Symbioses between ciliates and bacteria inhabiting reduced marine sediments in mangroves of Guadeloupe.

Alors que la Guadeloupe possède la plus grande bordure littorale de mangroves des Petites Antilles, la microfaune et la microflore bactérienne marine associées à cet écosystème sont très mal connues. Pourtant, ces diverses communautés de micro-organismes sont à la base du réseau trophique des sédiments marins de mangrove. En effet, grâce à leurs activités basées sur des processus hétérotrophes, ces micro-organismes vont permettre de dégrader la litière constituée de feuilles et branches de palétuviers tombées à la surface du sédiment. En condition anoxique, la dégradation des substrats végétaux par des bactéries sulfato-réductrices entraine la production de sulfures qui vont soutenir l’activité de bactéries chimiosynthétiques. Les protistes ciliés sont des micro-organismes eucaryotes unicellulaires caractérisés par la présence de cils sur la surface cellulaire et appartenant au micro-zooplancton. Leur mode de nutrition basé sur la phagocytose permet non seulement de favoriser la reminéralisation de la biomasse microbienne, ce qui augmente le transfert de nutriments à d'autres organismes du réseau trophique, mais facilite surtout l’émergence de nombreuses associations symbiotiques. Les résultats obtenus durant cette thèse ont permis de mettre en évidence la présence d’associations symbiotiques entre des bactéries sulfo-oxydantes ou hétérotrophes et des espèces de protistes ciliés faisant parties du périphyton de mangrove.While Guadeloupe has the largest coastal edge of mangroves in the Lesser Antilles, the microfauna and marine bacterial microflora associated with this ecosystem are very poorly understood. However, these diverse communities of microorganisms are at the base of the marine mangrove sediment food web. Indeed, thanks to their activities based on heterotrophic processes, these micro-organisms will make it possible to degrade the litter composed of mangrove leaves and branches that have fallen to the surface of the sediment. In anoxic conditions, the degradation of plant substrates by sulfate-reducing bacteria leads to the production of sulfides that will support the activity of chemosynthetic bacteria. Ciliates protists are unicellular eukaryotic microorganisms characterized by the presence of cilia on the cell surface and belonging to micro-zooplankton. Their phagocytosis-based nutrition not only promotes the remineralization of microbial biomass, which increases the transfer of nutrients to other organisms in the food web, but also facilitates the emergence of many symbiotic associations. The results obtained during this thesis allowed to highlight the presence of symbiotic associations between sulfur-oxidizing or heterotrophic bacteria and ciliates protist species that are part of the mangrove periphyton

First Description of Sulphur-Oxidizing Bacterial Symbiosis in a Cnidarian (Medusozoa) Living in Sulphidic Shallow-Water Environments

<div><p>Background</p><p>Since the discovery of thioautotrophic bacterial symbiosis in the giant tubeworm <i>Riftia pachyptila</i>, there has been great impetus to investigate such partnerships in other invertebrates. In this study, we present the occurrence of a sulphur-oxidizing symbiosis in a metazoan belonging to the phylum Cnidaria in which this event has never been described previously.</p><p>Methodology/Principal Findings</p><p>Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) observations and Energy-dispersive X-ray spectroscopy (EDXs) analysis, were employed to unveil the presence of prokaryotes population bearing elemental sulphur granules, growing on the body surface of the metazoan. Phylogenetic assessments were also undertaken to identify this invertebrate and microorganisms in thiotrophic symbiosis. Our results showed the occurrence of a thiotrophic symbiosis in a cnidarian identified as <i>Cladonema</i> sp.</p><p>Conclusions/Significance</p><p>This is the first report describing the occurrence of a sulphur-oxidizing symbiosis in a cnidarian. Furthermore, of the two adult morphologies, the polyp and medusa, this mutualistic association was found restricted to the polyp form of <i>Cladonema</i> sp.</p></div

Phylogenetic tree.

<p>Maximum Likelihood tree displaying the phylogenetic relationships between <i>Cladonema</i> sp. Guadeloupe F.W.I. (<b>in bold</b>) with other capitata hydrozoans based on the analysis of partial 18S rRNA gene sequences of 895 nucleotides. <i>Bellonella rigida</i> was used as the outgroup. Only bootstrap values of more than 60% are shown at each node. The scale bar corresponds to 0.01 changes per nucleotide.</p

Ultrastructural analysis of a Medusa.

<p>Light micrograph of the medusoid stage of <i>Cladonema</i> sp. (A) shows that the tentacles extended around the bell while SEM image (D) shows that the tentacles were retracted due to the chemical fixation artefact. No bacterium was observed on the bell surface of the medusa (B, D) or on its tentacles (C-D). The two insets clearly confirmed the absence of ectosymbiotic bacteria on the bell surface (B) and on the tentacles (C) of the medusa.</p

Structural analysis of polyp stage of <i>Cladonema</i> sp.

