Diss. (sammanfattning) Stockholm : Karol. inst., 2001. / Härtill 6 uppsatser.

Thesis (Ph. D.)--Oregon State University, 1998. / Typescript (photocopy). Includes bibliographical references. Also available online.

Thesis (M. Sc.)--University of Hong Kong, 2006.

abstract: The oceans play an essential role in global biogeochemical cycles and in regulating climate. The biological carbon pump, the photosynthetic fixation of carbon dioxide by phytoplankton and subsequent sequestration of organic carbon into deep water, combined with the physical carbon pump, make the oceans the only long-term net sink for anthropogenic carbon dioxide. A full understanding of the workings of the biological carbon pump requires a knowledge of the role of different taxonomic groups of phytoplankton (protists and cyanobacteria) to organic carbon export. However, this has been difficult due to the degraded nature of particles sinking into particle traps, the main tools employed by oceanographers to collect sinking particulate matter in the ocean. In this study DNA-based molecular methods, including denaturing gradient gel electrophoresis, cloning and sequencing, and taxon-specific quantitative PCR, allowed for the first time for the identification of which protists and cyanobacteria contributed to the material collected by the traps in relation to their presence in the euphotic zone. I conducted this study at two time-series stations in the subtropical North Atlantic Ocean, one north of the Canary Islands, and one located south of Bermuda. The Bermuda study allowed me to investigate seasonal and interannual changes in the contribution of the plankton community to particle flux. I could also show that small unarmored taxa, including representatives of prasinophytes and cyanobacteria, constituted a significant fraction of sequences recovered from sediment trap material. Prasinophyte sequences alone could account for up to 13% of the clone library sequences of trap material during bloom periods. These observations contradict a long-standing paradigm in biological oceanography that only large taxa with mineral shells are capable of sinking while smaller, unarmored cells are recycled in the euphotic zone through the microbial loop. Climate change and a subsequent warming of the surface ocean may lead to a shift in the protist community toward smaller cell size in the future, but in light of these findings these changes may not necessarily lead to a reduction in the strength of the biological carbon pump. / Dissertation/Thesis / Ph.D. Biology 2011

abstract: The ocean sequesters more than 25% of the carbon released by anthropogenic action every year, and oligotrophic oceans, such as the Sargasso Sea, are responsible for about 50% of the global carbon export. Pico- and nano-phytoplankton (cells < 5 µm), mostly unicellular eukaryotes (protists) and cyanobacteria, dominate the primary production in the Sargasso Sea; however, little is known about their contribution to the export of carbon into the deep ocean via sinking particles. The overall goal of this study is to examine the link between growth and grazing rates of pico- and nano-phytoplankton and the carbon export in the Sargasso Sea. I investigate three aspects: 1) how microzooplankton grazing and physical forcing affect taxon-specific primary productivity in this region, 2) how these microbial trophic dynamics impact their contribution to the export of particulate matter, and 3) how much pico-phytoplankton, specifically the pico-cyanobacteria Synechococcus and Prochlorococcus, contribute to the carbon export. I collected seawater samples within the sunlit (euphotic) zone, and sinking particles at 150 m depth using particle traps in the Sargasso Sea during the winter and summer seasons of 2011 and 2012. I conducted dilution experiments to determine the growth and grazing rates of the pico- and nano-phytoplankton community, and used 454 pyrosequencing and quantitative Polymerase Chain Reaction to measure the relative and absolute contribution of these primary producers to the plankton community within the euphotic zone and in the sinking particles. I found that micrograzing controls taxon-specific primary production, and that microbial trophic dynamics impact directly the taxonomical composition of the sinking particles. For the first time, I was able to quantify clade-specific carbon export of pico-cyanobacteria and found that, despite their small size, these tiny primary producers are capable of sinking from the surface to the deeper oceans. However, their contribution to the carbon flux is often less than one tenth of their biomass contribution in the euphotic zone. Our study provides a comprehensive approach to better understand the role of pico- and nano-phytoplankton in the carbon cycle of oligotrophic oceans, and a baseline to study changes in the carbon export in future warmer oceans. / Dissertation/Thesis / Doctoral Dissertation Natural Science 2016

