Elemental composition and degree of homeostasis of fungi: are aquatic hyphomycetes more like metazoans, bacteria or plants?

Ecological stoichiometry generally assumes that heterotrophs have a higher degree of ele- mental homeostasis than autotrophs. Differences between fixed consumer nutrient requirements and nutrients available in resources allow prediction of the intensity of nutrient recycling ensured by heterotrophs. Despite their fundamental role in detritus decomposition, extremely few data are currently available on fungal elemental composition. In this study, we quantified the degree of elemental homeostasis of aquatic hyphomycetes used as model organisms. Contrary to metazoans, but similar to plants, aquatic hyphomy- cetes exhibited highly plastic elemental compositions. Mycelium also reached far higher C/ nutrient ratios than reported for bacteria. Our results suggest that non-homeostasis of fungi should be explicitly included in stoichiometric models dealing with nutrient recycling, and that the discrepancy in homeostasis between some bacterial strains and fungi should cer- tainly be considered when investigating interactions between both groups of decomposers

Questioning the roles of resources nutritional quality in ecology

International audienceOur understanding of ecosystem functioning is strongly linked to the study ofpredator–prey relationships and food web structures. However, trophic ecology hasoften focused on identifying taxonomic relationships and quantifying the biomassor energy ingested by consumers, but has often failed to integrate the importanceof the nutritional quality of resources in ecological dynamics. Underlying this gap isthe multi-dimensional nature of resource quality which has hampered any consensuson the definition of resource nutritional quality. In this special issue, we aimed atgathering a subset of articles exemplifying the diversity of variables by which resourcesquality is quantified, the diversity of research topics that can be tackled in ecology– from physiological or evolutionary aspects to ecosystem processes – and proposesome perspectives on the integration of nutritional quality within broader ecologicalconcepts. Using a semi-automated literature analysis, we map the current landscapeof the ‘resources nutritional quality’ research of the last 30 years. We depict how ithas been quantified through physical, biological or chemical indicators, the use ofthese parameters being largely dependent on the type of ecosystem studied and on theinvestigated ecological process. We then position the articles published in this specialissue of Oikos within this landscape, showing they cover a small but relatively wellrepresentative subset of the domains of resources quality-related issues. Articles in thisspecial issue browse a range of individual and population-level approaches (embracingevolutionary questions) to community related questions, include methodologicalissues and ecosystem-wide approaches using trophic quality indicators as tracers ofresources origin. Based on these studies and on the literature review, we identify a nonexhaustivelist of challenges and perspectives of research that we consider of highestpriority in the large topic of trophic ecology

High-Rate Capable Floating Strip Micromegas

We report on the optimization of discharge insensitive floating strip Micromegas (MICRO-MEsh GASeous) detectors, fit for use in high-energy muon spectrometers. The suitability of these detectors for particle tracking is shown in high-background environments and at very high particle fluxes up to 60MHz/cm$^2$. Measurement and simulation of the microscopic discharge behavior have demonstrated the excellent discharge tolerance. A floating strip Micromegas with an active area of 48cm$\times$50cm with 1920 copper anode strips exhibits in 120GeV pion beams a spatial resolution of 50$\mu$m at detection efficiencies above 95%. Pulse height, spatial resolution and detection efficiency are homogeneous over the detector. Reconstruction of particle track inclination in a single detector plane is discussed, optimum angular resolutions below $5^\circ$ are observed. Systematic deviations of this $\mu$TPC-method are fully understood. The reconstruction capabilities for minimum ionizing muons are investigated in a 6.4cm$\times$6.4cm floating strip Micromegas under intense background irradiation of the whole active area with 20MeV protons at a rate of 550kHz. The spatial resolution for muons is not distorted by space charge effects. A 6.4cm$\times$6.4cm floating strip Micromegas doublet with low material budget is investigated in highly ionizing proton and carbon ion beams at particle rates between 2MHz and 2GHz. Stable operation up to the highest rates is observed, spatial resolution, detection efficiencies, the multi-hit and high-rate capability are discussed.Comment: Presented at ICHEP 2014, accepted for publication in Nuclear Physics B Proceedings Supplement

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

Benthic algae stimulate leaf litter decomposition in detritus-based headwater streams: a case of aquatic priming effect?

