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Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

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  • Publication
    Métadonnées seulement
    To What Extent Do Food Preferences Explain the Trophic Position of Heterotrophic and Mixotrophic Microbial Consumers in a Sphagnum Peatland?
    (2013)
    Jassey, Vincent E. J.
    ;
    Meyer, Caroline
    ;
    Dupuy, Christine
    ;
    Bernard, Nadine
    ;
    Mitchell, Edward 
    ;
    Toussaint, Marie-Laure
    ;
    Metian, Marc
    ;
    Chatelain, Auriel P.
    ;
    Gilbert, Daniel
    Although microorganisms are the primary drivers of biogeochemical cycles, the structure and functioning of microbial food webs are poorly studied. This is the case in Sphagnum peatlands, where microbial communities play a key role in the global carbon cycle. Here, we explored the structure of the microbial food web from a Sphagnum peatland by analyzing (1) the density and biomass of different microbial functional groups, (2) the natural stable isotope (delta C-13 and delta N-15) signatures of key microbial consumers (testate amoebae), and (3) the digestive vacuole contents of Hyalosphenia papilio, the dominant testate amoeba species in our system. Our results showed that the feeding type of testate amoeba species (bacterivory, algivory, or both) translates into their trophic position as assessed by isotopic signatures. Our study further demonstrates, for H. papilio, the energetic benefits of mixotrophy when the density of its preferential prey is low. Overall, our results show that testate amoebae occupy different trophic levels within the microbial food web, depending on their feeding behavior, the density of their food resources, and their metabolism (i.e., mixotrophy vs. heterotrophy). Combined analyses of predation, community structure, and stable isotopes now allow the structure of microbial food webs to be more completely described, which should lead to improved models of microbial community function.
  • Publication
    Métadonnées seulement
    Above- and belowground linkages in Sphagnum peatland: climate warming affects plant-microbial interactions
    (2013)
    Jassey, Vincent E. J.
    ;
    Chiapusio, Genevieve
    ;
    Binet, Philippe
    ;
    Buttler, Alexandre
    ;
    Laggoun-Defarge, Fatima
    ;
    Delarue, Frederic
    ;
    Bernard, Nadine
    ;
    Mitchell, Edward 
    ;
    Toussaint, Marie-Laure
    ;
    Francez, Andre-Jean
    ;
    Gilbert, Daniel
    Peatlands contain approximately one third of all soil organic carbon (SOC). Warming can alter above- and belowground linkages that regulate soil organic carbon dynamics and C-balance in peatlands. Here we examine the multiyear impact of in situ experimental warming on the microbial food web, vegetation, and their feedbacks with soil chemistry. We provide evidence of both positive and negative impacts of warming on specific microbial functional groups, leading to destabilization of the microbial food web. We observed a strong reduction (70%) in the biomass of top-predators (testate amoebae) in warmed plots. Such a loss caused a shortening of microbial food chains, which in turn stimulated microbial activity, leading to slight increases in levels of nutrients and labile C in water. We further show that warming altered the regulatory role of Sphagnum-polyphenols on microbial community structure with a potential inhibition of top predators. In addition, warming caused a decrease in Sphagnum cover and an increase in vascular plant cover. Using structural equation modelling, we show that changes in the microbial food web affected the relationships between plants, soil water chemistry, and microbial communities. These results suggest that warming will destabilize C and nutrient recycling of peatlands via changes in above- and belowground linkages, and therefore, the microbial food web associated with mosses will feedback positively to global warming by destabilizing the carbon cycle. This study confirms that microbial food webs thus constitute a key element in the functioning of peatland ecosystems. Their study can help understand how mosses, as ecosystem engineers, tightly regulate biogeochemical cycling and climate feedback in peatlands
  • Publication
    Métadonnées seulement
    Can pollution bias peatland paleoclimate reconstruction?
    (2012)
    Payne, Richard J.
    ;
    Mitchell, Edward 
    ;
    Hung, Nguyen-Viet
    ;
    Gilbert, Daniel
    Peatland testate amoebae are widely used to reconstruct paleohydrological/climatic changes, but many species are also known to respond to pollutants. Peatlands around the world have been exposed to anthropogenic and intermittent natural pollution through the late Holocene. This raises the question: can pollution lead to changes in the testate amoeba paleoecological record that could be erroneously interpreted as a climatic change? To address this issue we applied testate amoeba transfer functions to the results of experiments adding pollutants (N, P, S, Pb, O-3) to peatlands and similar ecosystems. We found a significant effect in only one case, an experiment in which N and P were added, suggesting that pollution-induced biases are limited. However, we caution researchers to be aware of this possibility when interpreting paleoecological records. Studies characterising the paleoecological response to pollution allow pollution impacts to be tracked and distinguished from climate change. (c) 2012 University of Washington. Published by Elsevier Inc. All rights reserved.
