Résultat de la recherche
Highways and subways: a story of fungi and bacteria in soils
Traditionnellement, bactériologues et mycologues mènent leurs recherches séparément, et ce malgré le fait que bactéries et champignons coexistent dans pratiquement tous les écosystèmes. Parmi les nombreuses interactions qui peuvent avoir lieu entre champignons et bactéries, cette thèse étudie les autoroutes fongiques, une interaction dans laquelle les bactéries sont capables de se déplacer le long d’hyphes de champignons. Nous avons investigué différents sols, avec une attention particulière portée à la voie oxalate-carbonate. Dans cette voie métabolique, il semblerait que la dispersion de bactéries le long d’hyphes fongiques soit importante pour l’activité oxalotrophe des bactéries. Nous avons développé un nouvel outil, appelé colonne pour autoroutes fongiques, destiné à isoler spécifiquement des champignons et des bactéries capables de se déplacer le long d’hyphes fongiques. Une approche à la fois cultivable et moléculaire nous a permis d’avoir un aperçu de la diversité des champignons et des bactéries interagissant par autoroutes fongiques dans les sols. Nos résultats suggèrent que la migration de bactéries le long des hyphes peut être soit extra-hyphale (autoroutes fongiques), soit intra-hyphale (métros fongiques). Nous avons observé que le déplacement de bactéries le long des hyphes fongiques est dépendant du milieu de culture, mais nous n’avons pas pu trouver l’élément régissant ce comportement différentiel. Nos résultats montrent que les autoroutes - et métros - fongiques doivent être considérés comme cruciaux pour la colonisation de nouveaux environnements par les bactéries. De plus, ces interactions jouent un rôle important dans la structuration des communautés bactériennes dans les sols. Nous avons également démontré que des champignons présents dans la voie oxalate-carbonate sont capables de dissoudre l’oxalate de calcium, ainsi que de disperser des bactéries non-oxalotrophes, redéfinissant ainsi le rôle de chaque organisme dans la voie oxalate-carbonate. Finalement, nous avons observé une propriété surprenante de certains champignons, que l’on pourrait relier a de la fixation d’azote, mais nous n’avons pu ni prouver, ni démentir, cette capacité. Cette thèse permet d’entrevoir la complexité des interactions champignons-bactéries, et démontre qu’il est important de considérer les deux groupes d’organismes lors d’études de leur écologie dans les sols., Traditionally, bacteriologists and mycologists have conducted study of their respective fields separately, despite of the fact that bacteria and fungi co-exist in almost every type of ecosystem. Amongst the numerous types of interactions possibly occurring between fungi and bacteria, this thesis focuses on fungal highways, a mechanism by which bacteria disperse along fungal hyphae. We investigated various soils, with special focus on soils under the influence of the oxalate-carbonate pathway. In this pathway, fungal-driven bacterial dispersal seems to be relevant for bacterial oxalotrophic activity. We developed and validated a device called fungal highway column, intended for the targeted isolation of fungi and bacteria migrating along fungal hyphae. Culture-based and culture-independent approaches allowed for an insight into the diversity of fungi and bacteria able to interact via fungal highways in soils. Our findings propose that migration of bacteria along hyphae might be either extrahyphal (fungal highways) or intrahyphal (fungal subways). We observed that bacterial migration along fungal hyphae is dependent on the culture medium, but we could not determine the element(s) triggering this differential behavior. Our results show that fungal highways (and subways) should be considered as crucial for the colonization of novel environments by bacteria, and that they play an important role in the structuring of bacterial communities in soils. As well, we demonstrated that fungi present in the oxalate-carbonate pathway are able to dissolve calcium oxalate and to disperse non-oxalotrophic bacteria, thus redefining the role of the microbial components in this pathway. Finally, we observed surprising properties of fungi, resembling to nitrogen fixation, but we could neither prove nor disprove this ability. This thesis gives an insight into the complexity of fungal-bacterial interactions, and demonstrates the importance of considering both groups of organisms for the study of their ecology in soils.
Protection of metal artifacts with the formation of metal–oxalates complexes by Beauveria bassiana
Several fungi present high tolerance to toxic metals and some are able to transform metals into metal-oxalate complexes. In this study, the ability of Beauveria bassiana to produce copper oxalates was evaluated. Growth performance was tested on various copper-containing media. B. bassiana proved highly resistant to copper, tolerating concentrations of up to 20 g L-1, and precipitating copper oxalates on all media tested. Chromatographic analyses showed that this species produced oxalic acid as sole metal chelator. The production of metal-oxalates can be used in the restoration and conservation of archeological and modern metal artifacts. The production of copper oxalates was confirmed directly using metallic pieces (both archeological and modern). The conversion of corrosion products into copper oxalates was demonstrated as well. In order to assess whether the capability of B. bassiana to produce metal-oxalates could be applied to other metals, iron and silver were tested as well. Iron appears to be directly sequestered in the wall of the fungal hyphae forming oxalates. However, the formation of a homogeneous layer on the object is not yet optimal. On silver, a co-precipitation of copper and silver oxalates occurred. As this greenish patina would not be acceptable on silver objects, silver reduction was explored as a tarnishing remediation. First experiments showed the transformation of silver nitrate into nanoparticles of elemental silver by an unknown extracellular mechanism. The production of copper oxalates is immediately applicable for the conservation of copper-based artifacts. For iron and silver this is not yet the case. However, the vast ability of B. bassiana to transform toxic metals using different immobilization mechanisms seems to offer considerable possibilities for industrial applications, such as the bioremediation of contaminated soils or the green synthesis of chemicals.