Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1966. / Typescript. Vita. Nitrogen fixation by members of the tribe Klebsielleae [by] M.C. Mahl ... [et al.] : leaves 7-12. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin, 1967. / "Nitrogen fixation by growing cells and cell-free extracts of the of the bacillaceae [by] D.F. Witz, R.W. Detroy and P.W. Wilson, [reprinted from] Archiv für mikrobiologie 55, 369-381, 1967" inserted between leaves [9]-[23]. Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Magister Scientiae - MSc / The microbial life of sub-Antarctic islands plays a key role in the islands ecosystem, with microbial activities providing the majority of nutrients available for primary production. Knowledge of microbial diversity is still in its infancy and this is particularly true regarding the diversity of micro-organisms in the Antarctic and sub-Antarctic regions. One particularly important functional group of micro-organisms is the diazotrophs, or nitrogen-fixing bacteria and archaea. This group have not been well studied in the sub-Antarctic region, but play an important role in the nutrient cycling of the island. This thesis explored the diversity of nitrogen-fixing organisms in the soil of different ecological habitats on the sub-Antarctic Marion Island. / South Africa

No description available.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Biology. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / Ph.D.

No description available.

Genome plasticity in soil bacteria is predicted to be evolutionarily advantageous, allowing bacteria to sample genetic variation for adaptation to local soil ecology. In the field population of mesorhizobia where the symbiosis island (ICEMlSym[R7A]; an I̲ntegrative C̲onjugative E̲lement) was first identified, individual members were found to have significant chromosomal variation downstream of the phe-tRNA gene or phe-tRNA integrated ICEMlSym[R7A]. However, the nature of this genetic variation and whether it contributed to the adaptation of the indigenous mesorhizobia to their field environment were unknown. This work focused on a nodule isolate, Mesorhizobium sp. strain R88B, a member of the indigenous mesorhizobial population that received ICEMlSym[R7A] from strain R7A. The region downstream of ICEMlSym[R7A] was sequenced, revealing three distinct regions of non-conserved DNA, totalling 34.5 kb. Integrated directly downstream of ICEMlSym[R7A] was IMEMlAdh[R88B], a 24.3-kb novel I̲ntegrative M̲obilisable E̲lement. Using a PCR-based assay, it was shown that the IMEMlAdh[R88B] integrase could excise not only IMEMlAdh[R88B], but also a dual-IMEMlAdh[R88B]/ICEMlSym[R7A] hybrid, indicating the potential mobility of IMEMlAdh[R88B], and a likely evolutionary intermediate of a novel ICE. However, a functional role for MadA, (a putative adhesin and the sole adaptive trait encoded on IMEMlAdh[R88B]) was not discovered. Southern hybridisations with the mesorhizobial population provided evidence for the existence of a novel family of IMEs in the mesorhizobia, which, by diversifying their internal sequences, provide allele-specific variation to the population. The two other regions downstream of IMEMlAdh[R88B] possessed no obvious mobile genetic element structures, and only the region adjacent to the core-chromosome encoded ORFs with putative functions. Mutation of two of these ORFs, fhuD1 and fhuB1, identified their function as two of the four components of a ferrichrome ABC-uptake (Fhu) system. Using genetic screens, the remaining components of this transporter were mapped to two separate loci. Thus, the functional transporter in R88B was a composite of at least two independently-acquired Fhu systems. The genetic screens also revealed that ferrichrome utilisation was dependent on a TonB energy-transduction system encoded downstream of the Fhu ATPase gene, fhuC. Expression studies on the three fhu loci demonstrated that, despite their separate acquisition, their expression was coordinately up-regulated in response to low-iron conditions. Bioinformatics on the predicted promoter regions of the fhu genes identified the binding site of the rhizobial Fur analogue, RirA, which is likely to be responsible for this expression profile. Southern hybridisations of DNA isolated from members of the mesorhizobial population revealed the three fhu loci were not conserved in the mesorhizobial population. The presence of FhuA was the best predictive marker for the trait. It is proposed that multiple rounds of acquisitions and recombinations, both illegitimate and legitimate, formed this transporter, with the constant need for iron offset by the negative selection pressure of FhuA being a target for phage. None of the Fhu-specific genes was present in the sequenced M. loti strain MAFF303099 though flanking sequences were, further emphasizing the role of genome microevolution in forming the Fhu phenotype.