  • Publication
    Métadonnées seulement
    Functional microbial diversity in regenerating cutover peatlands responds to vegetation succession
    (2008)
    Artz, Rebekka
    ;
    Chapman, Steve
    ;
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Bortoluzzi, Estelle
    ;
    Gilbert, Daniel
    ;
    Yli-Petays, Mika
    ;
    Vasander, Harri
    ;
    Francez, André-Jean
    1. While establishment of vegetation is the most visual indicator of regeneration on cutover peatland, the reinstatement of belowground functions is less well understood. Vegetation succession results in differences in peat quality in terms of C availability. The respiratory response of the soil microbial community to ecologically relevant substrates (community-level physiological profile, CLPP) such as those found in rhizosphere exudates and litter hydrolysates, is thought to reflect the activity and functional diversity of the soil microbial community, especially those involved in turnover of soluble photosynthate-derived C. 2. The relationship between CLPP and typical regeneration stages was investigated at five European peatlands, each with up to five sites representing a gradient of natural regeneration stages. We aimed to determine whether unaided revegetation consistently affected soil microbial CLPP, which environmental factors explained variation in CLPP on the scale of individual peatlands, and if these factors were consistent across different peatlands. 3. Within each peatland, a decomposition index based on diagnostic bands in Fourier transform-infrared spectra indicated that regeneration had generally started from a common base and that the influence of vegetation on the decomposition index declined with depth. In parallel, differences in vegetation cover between regeneration stages resulted in significantly different CLPP, but this effect decreased rapidly with soil depth. The magnitudes of the effect of vegetation succession versus soil depth appeared to be linked with the age range of the regeneration gradients. Hence, the effect of vegetation on CLPP is effectively diluted due to the remaining organic matter. Specific plant species described significant proportions of CLPP variability but these species were not consistent across peatland types. The effects of soil depth appeared to be peatland-specific. 4. Synthesis and applications. Together, the results indicate significant responses of the microbial community to vegetation succession, with the strength of the effect probably dependent on quantities of labile C allocation to the soil microbial community. Therefore, particularly in the early stages of regeneration of cutover peatlands, CLPP could provide vital information about the relative importance of different plant functional types on potential rates of labile C turnover.
  • Publication
    Métadonnées seulement
    Cut-over peatland regeneration assessment using organic matter and microbial indicators (bacteria and testate amoebae)
    (2008)
    Laggoun-Défarge, Fatima
    ;
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Disnar, Jean-Robert
    ;
    Comont, Laure
    ;
    Warner, Barry
    ;
    Buttler, Alexandre
    1. Cut-over peatlands cover large surfaces of high potential value for enhancing biodiversity and carbon sequestration if successfully restored. Unfortunately, evaluation of restoration success is not straightforward. We assessed the bioindicator value of organic matter (OM), testate amoebae (protozoa) and bacteria in peat from two regeneration stages and a reference site of a cut-over bog. 2. Contrasting biochemical signatures of peat OM were observed along the regenerating profiles, allowing clear differentiation between the newly regenerated peat and the old peat. Where peat macrofossils were absent sugar biomarkers were used to infer peat botanical origin and OM alteration. 3. Over the succession, the OM composition of the new peat differed. Peat from the more recent stage was dominated by Sphagnum-derived tissues and characterized by lower carbohydrate preservation and higher bacterial biomass than the advanced regeneration stage. 4. Surface testate amoeba communities also changed from the recent to the advanced stages of regeneration, indicating a shift from wet and moderately acidic conditions to drier and more acidic conditions. Over this regeneration sequence (i) the biomass and average size of species declined but were higher at the unexploited site and (ii) species richness and diversity increased but density declined. 5. Synthesis and applications. Although secondary succession in the cut-over bog led to an ecosystem similar to that of the reference site in terms of surface vegetation, OM and testate amoebae continued to reflect disturbances associated with peat harvesting. Nevertheless, the described dynamics of both microbial and biochemical variables over the succession showed similarities between the advanced stage and the reference site: a higher testate amoeba diversity was associated with better carbohydrate preservation and a more heterogeneous botanical composition of the peat. The inferred water table depth and pH based on testate amoebae indicators proved to be an alternative approach for assessing restoration processes, in contrast to labour-intensive repeated measurements in the field. The botanical and biochemical composition of peat OM provided additional information on past anthropogenic perturbations of the bog and could be used for restoration monitoring. The combination of several indicators therefore provides a more complete assessment of ecological conditions that could be valuable for the management of cut-over peatlands.
