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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
    Accès libre
    Functional microbial diversity in regenerating cutover peatlands responds to vegetation succession
    (2008)
    Artz, Rebekka R. E.
    ;
    Chapman, Stephen J.
    ;
    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
    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
    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
    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
  • Publication
    Accès libre
    Exploitation of northern peatlands and biodiversity maintenance: a conflict between economy and ecology
    (2003)
    Chapman, Steve
    ;
    Buttler, Alexandre
    ;
    Francez, André-Jean
    ;
    Laggoun-Défarge, Fatima
    ;
    Vasander, Harri
    ;
    Schloter, Michael
    ;
    Combe, Jean
    ;
    Grosvernier, Philippe
    ;
    Harms, Hauke
    ;
    Epron, Daniel
    ;
    Gilbert, Daniel
    ;
    Mitchell, Edward 
    Peatlands are ecosystems of exceptional conservation value because of their beauty, biodiversity, importance in global geochemical cycles, and the paleoenvironmental records they preserve. Commercial extraction and drainage for forestry or agriculture have caused the destruction of many peatlands, especially in or close to urban areas of the northern temperate zone. Are these commercial and environmental interests irreconcilable? A close analysis suggests that limited peat extraction may actually increase biodiversity in some cases, and may be sustainable over the long term. As we learn more about how peatlands spontaneously regenerate following disturbance, and what conditions govern the re-establishment of a diverse community and the ability to sequester carbon, we increase our chances of being able to restore damaged peatlands. Preserving the chronological records hidden in the peat profile, the natural heritage value of peatlands, and the bulk of sequestered carbon, however, will remain incompatible with any form of exploitation.
  • Publication
    Métadonnées seulement
  • Publication
    Accès libre
    Population Dynamics and Food Preferences of the Testate Amoeba Nebela tincta major-bohemica-collaris Complex (Protozoa) in a Sphagnum Peatland
    (2003)
    Gilbert, Daniel
    ;
    Mitchell, Edward 
    ;
    Bourdier, Gilles
    ;
    Francez, André-Jean
    Population dynamics and food preferences of the testate amoeba species complex Nebela tincta major-bohemica-collaris ("Nebela collaris sensu lato") were described from a Sphagnum peatland over one growing season. The average abundance of Nebela collaris sensu lato was 29582 ind. l-1 active, and 2263 ind. l-1 encysted forms. On average, 17.4% of Nebela collaris sensu lato specimens were observed associated with prey, 71% of which could not be identified because of their poor preservation state. Among the identified prey, those most frequently ingested were micro-algae (45% of the total predator-prey associations, especially diatoms: 33%), and spores and mycelia of fungi (36%). Large ciliates, rotifers and small testate amoebae were also ingested, but mainly in summer. The seasonal variations in the proportions of prey categories in the ecosystem and the percentage of identifiable prey lead us to hypothesise that (1) Nebela collaris sensu lato ingest mainly immobile, senescent or dead organisms, and (2) that the more mobile micro-organisms such as ciliates and microMetazoa become more accessible, in relatively dry conditions, when the water film is thin.
  • Publication
    Accès libre
    Le régime alimentaire des Thécamoebiens (Protista, Sarcodina)
    (2000)
    Gilbert, Daniel
    ;
    Amblard, Christian
    ;
    Bourdier, Gilles
    ;
    Francez, André-Jean
    ;
    Mitchell, Edward 
    Les amibes à thèques constituent un groupe de protozoaires libres, dont la caractéristique commune est la présence d'une thèque sur la morphologie de laquelle est basée une grande partie de la systématique. Leur détermination étant relativement aisée, ces microorganismes représentent une communauté de choix pour l'étude des écosystèmes, et sont des indicateurs biologiques de la qualité des milieux. De récents travaux ont notamment souligné l'importance de ces microorganismes dans la structure et le fonctionnement des communautés microbiennes à la surface des tourbières à sphaignes. Cependant, de nombreux aspects de leur biologie et de leur écologie, tel que leur rôle trophique, demeurent encore mal connus. À ce sujet, les investigations menées au cours de ces dernières années ont montré que les thécamoebiens peuvent utiliser des ressources trophiques très variées: bactéries, champignons, microalgues, protozoaires, petits métazoaires, cellules végétales, matière organique particulaire détritique, et, probablement, substances solubles. Il est toutefois probable que la plupart des espèces ont un régime alimentaire peu diversifié et ne consomment plusieurs types de proies qu'en cas de nécessité. Par ailleurs, il semble que les proies les plus mobiles ou les plus grosses ne sont ingérées que lorsqu'elles sont sénéscentes. Dans la littérature, peu de données sont disponibles quant à la reconnaissance des proies par les thécamoebiens. Il est, cependant, possible que certaines espèces détectent leur proies par chimiotactisme. L'objet de cet article est de présenter la diversité du régime alimentaire des thécamoebiens à partir d'une synthèse bibliographique agrémentée de nos propres observations., Tcslate amoeba are a heterogeneous and most likely polyphyletic group of free-living protists. Their common characteristic is the presence of a test. Testate amoebae have been observed in almost every aqualic habitat, mainly in the periphytic zone of fresh and salt water bodies, soils and mosses. These protozoa are of great interest in the study of ecosystems. Indeed, the presence of a rigid test makes their identification relatively easy and they are good biological indicators. Furthermore, these micro-organisms play a key role in the structure and the functioning of microbial communities at the surface of Sphagnum peatlands. Nevertheless, testate amoebae remain globally little studied. More precisely, only limited data exist on their feeding habits although this is a necessary preliminary key aspect to understand their role in microbiai trophic networks. The aim of this review therefore is to allow a synthesis of the observations at hand in the literature on the feeding behaviour of testate amoebae. An important part of this review is devoted to our own observations on the species living in Sphagnum. Although testate amoebae arc generally believed to be heterotrophic. several species arc mixotrophic. Strict heterotrophy however remains the dominant trophic mode of testate amoebae. In the literature, several authors consider testate amoebae as largely bacterivorous. It seems however that many species ingest other kinds of prey: paniculate organic matter, microalgae, plant cells, prolists, fungi and small metazoa. Furthermore, some species are polyphagous while others seem to have more specialised feeding habits. Little is known on prey identification by testate amoebae. In particular, it is difficult to tell if mechanisms of distant detection (e.g. chimiotactism) exist or if a physical contact is necessary. Our observations on living and on fixed individuals suggest that both mechanisms exist. Furthermore, it seems that the food selectivity by testate amoebae is influenced by the physiological state of prey organisms at the moment of their ingestion. As for naked amoebae, the ingestion of prey is done by invagination of the cytoplasmic membrane. However, the particularity of testate amoebae is that the ingestion and egestion lake place at the level of the pseudostome. but no clearly defined cytostome or cytoprocte exist. The ability of some testate amoebae species to ingest a wide range of prey sizes (0,2 to 1 000 μm) and of very different kinds likely represents an competitive advantage in some environments. In these conditions, it seems important to improve our knowledge on the biology and the ecology of these protozoa.
  • Publication
    Accès libre
    Above- and belowground linkages in Sphagnum peatland: climate warming affects plant-microbial interactions
    Jassey, Vincent E. J.
    ;
    Chiapusio, Geneviève
    ;
    Binet, Philippe
    ;
    Buttler, Alexandre
    ;
    Laggoun-Défarge, Fatima
    ;
    Delarue, Frédéric
    ;
    Bernard, Nadine
    ;
    Mitchell, Edward 
    ;
    Toussaint, Marie-Laure
    ;
    Francez, André-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.