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Accumulation and Dissolution of Magnetite Crystals in a Magnetically Responsive Ciliate

Résumé : Magnetotactic bacteria (MTB) represent a group of microorganisms that are widespread in aquatic habitats and thrive at oxic-anoxic interfaces. They are able to scavenge high concentrations of iron thanks to the biomineralization of magnetic crystals in their unique organelles, the so-called magnetosome chains. Although their biodiversity has been intensively studied, their ecology and impact on iron cycling remain largely unexplored. Predation by protozoa was suggested as one of the ecological processes that could be involved in the release of iron back into the ecosystem. Magnetic protozoa were previously observed in aquatic environments, but their diversity and the fate of particulate iron during grazing are poorly documented. In this study, we report the morphological and molecular characterizations of a magnetically responsive MTB-grazing protozoan able to ingest high quantities of MTB. This protozoan is tentatively identified as $Uronema\ marinum$, a ciliate known to be a predator of bacteria. Using light and electron microscopy, we investigated in detail the vacuoles in which the lysis of phagocytized prokaryotes occurs. We carried out high-resolution observations of aligned magnetosome chains and ongoing dissolution of crystals. Particulate iron in the ciliate represented approximately 0.01% of its total volume. We show the ubiquity of this interaction in other types of environments and describe different grazing strategies. These data contribute to the mounting evidence that the interactions between MTB and protozoa might play a significant role in iron turnover in microaerophilic habitats.
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Caroline Monteil, Nicolas Menguy, Sandra Preveral, Alan Warren, David Pignol, et al.. Accumulation and Dissolution of Magnetite Crystals in a Magnetically Responsive Ciliate. Applied and Environmental Microbiology, American Society for Microbiology, 2018, 84 (8), pp.e02865-17. ⟨10.1128/AEM.02865-17⟩. ⟨cea-01950914⟩

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