  • Publication
    Métadonnées seulement
    Relationship between testate amoeba (Protist) communities and atmospheric heavy metals accumulated in Barbula indica (Bryophyta) in vietnam
    (2007)
    Nguyen-Viet, Hung
    ;
    Bernard, Nadine
    ;
    Mitchell, Edward 
    ;
    Cortet, J
    ;
    Badot, Pierre-Marie
    ;
    Gilbert, Daniel
    We studied the relationships between testate amoeba communities and heavy metal (Pb, Cd, Zn, Ni, Cu, Mn, and Fe) concentrations in the moss Barbula indica sampled at 29 sites in and around the city of Hanoi (Vietnam). Our first approach was to compare the heavy metal concentrations and testate amoeba variables between the city (zone 1) and the surrounding (zone 2). Mean moss concentrations of Pb, Cd, Zn, Ni, and Cu were significantly higher and testate amoeba species richness and abundance were significantly lower in zone 1 and the abundance of eight taxa differed significantly between the two zones. We then studied the correlation between heavy metals and testate amoebae. Species richness and abundance were correlated negatively to Pb concentration. Shannon H' was negatively correlated to both Pb and Cd. The abundance of several species was negatively correlated with Pb, Cd, Zn, and Ni; however, at the community level, Pb emerged as the only significant variable in a redundancy analysis. Our results suggest that testate amoebae are sensitive to and may be good bioindicators for heavy metal pollution, especially lead. Further research is needed to understand the causal relationships underlying the observed patterns.
  • Publication
    Métadonnées seulement
    Effects of experimental lead pollution on the microbial communities associated with Sphagnum fallax (Bryophyta)
    (2007)
    Nguyen-Viet, Hung
    ;
    Gilbert, Daniel
    ;
    Mitchell, Edward 
    ;
    Badot, Pierre-Marie
    ;
    Bernard, Nadine
    Ecotoxicological studies usually focus on single microbial species under controlled conditions. As a result, little is known about the responses of different microbial functional groups or individual species to stresses. In an aim to assess the response of complex microbial communities to pollution in their natural habitat, we studied the effect of a simulated lead pollution on the microbial community (bacteria, cyanobacteria, protists, fungi, and micrometazoa) living on Sphagnum fallax. Mosses were grown in the laboratory with 0 (control), 625, and 2,500 mu g L-1 of Pb2+ diluted in a standard nutrient solution and were sampled after 0, 6, 12, and 20 weeks. The biomasses of bacteria, microalgae, testate amoebae, and ciliates were dramatically and significantly decreased in both Pb addition treatments after 6, 12, and 20 weeks in comparison with the control. The biomass of cyanobacteria declined after 6 and 12 weeks in the highest Pb treatment. The biomasses of fungi, rotifers, and nematodes decreased along the duration of the experiment but were not significantly affected by lead addition. Consequently, the total microbial biomass was lower for both Pb addition treatments after 12 and 20 weeks than in the controls. The community structure was strongly modified due to changes in the densities of testate amoebae and ciliates, whereas the relative contribution of bacteria to the microbial biomass was stable. Differences in responses among the microbial groups suggest changes in the trophic links among them. The correlation between the biomass of bacteria and that of ciliates or testate amoebae increased with increasing Pb loading. We interpret this result as an effect on the grazing pathways of these predators and by the Pb effect on other potential prey (i.e., smaller protists). The community approach used here complements classical ecotoxicological studies by providing clues to the complex effect of pollutant-affecting organisms both directly and indirectly through trophic effects and could potentially find applications for pollution monitoring.
  • Publication
    Métadonnées seulement
    Reconciling commercial exploitation of peat with biodiversity in peatland ecosystems
    (2006)
    Artz, Rebekka
    ;
    Campbell, Colin
    ;
    Francez, André-Jean
    ;
    Laggoun-Défarge, Fatima
    ;
    Vasander, Harri
    ;
    Schloter, Michael
    ;
    Epron, Daniel
    ;
    Gilbert, Daniel
    ;
    Mitchell, Edward 
    ;
    Schwarz, G
    ;
    Chapman, Steve
  • Publication
    Métadonnées seulement
    Biochemical characteristics of peat organic matter and distribution of testate amoebae in two naturally regenerating cutover Sphagnum peatlands of the Jura Mountains
    (2004)
    Laggoun-Défarge, Fatima
    ;
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Warner, Barry
    ;
    Comont, Laure
    ;
    Disnar, Jean-Robert
    ;
    Buttler, Alexandre
  • Publication
    Métadonnées seulement
    Reconciling commercial exploitation of peat with biodiversity in peatland ecosystems (EU Project RECIPE)
    (2004)
    Mitchell, Edward 
    ;
    Chapman, Steve
    ;
    Buttler, Alexandre
    ;
    Combe, Jean
    ;
    Francez, André-Jean
    ;
    Gilbert, Daniel
    ;
    Harms, Hauke
    ;
    Laggoun-Défarge, Fatima
    ;
    Schloter, Michael
    ;
    Vasander, Harri