Schmidt Christiane

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Schmidt
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Christiane
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  • Article
    Deposit-feeding of Nonionellina labradorica (foraminifera) from an Arctic methane seep site and possible association with a methanotroph
    (European Geosciences Union, 2022-08-30) Schmidt, Christiane ; Geslin, Emmanuelle ; Bernhard, Joan M. ; LeKieffre, Charlotte ; Svenning, Mette Marianne ; Roberge, Hélène ; Schweizer, Magali ; Panieri, Giuliana
    Several foraminifera are deposit feeders that consume organic detritus (dead particulate organic material with entrained bacteria). However, the role of such foraminifera in the benthic food web remains understudied. Foraminifera feeding on methanotrophic bacteria, which are 13C-depleted, may cause negative cytoplasmic and/or calcitic δ13C values. To test whether the foraminiferal diet includes methanotrophs, we performed a short-term (20 h) feeding experiment with Nonionellina labradorica from an active Arctic methane-emission site (Storfjordrenna, Barents Sea) using the marine methanotroph Methyloprofundus sedimenti and analysed N. labradorica cytology via transmission electron microscopy (TEM). We hypothesised that M. sedimenti would be visible post-experiment in degradation vacuoles, as evidenced by their ultrastructure. Sediment grains (mostly clay) occurred inside one or several degradation vacuoles in all foraminifers. In 24 % of the specimens from the feeding experiment degradation vacuoles also contained bacteria, although none could be confirmed to be the offered M. sedimenti. Observations of the apertural area after 20 h incubation revealed three putative methanotrophs, close to clay particles, based on bacterial ultrastructural characteristics. Furthermore, we noted the absence of bacterial endobionts in all examined N. labradorica but confirmed the presence of kleptoplasts, which were often partially degraded. In sum, we suggest that M. sedimenti can be consumed via untargeted grazing in seeps and that N. labradorica can be generally classified as a deposit feeder at this Arctic site.
  • Article
    Ammonium and sulfate assimilation is widespread in benthic foraminifera
    (Frontiers Media, 2022-07-20) LeKieffre, Charlotte ; Jauffrais, Thierry ; Bernhard, Joan M. ; Filipsson, Helena L. ; Schmidt, Christiane ; Roberge, Hélène ; Maire, Olivier ; Panieri, Giuliana ; Geslin, Emmanuelle ; Meibom, Anders
    Nitrogen and sulfur are key elements in the biogeochemical cycles of marine ecosystems to which benthic foraminifera contribute significantly. Yet, cell-specific assimilation of ammonium, nitrate and sulfate by these protists is poorly characterized and understood across their wide range of species-specific trophic strategies. For example, detailed knowledge about ammonium and sulfate assimilation pathways is lacking and although some benthic foraminifera are known to maintain intracellular pools of nitrate and/or to denitrify, the potential use of nitrate-derived nitrogen for anabolic processes has not been systematically studied. In the present study, NanoSIMS isotopic imaging correlated with transmission electron microscopy was used to trace the incorporation of isotopically labeled inorganic nitrogen (ammonium or nitrate) and sulfate into the biomass of twelve benthic foraminiferal species from different marine environments. On timescales of twenty hours, no detectable 15N-enrichments from nitrate assimilation were observed in species known to perform denitrification, indicating that, while denitrifying foraminifera store intra-cellular nitrate, they do not use nitrate-derived nitrogen to build their biomass. Assimilation of both ammonium and sulfate, with corresponding 15N and 34S-enrichments, were observed in all species investigated (with some individual exceptions for sulfate). Assimilation of ammonium and sulfate thus can be considered widespread among benthic foraminifera. These metabolic capacities may help to underpin the ability of benthic foraminifera to colonize highly diverse marine habitats.