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A Bacteria-Plant Model System to Study Nitrogen Fixation in Mangrove Ecosystems


Alfaro-Espinoza,  Gabriela
IMPRS MarMic, Max Planck Institute for Marine Microbiology, Max Planck Society;

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Alfaro-Espinoza, G. (2014). A Bacteria-Plant Model System to Study Nitrogen Fixation in Mangrove Ecosystems. PhD Thesis, Jacobs University, Bremen, Germany.

Cite as: http://hdl.handle.net/21.11116/0000-0001-C4D2-4
Mangrove ecosystems are highly productive and rich in organic matter. However, they are considered low-nutrient environments, being nitrogen one of the main nutrients limiting mangrove growth. Nitrogen-fixing bacteria are one of the main inputs of nitrogen to these forests. High nitrogen fixation rates have been detected in mangrove sediments and roots. Moreover, studies indicated that the relationship between diazotrophs and mangroves might be mutualistic. Several diazotrophs from mangrove roots and the associated rhizosphere have been isolated or identify by phylogenetic studies of the nitrogenase-coding nifH gene. However, our knowledge about the molecular signals and cellular mechanisms that govern diazotroph-mangrove interactions is scarce. Thus, in this thesis a diazotroph-mangrove model system was established to better understand the importance of this interaction for the ecosystem and how environmental changes could impact this organismal interplay. For this, nitrogen-fixing bacteria were isolated from mangrove roots. Moreover, root colonization pattern of the selected nitrogen fixer and the impact of some of the environmental factors that could affect its nitrogen fixation were investigated. The first result showed that the diazotroph M. mangrovicola Gal22 was a prominent candidate for the establishment of the model system. This bacterium was shown to possess all features and requirements needed for being a representative model organism. Thus a detail phenotypic and genotypic characterization of strain Gal22 was conducted. A strong mangrove root colonization was observed by M. mangrovicola in the absence of carbon and nitrogen sources, suggesting that root exudates were instrumental in establishing the interaction. Nitrogen fixation by this diazotroph was increased in the presence of the roots, supporting the above results. Finally, changes in environmental factors such as temperature and salinity had a rather minor impact on nitrogen fixation by M. mangrovicola thereby underlining the environmental robustness of this organism. Only very high temperatures showed a more pronounced effect inhibiting nitrogenase activity. The remarkable wide range of salinity under which Gal22 could conduct nitrogen fixation indicated its environmental versatility and may suggest that this bacterium can outcompete other bacterial organisms in the mangrove rhizosphere undergoing tidal and long-term changes in salinity. Overall, the results presented in this work together with the described model interaction system provide a solid basis for future investigations on the relationship between diazotrophs and mangrove plants as well as on the cycling of nitrogen in coastal environments.