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Abstract:
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.