In detritus-based ecosystems, autochthonous primary production contributes very little to the detritus pool. Yet primary producers may still influence the functioning of these ecosystems through complex interactions with decomposers and detritivores. Recent studies have suggested that, in aquatic systems, small amounts of labile carbon (C) (e.g., producer exudates), could increase the mineralization of more recalcitrant organic-matter pools (e.g., leaf litter). This process, called priming effect, should be exacerbated under low- nutrient conditions and may alter the nature of interactions among microbial groups, from competition under low-nutrient conditions to indirect mutualism under high-nutrient conditions. Theoretical models further predict that primary producers may be competitively excluded when allochthonous C sources enter an ecosystem. In this study, the effects of a benthic diatom on aquatic hyphomycetes, bacteria, and leaf litter decomposition were investigated under two nutrient levels in a factorial microcosm experiment simulating detritus- based, headwater stream ecosystems. Contrary to theoretical expectations, diatoms and decomposers were able to coexist under both nutrient conditions. Under low-nutrient conditions, diatoms increased leaf litter decomposition rate by 20% compared to treatments where they were absent. No effect was observed under high-nutrient conditions. The increase in leaf litter mineralization rate induced a positive feedback on diatom densities. We attribute these results to the priming effect of labile C exudates from primary producers. The presence of diatoms in combination with fungal decomposers also promoted decomposer diversity and, under low-nutrient conditions, led to a significant decrease in leaf litter C:P ratio that could improve secondary production. Results from our microcosm experiment suggest new mechanisms by which primary producers may influence organic matter dynamics even in ecosystems where autochthonous primary production is low

Using plant litter decomposition as an indicator of ecosystem response to soil contamination

The inventory and remediation of contaminated sites have emerged as top environmental priorities worldwide. A large body of evidence has accumulated to show how soil contamination affects biological communities and ecological processes. This knowledge has yet to be used for the development of indicators of soil quality that are meaningful to end-users and are easy to implement in soil quality assessment schemes. In this study, we used quantifiable measures of litter decomposition, a key biophysical process, as indicators of the ecological impact of soil contamination by trace metals and hydrocarbons. We conducted a litterbag experiment with coarse and fine mesh bags to compare highly vs. minimally contaminated sites within eight locations representative of a wide array of environmental conditions and types of pollution. Contrary to the common assumption that soil contamination hampers soil functions, idiosyncratic responses were detected for litter decomposition rate and decomposer activity metrics. A negative relationship between detritivore and microbial responses to soil contamination indicates that wherever the activity of one group of decomposers is reduced, increase in activity of the other group may ensure litter decomposition to proceed at rate similar or higher than baseline rate. This finding may indicate that compensatory dynamics in soil communities is important in determining ecosystem stability against chemical stressors. As litter decomposition may inform on the capacity of terrestrial ecosystems to cope with soil contamination, it may be a useful complement to chemical soil analyses in routine soil quality assessment schemes

Self-Secured PUF: Protecting the Loop PUF by Masking

Physical Unclonable Functions (PUFs) provide means to generate chip individual keys, especially for low-cost applications such as the Internet of Things (IoT). They are intrinsically robust against reverse engineering, and more cost-effective than non-volatile memory (NVM). For several PUF primitives, countermeasures have been proposed to mitigate side-channel weaknesses. However, most mitigation techniques require substantial design effort and/or complexity overhead, which cannot be tolerated in low-cost IoT scenarios. In this paper, we first analyze side-channel vulnerabilities of the Loop PUF, an area efficient PUF implementation with a configurable delay path based on a single ring oscillator (RO). We provide side-channel analysis (SCA) results from power and electromagnetic measurements. We confirm that oscillation frequencies are easily observable and distinguishable, breaking the security of unprotected Loop PUF implementations. Second, we present a low-cost countermeasure based on temporal masking to thwart SCA that requires only one bit of randomness per PUF response bit. The randomness is extracted from the PUF itself creating a self-secured PUF. The concept is highly effective regarding security, low complexity, and low design constraints making it ideal for applications like IoT. Finally, we discuss trade-offs of side-channel resistance, reliability, and latency as well as the transfer of the countermeasure to other RO-based PUFs

