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.

Trifolium ambiguum (Caucasian clover) is being released in New Zealand for use in areas where growth of T. repens (white clover) is marginal. Although closely related to T. repens, T. ambiguum has unique and highly specific nodulation requirements and as rhizobial strains capable of effectively nodulating T. ambiguum are not naturally found in New Zealand soils, they must be introduced with the seed. Rhizobium leguminosarum bv. trifolii strains such as ICC105 form effective nodules on T. ambiguum but ineffective (Fix⁻) nodules on T. repens. The T. repens nodules nevertheless develop normally and contain bacteroids. R. l. bv. trifolii strains that are effective on T. repens such as NZP561, fail to nodulate T. ambiguum. As the host-specific nitrogen fixation defect of Caucasian clover rhizobia on T. repens has potentially adverse agronomic implications, the genetic basis for this Fix⁻ phenotype was investigated. Rhizobium leguminosarum bv. trifolii strain ICC105 was converted to Fix⁺ on T. repens by the introduction of an 18-kb fragment of DNA from a white clover rhizobial strain (NZP514) symbiotic plasmid. This fragment contained several nif and fix genes, including nifHDKEN, fixABCX, nifA, nifB, fdxN and fixU. Tn5 mutation of these white clover rhizobial genes demonstrated that most were required to impart the Fix⁺ phenotype on T. repens to ICC105, with the exception of nifA. Mutagenesis of the ICC105 nifA gene and subsequent complementation with various combinations of the white clover rhizobia nif/fix genes as well as transcriptional lacZ fusion studies of the ICC105 nifA and nifH genes demonstrated that ICC105 nifA is expressed and functional during the ineffective nodulation of T. repens and able to activate expression of nifHDKEN and fixABCX operons derived from white clover rhizobium but not from ICC105. Sequence analysis and comparison of the intergenic region between the divergently transcribed nif/fix operons revealed a conserved 111-bp region found between the nifH/fixA promoters of Caucasian clover rhizobia, but not in white clover rhizobia. Attempts to modify this region in ICC105 failed in creating a strain which was Fix⁺ on T. repens; however recombination of the nifHD/fixAB region from a white clover rhizobium into the ICC105 genome produced several strains with a �swapped� nitrogen fixation phenotype (i.e. Fix⁺ on T. repens and Fix⁻ on T. ambiguum). A hypothesis was therefore proposed by which differences in the nifH/fixA promoter regions of Caucasian clover rhizobia and white clover rhizobia modulate the expression of the upstream genes in response to the particular plant host they are nodulating. The incompatibility between the symbiotic plasmid of R. l. bv. trifolii ICC105 and the white clover rhizobium symbiotic plasmid cointegrate, pPN1, was also investigated and potential regions of each plasmid involved in this incompatibility were identified. The research presented in this thesis has contributed to the genetic knowledge of the nitrogen fixation genes, and regulation of these genes in R. l. bv. trifolii. It has also provided progress towards the goal of creating a suitable inoculant strain for T. ambiguum that is able to fix nitrogen in symbiosis with both T. repens and T. ambiguum.

Thesis (M.Sc.) -- University of Western Sydney, Hawkesbury, 1999. / Thesis submitted for the degree of Master of Science. Includes bibliographical references (leaves 113-131).

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on May 7, 2009) Includes bibliographical references.

Thesis (Ph. D.)--Texas State University--San Marcos, 2009. / Vita. Includes bibliographic references (leaves 19-24, 46-50, 69-74, 97-101,127-131, 141-144). Also available on microfilm.

Thesis (Ph. D.)--Washington State University, August 2009. / Title from PDF title page (viewed on Aug. 28, 2009). "Department of Crop and Soil Sciences." Includes bibliographical references.

Thesis (Ph.D.) - University of Queensland, 2005. / Includes bibliography.

Thesis (M.S.)--University of North Carolina Wilmington, 2007. / Includes bibliographical references (leaves: 120-121)

A comprehensive field and laboratory study of nitrogen uptake in intertidal seaweeds was undertaken. Methods for measuring nitrogen uptake rates were evaluated. Short initial periods of rapid ammonium uptake were common in nitrogen deficient plants. The presence of ammonium inhibited nitrate uptake, but a certain degree of nitrogen starvation overcame this suppression. Laboratory studies with Porphyra perforata showed that nitrogen starved cultures maintained rapid initial ammonium uptake rates. The nitrate uptake system did not remain activated. Nitrogen starvation also resulted in a general decrease in soluble nitrogen content and a transient increase in nitrate reductase activity. The effectiveness of in vitro and in vivo nitrate reductase assays was investigated. The rate of nitrite production in the in vivo assay varied with incubation time. Therefore, the in vitro assay was used. Nitrate grown cultures of Porphyra perforata maintained high ammonium uptake rates. It was suggested that the rate of nitrate reduction was limiting the supply of nitrogen for further assimilation which may control ammonium uptake. Ammonium arid ammonium plus nitrate grown cultures had very low nitrogen uptake rates and nitrate reductase activities. Field studies with Gracilaria verrucosa confirmed that growth on ammonium inhibited nitrate uptake, nitrate accumulation and nitrate reductase activity. The presence of ammonium did not inhibit nitrate uptake rates in severely starved populations. All populations maintained high ammonium uptake rates suggesting that they were nitrogen limited at this time (August). Ammonium and nitrate uptake were saturable in the high intertidal G. verrucosa population but not in the low intertidal population. An investigation was made into the effect of nitrogen source and periodic exposure to air on growth, development and nitrogen uptake in Fucus distichus germlings. Gamete release, fertilization, germination and germling growth had no requirement for a specific form of nitrogen. Periodic exposure to air increased secondary rhizoid development. Ammonium and nitrate uptake rates of the germlings were much higher than for the mature thalli, but the affinity for nitrate was similar. The germlings showed saturable uptake kinetics but the mature thalli did not. The presence of ammonium inhibited nitrate uptake by the mature plants but not by the germlings. Mild desiccation enhanced nutrient uptake rates in several intertidal seaweeds. This uptake response occurred when growth was limited by that particular nutrient and when the thallus had been exposed to periodic desiccation for several weeks. The degree of enhancement, the percent desiccation producing maximum uptake rates and the tolerance to higher degrees of desiccation were related to intertidal location. This was shown to be an intraspecific as well as an interspecific adaptation. Transplant experiments with G. verrucosa showed that enhanced nutrient uptake rates after desiccation were related to intertidal height and not geographic location and that this response could be induced in approximately five weeks. It was suggested that this enhanced uptake response was an adaptation to nitrogen procurement and C/N homeostasis following periodic exposure when carbon was assimilated but when other nutrients were not available. / Science, Faculty of / Botany, Department of / Graduate

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