Fungal alteration of the elemental composition of leaf litter affects shredder feeding activity

1. Leaf litter from riparian vegetation provides the main source of matter and energy for food webs of small forest streams. Shredding macroinvertebrates mostly feed on this litter when it has been colonised and conditioned by microorganisms, especially by aquatic hyphomycetes. Since shredders feed selectively, they must make foraging decisions based on the physical and chemical characteristics of the food resource, which can change depending on the identity of fungal species. 2. Here, we addressed the effect of changes in fungal assemblage structure on the elemental composi- tion of oak (Quercus robur) leaf litter and how variation in litter quality affects the feeding of a stream shredder. Leaf discs were incubated in microcosms for 2 weeks, inoculated with various fungal assemblages comprised of three species each, and offered to a shredder (Schizopelex festiva, Trichoptera: Sericostomatidae) as food. 3. This shredder ate more leaves with a high mycelial biomass, which depended on fungal assemblage composition. Leaf litter conditioned by different fungal assemblages resulted in different litter N and P concentrations. Mycelial biomass was positively related to litter P concentration, with the lowest and highest P concentrations differing by 40% at most, but not to litter N concentration, even though the lowest and highest N concentrations differed by as much as 35%. The caddisfly larvae ate more leaves with a low C/P ratio. 4. These findings suggest a key role of litter P concentration in eliciting fungal conditioning effects on shredder-mediated litter decomposition

Effect of acidification on leaf litter decomposition in benthic and hyporheic zones of woodland streams

Anthropogenic acidification has deleterious effects on both structure and functioning of surface water ecosystems. This study examined how it may affect the leaf decomposition rate and the community structure and activity of decomposers in both benthic and hyporheic zones of five headwater streams along an acidification gradient from highly acidic (pH 4.6) to circumneutral (pH 7.4). Overall, responses to acidification in hyporheic zones were less pronounced, but followed the same pattern as in their benthic counterparts. Leaf decomposition was much faster in the circumneutral stream, both in the hyporheic and benthic zones (k = 0.0068 and 0.0534 d−1, respectively), than in the most acidic one (k = 0.0016 and 0.0055 d−1, respectively), and correlated well with the acidic gradient in both compartments. Interestingly, leaf litter decomposition was less affected by acidification in hyporheic compared to benthic compartments, likely due to the relatively low sensitivity of fungi, which were the main decomposers of buried coarse particulate organic matter. These results argue in favour of conserving hyporheic habitats in acidified streams as they can maintain matter and species fluxes that are essential to the ecosystem

Litter identity mediates predator impacts on the functioning of an aquatic detritus-based food web

During past decades, several mechanisms such as resource quality and habitat complexity have been proposed to explain variations in the strength of trophic cascades across ecosystems. In detritus-based headwater streams, litter accumulations constitute both a habitat and a resource for detritivorous macroinvertebrates. Because litter edibility (which promotes trophic cascades) is usually inversely correlated with its structural complexity (which weakens trophic cascades), there is a great scope for stronger trophic cascades in litter accumulations that are dominated by easily degradable litter species. However, it remains unclear how mixing contrasting litter species (conferring both habitat complexity and high quality resource) may influence top–down controls on communities and processes. In enclosures exposed in a second-order stream, we manipulated litter species composition by using two contrasting litter (alder and oak), and the presence–absence of a macroinvertebrate predator (Cordulegaster boltonii larvae), enabling it to effectively exert predation pressure, or not, on detritivores (consumptive versus non-consumptive predation effects). Leaf mass loss, detritivore biomass and community structure were mostly controlled independently by litter identity and mixing and by predator consumption. However, the strength of predator control was mediated by litter quality (stronger on alder), and to a lesser extent by litter mixing (weaker on mixed litter). Refractory litter such as oak leaves may contribute to the structural complexity of the habitat for stream macroinvertebrates, allowing the maintenance of detritivore communities even when strong predation pressure occurs. We suggest that considering the interaction between top–down and bottom–up factors is important when investigating their influence on natural communities and ecosystem processes in detritus-based ecosystems

Allelopathic inhibition of primary producer growth and photosynthesis by aquatic fungi

Autochthonous primary production is generally much reduced in forested headwater streams. Several hypotheses have been proposed for explaining this observation, among them, the low light intensity, or the strong constraints exerted by stream current. Allelopathic inhibition of competitors is a common ecological process in aquatic environments. Aquatic hyphomycetes are known to chemically inhibit bacteria and other fungi (including other aquatic hyphomycetes) but a possible allelopathic effect of aquatic hyphomycetes on primary producers has never been tested. The inhibitory effect of twelve aquatic hyphomycete species was tested on three diatom species. Nine aquatic hyphomycete species exhibited anti-diatom activity. Up to 100% diatom growth inhibition was observed. Our study reveals that such allelopathic interactions might be common in streams and probably involve an array of fungal compounds. We propose that the generally reduced primary production observed in forested headwater streams is, among other factors, due to the inhibition of primary producers by allelopathic compounds released by aquatic hyphomycetes

