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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
    The curse of taxonomic uncertainty in biogeographical studies of free-living terrestrial protists: a case study of testate amoebae from Amsterdam Island
    (2009)
    Heger, Thierry J
    ;
    Mitchell, Edward 
    ;
    Ledeganck, Pieter
    ;
    Vincke, Sofie
    ;
    Van de Vijver, Bart
    ;
    Beyens, Louis
    Aim A current debate in microbial biogeography contrasts two views concerning the distribution of free-living microorganisms. The first view assumes a ubiquitous distribution, while the second assumes that at least some species have limited geographical distributions. We tested for limited geographical distributions by identifying testate amoebae morphospecies from an extremely remote oceanic island where the potential for endemism is high. Location Amsterdam Island, Indian Ocean. Methods Sixty moss and water samples collected from the top of the volcano to the lowland were investigated for their testate amoeba content. Due to taxonomic uncertainties among the Argynnia (Nebela) dentistoma species complex (including A. antarctica), we also performed light and scanning electron microscopy investigations on the shell ultrastructure and biometric analyses on several specimens of this taxon. Results We identified a total of 43 testate amoeba taxa belonging to 15 genera. Only four testate amoeba taxa had previously been recorded on this island. Testate amoeba communities of Amsterdam Island are dominated by cosmopolitan ubiquitous euglyphid taxa such as Trinema lineare, Assulina muscorum and Corythion dubium. The length and width ranges for Argynnia dentistoma on Amsterdam Island overlap with other records of this species and of A. antarctica, suggesting that A. antarctica is not a distinct taxon. Main conclusions Although Amsterdam Island is among the most remote islands in the world, an extensive inventory of testate amoeba morphospecies provided no clear evidence for endemism. On the one hand, our detailed morphometric analysis of the A. dentistoma complex revealed that A. antarctica, a morphospecies previously suggested to display endemism, cannot be confidently distinguished from the cosmopolitan morphospecies A. dentistoma. On the other hand, five morphotaxa could not be identified with certainty and might represent new species, potentially with limited distribution. These examples illustrate how taxonomic uncertainties undermine biogeographical studies of testate amoebae. In order to allow better interpretation of morphology-based testate amoeba distribution data, an assessment of genetic diversity among and within morphotaxa in relation to geographical distance for some common testate amoebae should be given high priority.
  • Publication
    Métadonnées seulement
    Ribosomal RNA genes challenge the monophyly of the hyalospheniidae (Amoebozoa : Arcellinida)
    (2008)
    Lara, Enrique 
    ;
    Heger, Thierry J
    ;
    Ekelund, Flemming
    ;
    Lamentowicz, Mariusz
    ;
    Mitchell, Edward 
    To date only five partial and two complete SSU rRNA gene sequences are available for the lobose testate amoebae (Arcellinida). Consequently, the phylogenetic relationships among taxa and the definition of species are still largely dependant on morphological characters of uncertain value, which causes confusion in the phylogeny, taxonomy and the debate on cosmopolitanism of free-living protists. Here we present a SSU rRNA-based phylogeny of the Hyalospheniidae including the most common species. Similar to the filose testate amoebae of the order Euglyphida the most basal clades have a terminal aperture; the ventral position of the pseudostome appears to be a derived character. Family Hyalospheniidae appears paraphyletic and is separated into three clades: (1) Heleopera sphagni, (2) Heleopera rosea and Argynnia dentistoma and (3) the rest of the species from genera Apodera, Hyalosphenia, Porosia and Nebela. Our data support the validity of morphological characters used to define species among the Hyalospheniidae and even suggest that taxa described as varieties may deserve the rank of species (e.g. N. penardiana var. minor). Finally our results suggest that the genera Hyalosphenia and Nebela are paraphyletic, and that Porosia bigibbosa branches inside the main Nebela clade. (c) 2007 Elsevier GmbH. All rights reserved.
  • Publication
    Métadonnées seulement
    Relationships among testate amoebae (Protozoa), vegetation and water chemistry in five Sphagnum-dominated peatlands in Europe
    (2000)
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Rydin, Hakan
    ;
    Albinsson, Christer
    ;
    Greenup, Alisson
    ;
    Heijmans, Monique
    ;
    Hoosbeek, Marcel
    ;
    Saarinen, Timo
    To study the relationships between groups of organisms and the degree to which these relationships are consistent across major climatic gradients, we analysed the testate amoeba (Protozoa) communities, vegetation and water chemistry of one peatland in five countries: Switzerland, The Netherlands, Great Britain, Sweden and Finland, as part of the BERI (Bog Ecosystem Research Initiative) project. The relationships between the different data sets and subsets were investigated by means of detrended correspondence analysis, canonical correspondence analysis and Mantel permutation tests. The comparison of data on vegetation and testate amoebae showed that inter-site differences are more pronounced for the vegetation than for the testate amoebae species assemblage. Testate amoebae are a useful tool in multi-site studies and in environmental monitoring of peatlands because: (1) the number of species in Sphagnum-dominated peatlands is much higher than for mosses or vascular plants; (2) most peatland species are cosmopolitan in their distributions and therefore less affected than plants by biogeographical distribution patterns, thus differences in testate amoeba assemblages can be interpreted primarily in terms of ecology; (3) they are closely related to the ecological characteristics of the exact spot where they live, therefore they can be used to analyse small-scale gradients that play a major role in the functioning of peatland ecosystems. This study revealed the existence of small-scale vertical gradients within the vegetation and life-form niche separation in response to water chemistry. The deep-rooted plants such as Carex spp. and Eriophorum spp. are related to the chemistry of water sampled at or near the ground water table, whereas the mosses are not. Testate amoebae wereshown to be ecologically more closely related to the chemistry of water sampled at or near the water table level and to the mosses than to the deep-rooted plants.
  • Publication
    Métadonnées seulement
    Can testate amoebae (Protozoa) and other micro-organisms help to overcome biogeographic bias in large scale global change research?
    (: Springer, 1999)
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Butler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Albinsson, Christer
    ;
    Rydin, Hakan
    ;
    Heijmans, Monique
    ;
    Hoosbeek, Marcel
    ;
    Greenup, Alisson
    ;
    Foot, Jonathan
    ;
    Saarinen, Timo
    ;
    Vasander, Harri
    ;
    Gobat, Jean-Michel 
    ;
    Visconti, Guido
    ;
    Beniston, Martin
    ;
    Iannorelli, Emilio D
    ;
    Barba, Diego
    To monitor global change, large scale long term studies are needed. Such studies often focus on vegetation, but most plant species have limited distribution areas. Micro-organisms by contrast are mostly cosmopolitan in their distributions. To study the relationships between organisation groups, we analysed the testate amoebae (Protozoa), vegetation, and water chemistry of five Sphagnum peatlands across Europe. Inter-site differences were more pronounced for the vegetation than for testate amoebae species assemblage. Testate amoebae represent a useful tool in multi-site studies and environmental monitoring of peatlands because: 1) the number of species is much higher than for plants, 2) most species are cosmopolitan and are therefore less affected by biogeographical distribution patterns than plants; thus differences in restate amoebae assemblages can be interpreted primarily in terms of ecology, 3) restate amoebae can be used to analyse and monitor small scale (cm) gradients that play a major role in the functioning of peatland ecosystems. We further studied the effect of elevated CO2 on microbial communities in the same peatlands. Elevated COL increased the biomass of heterotrophic bacteria and decreased the biomass of medium size protozoa (mostly small restate amoebae). These effects suggest changes in community functioning that may have feedback effects on other components of the ecosystem.