphomycetes belong to a phylogenetically heterogeneous group of true fungi that dominate the breakdown of leaves and other allochthonous detritus in streams and rivers (5, 15). Their activity increases the palatability of the substrate to detritus feeders. Macroinvertebrates discriminate between, and show preferences for, particular fungal species (2, 6, 26). To fully characterize the fungal contribution to leaf decay and invertebrate nutrition, the aquatic hyphomycete community must be subdivided into its constituent species. The same principle applies when investigating relationships between biodiversity and ecological functions-a topic that has attracted increasing attention in the last decade (18,33).Most of the fungal biomass on decaying leaves consists of vegetative (nonreproducing) hyphae that cannot be identified through conventional microscopy. To characterize community structure, leaf surfaces often are examined for sporulating structures (e.g., see references 7 and 30), or conidia released from the leaf are collected, counted, and identified (e.g., see reference 3). The obvious shortcoming to these protocols is that absence of conidia might be due to the absence of species or to the presence of nonsporulating mycelium. In the initial phase of fungal colonization, between the landing of conidia and their growth into a sporulating colony, newly arrived species will escape detection by traditional microscope-based techniques.Molecular approaches characterize nucleic acids that are present in all...

We conducted a microcosm experiment with monocultures and all possible combinations of four aquatic hyphomycete species, Articulospora tetracladia, Flagellospora curta, Geniculospora grandis and Heliscus submersus, to examine the potential effects of species richness on three functional aspects: leaf litter decomposition (leaf mass loss), fungal production (ergosterol buildup) and reproductive effort (released spores). Both species richness and identity significantly affected fungal biomass and conidial production (number and biomass of released spores), whereas only species identity had a significant effect on leaf mass loss. In mixed cultures, all measures of fungal functions were greater than expected from the weighted performances of participating species in monoculture. Mixed cultures outperformed the most active monoculture for biomass accumulation but not for leaf mass loss and conidial production. The three examined aspects of aquatic hyphomycete activity tended to increase with species richness, and a complementary effect was unequivocally demonstrated for fungal biomass. Our results also suggest that specific traits of certain species may have a greater influence on ecosystem functioning than species number.

1. We characterised the fungal communities of eight streams in Portugal, four bordered by native deciduous forest and four bordered by pure stands of Eucalyptus globulus. 2. Aquatic hyphomycete species richness and evenness, but not numbers of water‐borne conidia, of aquatic hyphomycetes were significantly lower in eucalypt bordered streams. 3. Multivariate analyses subdivided the fungal communities into two distinct groups corresponding to riparian vegetation. 4. Despite these differences in the dominant decomposer community, decay rates of eucalypt leaves (accounting for ≥98% of naturally occurring leaves in eucalypt bordered streams, absent in native forest) and chestnut leaves (occurring naturally in native forests) did not differ between the two groups of streams.

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