Felix Bärlocher

The Ecology of aquatic hyphomycetes

Aquatic hyphomycetes were discovered 50 years ago by C.T. Ingold. They remained a relatively obscure group until their role as intermediaries between deciduous leaves and stream invertebrates was established some 20 years ago. This book provides a summary and critical evaluation of the biology and ecology of these organisms. A special effort was made to evaluate the potential and actual insight that had been or will be derived from work in related disciplines such as the ecology of other fungal groups, stream ecology or population ecology. The topics treated include the basic life history of the fungi and the potential role of wood, a discussion of how the fungi has adjusted to life in running water, their interactions with invertebrates, the attachment and germination of their spores, what is known about sexual reproduction, how water chemistry may influence their distribution and activity, how they react to human degradation of the environment, and a summary of the research carried out on the Indian subcontinent.

The Role of Biodiversity in the Functioning of Freshwater and Marine Benthic Ecosystems

Abstract Empirical studies investigating the role of species diversity in sustaining ecosystem processes have focused primarily on terrestrial plant and soil communities. Eighteen representative studies drawn from post-1999 literature specifically examined how changes in biodiversity affect benthic ecosystem processes. Results from these small-scale, low-diversity manipulative studies indicate that the effects of changes in biodiversity (mostly synonymous with local species richness) are highly variable over space and time and frequently depend on specific biological traits or functional roles of individual species. Future studies of freshwater and marine ecosystems will require the development of new experimental designs at larger spatial and temporal scales. Furthermore, to successfully integrate field and laboratory studies, the derivation of realistic models and appropriate experiments will require approaches different from those already used in terrestrial systems.

Determining Diversity of Freshwater Fungi on Decaying Leaves: Comparison of Traditional and Molecular Approaches

ABSTRACT 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.

Leaf-conditioning by microorganisms

SummaryMany detritus-feeders prefer dead leaves which are colonized by microorganisms-i.e. conditioned leaves, over freshly fallen or sterile leaves. Traditionally, this has been attributed to the build-up of microbial cells on the substrate. Two experiments show that changes in the leaf itself, brought about by microbial excretions and secretions, or by a hydrolytic agent (hot HCl), can also increase its palatability to the detritus-feeding amphipod Gammarus pseudolimnaeus.

Fungi in freshwaters: ecology, physiology and biochemical potential

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.

Fungi in the diet of Gammarus pseudolimnaeus (Amphipoda)

Autumn-shed leaves which support a rich fungal population are an important source of food for many stream invertebrates. Two experiments were conducted to elucidate the relative importance of fungi and leaf substance in the nourishment of Gammarus pseudolimnaeus Bousfield. Mycelium of Tricladium angulatum Ingold proved to be a much better source of food for adult Gammarus than maple leaves with rich microbial growth. The weight increases of young Gammarus were compared when they could feed exclusively either on maple or elm leaves or on mycelium of one of ten fungi isolated from leaves in streams. The daily consumption of dry substance was about ten times higher in leaf diets than in fungal diets, but the largest weight increases were found in animals feeding on four of the fungi. Two of the fungi were clearly unsuitable as the sole source of food. Elm and maple leaves and the other four fungi fell between these extremes.

Taxon-specific fungal primers reveal unexpectedly high diversity during leaf decomposition in a stream

Traditional techniques for studying the fungal community composition in streams favour the detection and identification of aquatic hyphomycetes. Our objective was to use molecular techniques to determine the presence and contributions of other fungal groups. We designed primers specific for the ITS regions in Ascomycota, Basidiomycota, Chytridiomycota, Zygomycota and Oomycota. The primers were used to amplify DNA from linden, maple, and beech leaves, and birch wood submerged in a stream for 4 weeks in summer, autumn, winter and spring. The amplification products were separated by denaturing gradient gel electrophoresis. Ascomycota were present in large phylotype numbers (up to 21) on all substrates and all dates and represented ≥ 75 % of the fungal biomass. Basidiomycota were the second most abundant group in summer and autumn (up to 13 % on wood) and were absent only on linden and maple in spring. There were consistently large numbers of phylotypes from Chytridiomycota and their relative contribution to the microbial community peaked in winter on all substrates. Oomycota were present in summer and abundant only on wood. Zygomycota were present in low numbers and their estimated contribution to fungal biomass was ≤ 1%. Using primers to target individual groups facilitates a more balanced approach to studying fungal diversity in freshwater ecosystems.

Aquatic hyphomycete diversity and identity affect leaf litter decomposition in microcosms

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.

Hyporheic biofilms - a potential food source for interstitial animals

Glass-beads (diam. = 250 µm) were buried 10 cm deep in the sediment of a stream. After an exposure of eight weeks, bacterial densities on the beads varied between 2.7 × 105 and 2.4 × 107/cm2, and the length of the fungal mycelium between 0.2 and 5.3 mm/cm2. Bacterial densities did not show any correlation with the DOC content of the water, but were positively correlated with respiration on the beads. Fungal mycelium was negatively correlated with water temperature. Acid hydrolysis of stream-exposed beads released sugars and amino acids, whose combined carbon content exceeded that of the microbial cells by a factor of at least 4. Gut extracts of Gammarus tigrinus and Tipula caloptera released amino acids and sugars from stream-exposed beads.Glass-beads (diam. = 250 µm) were buried 10 cm deep in the sediment of a stream. After an exposure of eight weeks, bacterial densities on the beads varied between 2.7 × 105 and 2.4 × 107/cm2, and the length of the fungal mycelium between 0.2 and 5.3 mm/cm2. Bacterial densities did not show any correlation with the DOC content of the water, but were positively correlated with respiration on the beads. Fungal mycelium was negatively correlated with water temperature. Acid hydrolysis of stream-exposed beads released sugars and amino acids, whose combined carbon content exceeded that of the microbial cells by a factor of at least 4. Gut extracts of Gammarus tigrinus and Tipula caloptera released amino acids and sugars from stream-exposed beads.

Leaf Mass Loss Estimated by Litter Bag Technique

Leaf litter is a dominant component of coarse particulate organic matter in streams. In view of its central place in ecosystem functioning, estimates of its mass loss have been proposed to evaluate functional stream integrity. This chapter describes a standardized method to estimate mass loss by using litter bags, including statistical analysis of the data. Variable mesh sizes of the litter bags facilitate size-selective exclusion of macro-consumers and estimates of the relative contribution of microbes versus invertebrates to breakdown. Mass loss as a function of time is often approximated by a single exponential decay model. However, the fit can be poor, because different litter components are removed at different rates. The fit can often be improved by two-phase breakdown models, or by accounting for initial leaching. When comparing breakdown of two or more treatments, the recommended approach is to compare slopes of ln-transformed mass loss data by analysis of covariance. The response variable Y (litter mass remaining) is compared among treatments while statistically controlling for variation in Y caused by variation in the X variable (time). In mass loss studies, it tests for a significant interaction between the fraction of litter mass remaining among treatments and the covariate time.