Abstract:
This dissertation aimed to explore diazotrophs associated with various forestry tree species in South Africa. The comprehensive investigation includes an introduction to diazotrophs, covering essential aspects of biological nitrogen fixation, the evolutionary trajectory of diazotrophs, and their diverse symbiotic relationships. Emphasizing the substantial potential of diazotrophs in nitrogen-limited cropping, the study highlights the importance of a thorough exploration of their evolution, lifestyles, and genetic control mechanisms.
A significant portion of the dissertation focused the genus Bradyrhizobium, starting with the delineation of a novel Bradyrhizobium species, Bradyrhizobium xenonodulans, identified in association with Acacia dealbata and Acacia mearnsii in an invasive South African setting. The taxonomic methodology, employing genealogical concordance analysis and phenotypic analyses, establishes the novel status of this species.
The investigation also investigates the biogeographic origin of B. xenonodulans, considering the Accompanying Mutualist Hypothesis and the Generalist Host Hypothesis. Analysis of nodA and nifD sequences points to an Australian origin, indicating co-introduction with Acacia into South Africa. However, it is still possible that the bacterium is native to southern Africa, and that the strains examined acquired their Australian symbiotic loci from an unrelated bacterium previously introduced to the region. These findings contribute to understanding symbiotic relationships in invasive Acacia, underscoring the role of introduced symbionts in facilitating Acacia's establishment in non-native environments. The study sheds light on the intricacies of biogeographic patterns and provides insights into the origins of symbiotic loci within Bradyrhizobium species associated with invasive Acacia.
Furthermore, the dissertation delves into metagenomic soil data, challenging prevailing views by revealing that climatic conditions and soil properties exert limited influence on soil microbiome composition in Eucalyptus plantations. The study highlights the potential influence of specific plant species, particularly Eucalyptus, on microbiome composition, noting a consistent microbiome across varied plantation sites. Bradyrhizobium, a prevalent component of the soil microbiome, emerges as a key organism in nutrient cycling, particularly in the nitrogen cycle, despite the scarcity of nitrogen-fixing genes.