In the N2 fixing legume symbiosis, mineral nitrogen (N) not only decreases N2 fixation, but also delays and inhibits the formation and development of nodules. The purposes of this thesis were to elucidate the role of signaling in the mineral N effects on nodulation and nitrogen fixation in soybean [Glycine max (L.) Merr.] and to attempt to find ways to overcome this inhibition. The responses of soybean plants, in terms of daidzein and genistein synthesis and exudation, to different mineral N levels were studied. Daidzein and genistein distribution patterns varied with plant organs, mineral N levels, and plant development stages. Mineral N inhibited daidzein and genistein contents and concentrations in soybean root and shoot extracts, but did not affect root daidzein and genistein excretion in the same way. In both synthesis and excretion, daidzein and genistein were not affected equally by mineral N treatments. Variability existed among soybean cultivars in the responses of root daidzein and genistein contents and concentrations to mineral N levels. The amount of daidzein and genistein excreted by soybean roots did not always correspond to the daidzein and genistein contents and concentrations inside the roots. On the Bradyrhizobium japonicum side, nod gene expression was inhibited by mineral nitrogen. Genistein was used to pre-incubate B. japonicum cells or was applied directly into the plant growing medium. The results showed that genistein manipulation increased nodule weight and nodule nitrogen fixation under greenhouse conditions, but interactions existed among soybean cultivars, genistein concentrations and nitrate levels. Similar results were found under field conditions. Soybean yield was increased on sandy-loam soil by preincubation of B. japonicum cells with genistein. Addition of genistein beginning at the onset of nitrogen fixation also improved soybean nodulation and nitrogen fixation. Soybean cultivars had different sensitivities to genistein additi / Other studies also show that temperature affected genistein and daidzein content and concentration in soybean roots. The effect of temperature varied among soybean cultivars. Some PGPR strains can mitigate the negative effects of nitrate on soybean nodulation and nitrogen fixation, however, this is influenced by soybean genotype. Applying PGPR together with genistein preincubation of B. japonicum cells improved soybean nodulation and increased yield. The level of improvement varied among soybean cultivars and PGPR strains. Preincubation of B. japonicum cells with genistein improved strain competitiveness under greenhouse, but not field conditions. / Overall, these findings suggested that both plant-to-Bradyrhizobium and Bradyrhizobium-to-plant signals play important roles in the effects of mineral N on nodulation and N fixation. Signal manipulation could partially overcome the inhibitory effects of mineral N on soybean- B. japonicum N fixation symbiosis.

Nitrogen (N) generally limits primary production across large areas of the world's oceans. Allochthonous inputs of N (i.e., "new" N) via N2-fixing organisms (diazotrophs) are crucial for sustaining primary production and are often associated with net export of organic matter (OM) from surface waters. Diazotroph N (ND) contribution plays an integral role in supporting oceanic food webs and regulating the flux of OM into and through the oceans (e.g., the biological pump). Stable isotope techniques were used to trace the input and movement of new N through oceanic food webs. Laboratory experiments were performed to determine elemental and isotopic shifts of OM exposed to microbial and metazoan processing. δ15N of OM was typically higher when exposed to microbial communities, with no difference in δ15N of OM between experiments incubated at different temperatures (4°C and 25°C). In separate experiments, shrimp digestion did not alter the δ15N of OM through digestion, but the δ15N of macerated OM was enriched in 15N. Both of these experiments provide insight into the mechanisms driving variations in the δ15N of OM in the world's oceans. To assess the role of diazotrophs in oceanic food webs, we used the distribution of δ15N to quantify the relative ND contribution to suspended particle N (PN) and mesozooplankton N biomass (NZOOP) in the subtropical North Atlantic (STNA). Qualitatively, ND contribution was often high for both PN and NZOOP, with the highest contributions occurring in the mixed layer. Our results also indicate higher ND contribution to both PN and NZOOP in the western portion of the basin than in the east. ND contribution to larger mesozooplankton at depth further suggests that migrating mesozooplankton transport ND out of the mixed layer. Quantitatively, ND trophic transfer efficiency was lower than bulk N trophic transfer efficiency, suggesting low assimilation of ND by mesozooplankton. Overall, we estimated a ND pool turnover time on the order of weeks for our region of study. These findings demonstrate that ND is laterally and vertically variable in the STNA, and that the ND pool is sensitive to perturbations on short timescales. We discuss the global implications of our findings and their implications for the N cycle and elemental fluxes through oligotrophic oceans.