Rapid characterization of aquatic hyphomycetes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Protein fingerprinting using matrix-assisted laser desorption/ionization time-of-flight mass spec-trometry (MALDI-TOF MS) is a rapid, reliable, and economical method to characterize isolates of terrestrial fungi and other microorganisms. The objective of our study was to evaluate the suitability of MALDI-TOF MS for the identification of aquatic hyphomycetes, a polyphyletic group of fungi that play crucial roles in stream ecosystems. To this end, we used 34 isolates of 21 aquatic hyphomycete species whose identity was confirmed by spore morphology and internal transcribed spacer (ITS1-5.8S-ITS2 = ITS) nuc rDNA sequencing. We tested the efficiency of three protein extraction methods, including chemical and mechanical treatments using 13 different protocols, with the objective of producing high-quality MALDI-TOF mass spectra. In addition to extraction protocols, mycelium age was identified as a key parameter affecting protein extraction efficiency. The dendrogram based on mass-spectrum similarity indicated good and relevant taxonomic discrimination; the tree structure was comparable to that of the phylogram based on ITS sequences. Consequently, MALDI-TOF MS could reliably identify the isolates studied and provided greater taxonomic accuracy than classical morphological methods. MALDI-TOF MS seems suited for rapid characterization and identification of aquatic hyphomycete species

Beyond the water column: aquatic hyphomycetes outside their preferred habitat

Aquatic hyphomycetes have adapted to running waters by their uncommon conidial shape, which facilitates dispersal as well as adherence to plant substrata. However, they have been early and regularly reported to occur in a variety of environments other than their preferred habitat (e.g., in lentic freshwaters, brackish and marine environments, in terrestrial niches such as stream banks, dew, canopy waters and tree holes). In addition, several aquatic hyphomycetes have adapted to a mutualistic lifestyle which may involve plant defence, as endophytes in leaves, gymnosperm needles, orchids and terrestrial roots. There are several lines of evidence suggesting that aquatic hyphomycetes survive under terrestrial conditions due to their sexual states. Although exhibiting higher diversity in pristine streams, aquatic hyphomycetes can survive environmental stress, e.g., pollution or river intermittency. They also inhabit ground and hyporheic waters, where they appear to be subjected to both physical and physiological selection. Appropriate methods including molecular ones should provide a more comprehensive view of the occurrence and ecological roles of aquatic hyphomycetes outside their preferred habitat

Global Patterns and Controls of Nutrient Immobilization On Decomposing Cellulose In Riverine Ecosystems

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature

La ville à la campagne

To fulfil its needs, the city takes possession of various resources from the rural space, especially water. This article draws up an analysis of a conflict that arose in the 1960s-1970s following ground subsidences in the Mons-Borinage area, after new water catchments were installed by the Brussels Water Company and the national public works administration. Those subsidences resulted in considerable damage to buildings, whose owners haven’t received compensation until now. The article demonstrates how the city manages to preserve its catchments through the verbalization of its function, its position, and its prerogatives. Despite some compensations, in the end, the city’s public interest prevails against that of the rural space. This case illustrates the unequal balance of power between the two territories: as much in the conflict genesis as in its development and resolution, the city takes precedence over the country. Our analysis also stresses the necessity of considering the territorial effects of conflicts on natural resources

Roost selection and home range of an African insectivorous bat Nycteris thebaica (Chiroptera, Nycteridae)

Roost site selection, daily movement patterns and home range area of African bats are poorly known. We used radio-telemetry to investigate these parameters in the African bat Nycteris thebaica. The bats predominantly used antbear Orycteropus afer burrows or culverts as night roosts. Day roost sites included caves and antbear burrows. Individuals travelled an average of 1.1 km between day roosts and foraging areas, a distance similar to that predicted from a comparative study of aspect ratios. Foraging (home) ranges were relatively small and averaged 12.9 ha. The foraging range of each bat overlapped on average with the ranges of 4.3 neighbouring bats, and the area of overlap covered 49% of its foraging range. For a bat with low wingloading, N. thebaica is capable of large travelling distances, suggesting that it is a strong flyer. We suggest that antbear burrows are not limiting; however, it is likely that these bats travel beyond the foraging area for more optimal roosts such as caves

Organic-matter decomposition as a bioassessment tool of stream functioning: A comparison of eight decomposition-based indicators exposed to different environmental changes

International audienceOrganic-matter decomposition has long been proposed as a tool to assess stream functional integrity, but this indicator largely depends on organic-matter selection. We assessed eight decomposition-based indicators along two well-known environmental gradients, a nutrient-enrichment gradient (0.2-1.4 mg DIN/L) in central Portugal and an acidification gradient (pH: 4.69-7.33) in north-eastern France to identify the most effective organicmatter indicator for assessing stream functional integrity. Functional indicators included natural leaf litter (alder and oak) in 10-mm and 0.5-mm mesh bags, commercial tea (Lipton green and rooibos teas in 0.25-mm mesh bags), wood sticks (wood tongue depressors) and cotton strips. Biotic indices based on benthic macroinvertebrates (IPtIN for Portugal and IBGN for France) were calculated to compare the effectiveness of structural and functional indicators in detecting stream impairment and to assess the relationship between both types of indicators. The effectiveness of organic-matter decomposition rates as a functional indicator depended on the stressor considered and the substrate used. Decomposition rates generally identified nutrient enrichment and acidification in the most acidic streams. Decomposition rates of alder and oak leaves in coarse-mesh bags, green and rooibos teas and wood sticks were positively related with pH. Only decomposition rates of rooibos tea and wood sticks were related with DIN concentration; decomposition rates along the nutrient-enrichment gradient were confounded by differences in shredder abundance and temperature among streams. Stream structural integrity was good to excellent across streams; the IPtI N index was unrelated to DIN concentration, while the IBGN index was positively related with pH. The relationships between decomposition rates and biotic indices were loose in most cases, and only decomposition rates of alder leaves in coarse-mesh bags and green tea were positively related with the IBGN. Commercial substrates may be a good alternative to leaf litter to assess stream functional integrity, especially in the case of nutrient enrichment