Analysis and Protection of the Two-metric Helper Data Scheme

To compensate for the poor reliability of Physical Unclonable Function (PUF) primitives, some low complexity solutions not requiring error-correcting codes (ECC) have been proposed. One simple method is to discard less reliable bits, which are indicated in the helper data stored inside the PUF. To avoid discarding bits, the Two-metric Helper Data (TMH) method, which particularly applies to oscillation-based PUFs, allows to keep all bits by using different metrics when deriving the PUF response. However, oscillation-based PUFs are sensitive to side-channel analysis (SCA) since the frequencies of the oscillations can be observed by current or electromagnetic measurements. This paper studies the security of PUFs using TMH in order to obtain both reliable and robust PUF responses. We show that PUFs using TMH are sensitive to SCA, but can be greatly improved by using temporal masking and adapted extraction metrics. In case of public helper data, an efficient protection requires the randomization of the measurement order. We study two different solutions, providing interesting insights into trade-offs between security and complexity

Temperature and nutrient effects on the relative importance of brown and green pathways for stream ecosystem functioning: A mesocosm approach

In addition to global warming, aquatic ecosystems are currently facing multiple global changes among which include changes in nitrogen (N) loads. While several studies have investigated both temperature and N impacts on aquatic ecosystems independently, knowledge on their interactive effects remains scarce. In forested headwater streams, decomposition of leaf litter represents the main process ensuring the transfer of nutrients and energy to higher trophic levels, followed by autochthonous primary production, mainly ensured by phototrophic biofilms. The main aim of this study was to disentangle the independent and combined effects of temperature increase and nutrient availability on the relative importance of brown and green processes involved in stream functioning. We hypothesised that water temperature and nutrients would lead to a general increase in leaf‐litter decomposition and primary production, but that the intensity of these effects would be largely modulated by competitive interactions arising between microorganisms as well as by the top‐down control of microorganisms by macro‐invertebrates. Macro‐invertebrates would, in turn, be bottom‐up controlled by microbial resources quality. To test these hypotheses, we conducted a 56‐day experiment in artificial streams containing leaf litter, microbial decomposers and biofilm inoculum, and an assemblage of macro‐invertebrates. Two water inorganic N:phosphorus (P) ratios (33 and 100, molar ratios) and two temperatures (ambient, +2°C) were manipulated, each treatment being replicated three times. Fungal and biofilm growth as well as leaf‐litter decomposition and primary production were quantified. Top‐down impacts of invertebrate primary consumers on brown and green compartments were evaluated using exclosures while bottom‐up control was evaluated through the measurement of resource stoichiometry and fatty acid profiles, as well as quantification of macro‐invertebrate growth and survival. Contrary to expectations, microbial decomposition was not significantly stimulated by nutrient or temperature manipulations, while primary production was only improved under ambient temperature. In the + 2°C treatment with high N:P, greater biofilm biomass was associated with lower fungal development, which indicates competition for nutrients in these conditions. Temperature increased macro‐invertebrate growth and leaf‐litter consumption, but this effect was independent of any improvement of basal resource quality, suggesting that temperature mediated changes in consumer metabolism and activity was the main mechanism involved. Most of our hypotheses that were based on simplified laboratory observations have been rejected in our semi‐controlled mesocosms. Our study suggests that the complexity of biological communities might greatly affect the response of ecosystems to multiple stressors, and that interactions between organisms must be explicitly taken into account when investigating the impacts of global change on ecosystem functioning

Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms’ survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web