<p>(A) light micrograph of freshly collected polyps connected by a tube-like hydrocauli. The polyps appear white in colour especially for the tentacles. SEM image (B) shows that the polyp is composed of an entocodon (asterisk), a stalk (S), capitate (star) and filiform tentacles (arrows). A higher magnification of the polyp displays a filiform tentacle and small bacteria that can be clearly distinguished at the surface of the polyp (C). D and E show ectosymbiotic rod-shaped bacteria covering the polyp. Some of these are dividing (arrows) suggesting a high metabolism. The EDX spectrum obtained from the polyp (F) shows a peak of elemental sulphur suggesting that such bacteria are thioautotrophic (Cl: chloride, Na: sodium, Mg: magnesium, C: carbon, O: oxygen, S: elemental sulphur).</p

Candidatus Thiovulum sp. strain imperiosus: the largest free-living Epsilonproteobacteraeota Thiovulum strain lives in a marine mangrove environment

International audienceA large (47.75 ± 3.56 µm in diameter) Thiovulum bacterial strain forming white veils is described from a marine mangrove ecosystem. High sulfide concentrations (up to 8 mM of H 2 S) were measured on sunken organic matter (wood/bone debris) under laboratory conditions. This sulfur-oxidizing bacterium colonized the organic matter, forming a white veil. According to conventional scanning electron microscope (SEM) observations, bacterial cells are ovoid and slightly motile by numerous small flagella present on the cell surface. Large intracytoplasmic internal sulfur granules were observed, suggesting a sulfidic-based metabolism. Observations were confirmed by elemental sulfur distribution detected by energy-dispersive X-ray spectroscopy (EDXS) analysis using an environmental scanning electron microscope (ESEM) on non-dehydrated samples. Phylogenetic analysis of the partial sequence of 16S rDNA obtained from purified fractions of this Epsilonproteobacteraeota strain indicates that this bacterium belongs to the Thiovulaceae cluster and could be one of the largest Thiovulum ever described. We propose to name this species Candidatus Thiovulum sp. strain imperiosus

Ultrastructural analysis of polyp by TEM.

<p>Thin sections clearly show that the bacteria (arrows) are distributed exclusively on the body surface of polyps. No intracellular bacteria could be observed suggesting that there is no endosymbiosis in <i>Cladonema</i> sp. Higher magnification of the ectosymbionts (B) shows that the cytoplasm of the bacteria located outside the host tissue (b) contained two kinds of inclusions. The non-membrane-bound inclusions correspond to glycogen-like granules (white arrows) distributed throughout the cytoplasm while empty membrane-bound inclusions (curved arrow) correspond to sulphur granules probably located within the periplasm. The ectosymbionts appear to be fixed to the host cytoplasmic membrane (C) though atypical structures (see inset). Fig D displays a higher magnification of an atypical structure which is organized on two levels of “tubes” with a central tuft in contact with the bacteria. The nature of such “tubes” is unknown.</p

Phylogenetic tree.

<p>Maximum Likelihood tree displaying the phylogenetic relationships between the <i>Cladonema</i> sp. ectosymbiont (<b>in bold</b>) with other endo and ectosymbionts sulphur-oxidizing bacteria based on the analysis of 16S rRNA gene sequences of 926 nucleotides. <i>Methylocystis parvus</i> was used as the outgroup. Only bootstrap values of more than 50% are shown at each node. The white circles with or without black dot indicate whether symbionts are intra- or extracellular respectively. The scale bar corresponds to 0.02 changes per nucleotide.</p

Symbiotic status model in <i>Cladonema</i> sp.

<p>The diagram depicts a model to explain the distinctive symbiotic status existing between polyps and medusae of <i>Cladonema</i> sp. which seems to rely on an inverse gradient of sulphur and oxygen in the environment. The drawing shows that only the polypoid stage of the cnidarian living in a sulphide rich environment (at the oxic/anoxic interface) bears symbiotic sulphur-oxidizing bacteria while the medusoid stage located in the water column depleted in sulphide, is free of such ectosymbiosis. Such medusa were usually observed closed to sea grasses of <i>Caulerpa taxifolia</i> within the mangrove lagoon.</p

Protéger les cultures par la diversité végétale

International audienceL’avènement des engrais et des pesticides de synthèse a permis aux agriculteurs de s’affranchir des contraintes environnementales limitant les rendements et s’est accompagnée d’une simplification des parcelles et des paysages agricoles. Les impacts environnementaux et sanitaires de ce modèle dominant, ainsi que ses interrelations avec le changement climatique et l’érosion de la biodiversité, sont désormais bien établis par la communauté scientifique.La demande sociétale pour une agriculture répondant aux besoins alimentaires, dans le respect de l’environnement et de la santé humaine, est de plus en plus forte, mais la transition des systèmes de culture est insuffisamment engagée. Le manque d’alternatives efficaces aux pesticides de synthèse pour protéger les cultures est notamment pointé pour justifier cette inertie.Cette expertise scientifique collective dresse un bilan des stratégies de protection des cultures fondées sur la diversification végétale des parcelles et des paysages agricoles, et analyse les freins et les leviers de leur déploiement. Elle s’inscrit dans l’axe « Recherche-Innovation » du plan Écophyto 2+ qui soutient la production des connaissances et outils nécessaires à la réduction de l’utilisation des pesticides de synthèse.Cet ouvrage s’adresse aux enseignants-chercheurs et aux étudiants, ainsi qu’aux acteurs du monde agricole, aux gestionnaires du territoire, organismes et associations environnementales et à tout citoyen intéressé par ces questions