abstract: Microzooplankton, mainly heterotrophic unicellular eukaryotes (protists), play an important role in the cycling of nutrients and carbon in the sunlit (euphotic) zone of the world’s oceans. Few studies have investigated the microzooplankton communities in oligotrophic (low-nutrient) oceans, such as the Sargasso Sea. In this study, I investigate the seasonal and interannual dynamics of the heterotrophic protists, particularly the nanoflagellate, dinoflagellate, and ciliate communities, at the Bermuda Atlantic Time Series site and surrounding areas in the Sargasso Sea. In addition, I test the hypotheses that the community is controlled though bottom-up and top-down processes. To evaluate the bottom-up hypothesis, that the protists are controlled by prey availability, I test whether the protist abundance co-varies with the abundance of potential prey groups. Predation experiments with zooplankton were conducted and analyzed to test top-down control on the protists. I found distinguishable trends in biomass of the different protist groups between years and seasons. Nanoflagellates and dinoflagellates had higher biomass during the summer (28 ± 5 mgC/m2 and 44 ± 21 mgC/m2) than during the winter (17 ± 8 mgC/m2 and 30 ± 11 mgC/m2). Ciliates displayed the opposite trend with a higher average biomass in the winter (15 ± 9 mgC/m2) than in summer (5 ± 2 mgC/m2). In testing my bottom-up hypothesis, I found weak but significant positive grazer/prey relationships that indicate that nanoflagellates graze on picophytoplankton in winter and on the pico-cyanobacterium Prochlorococcus in summer. I found evidence that ciliates graze on Synechococcus in winter. I found weak but significant negative correlation between dinoflagellates and Prochlorococcus in summer. The predation experiments testing the top-down hypothesis did not show a clear top-down control, yet other studies in the region carried out during our investigation period support predation of the protists by the zooplankton. Overall, my results suggest a combination of bottom-up and top-down controls on these heterotrophic protists, however, further investigation is necessary to reveal the detailed trophic dynamics of these communities. / Dissertation/Thesis / Masters Thesis Biology 2016

No description available.

The adaptor protein 4 (AP4) heterotetrameric complex helps form vesicles and traffic cargo from the trans-Golgi Network. Tepsin, a member of the epsin family of trafficking proteins, is the only known accessory protein of AP4. My research focuses on understanding the interaction between AP4 and tepsin, and the function of tepsin. Through analysis of the largely unstructured C-terminus of tepsin, I identified a conserved motif that interacts specifically with AP4 β4. Mutagenesis of the motif affected binding in vitro and lead to loss of tepsin at the membrane in cells. I also showed tepsin binds a surface on β4 conserved across AP β subunits. Slight differences in the surfaces give rise to specific epsin family binding interactions. These data provided the first molecular description of the AP4-tepsin interaction. Unlike other epsins, tepsin contains a unique internal domain named VHS/ENTH-like (tVHS). I determined a high-resolution (1.85Ã) structure of the tVHS domain and found it is smaller than other VHS domains. The structure explains negative binding data between tVHS and typical VHS interactors such as ubiquitin and acidic dileucine motifs. The first structure of an internal VHS domain, it raised interesting questions about the evolution of the ENTH/ANTH/VHS domain family. We showed through phylogenetic analyses that tVHS clusters with VHS domains, but tepsin diverges early from other epsins, in line with my functional data. Commercial Y2H screens have revealed potential binding partners for tVHS and the N-terminal ENTH domain (tENTH). I have shown direct interactions between the tVHS domain and ISCA1, an iron-sulfur cluster assembly protein, as well as tENTH and c16orf62, a newly discovered retromer-like subunit, and we are validating the interactions in cells.

Preterm birth (PTB), or birth before 37 completed weeks of gestation, is the leading cause of newborn death worldwide. Despite its toll on human lives and economies, however, our understanding of its pathogenesis and prevention is limited. In this dissertation, I employed meta-analytical, comparative transcriptomic, and integrative genomic approaches to examine the pathways leading to prematurity. First, I synthesized the current landscape of transcriptomics research across PTB subtypes and uncovered gaps in our knowledge, particularly in cases that occur spontaneously (sPTB). Next, I sought to fill this gap by comparing gene expression profiles from placental tissue collected after human sPTB and term deliveries to identify genes whose dysregulation may contribute to sPTB pathogenesis. Furthermore, I refined these results with age matched control samples from rhesus macaque to tease apart sPTB-specific dysregulation from normal gestational age differences. Finally, I developed a desirability function-based approach to integrate the transcriptomics results with other heterogeneous sPTB âomics data. By applying this framework to GWAS, transcriptomics, epigenomics, and proteomics studies, I uncovered pathways related to homeostasis and muscle activity as candidates for future functional analyses.

Synaptogenesis requires coordinated intercellular communication between a presynaptic neuron and postsynaptic target cell. Neural development and maintenance is driven by a wide variety of molecules that necessarily traverse the specialized extracellular synaptomatrix separating partner cells. Extracellular matrix metalloproteinases (Mmps) shape the synaptomatrix environment and aberrant upregulation of Mmp function is causally linked to Fragile X syndrome (FXS), the most common heritable form of autism spectrum disorder and related intellectual disability. Therefore, researchers are interested in understanding how Mmps are regulated and function in the synaptomatrix to influence synaptogenesis and how this relates to the FXS synaptic disease state. A well-established Drosophila neuromuscular junction (NMJ) model system, containing just two conserved Mmps (Mmp1 and Mmp2), was used to fully interrogate Mmp functions in normal synapse development and in relationship to the FXS disease state. Synaptic Mmp1 and Mmp2 expression was co-dependently regulated and Mmp1 and Mmp2 co-suppressed each otherâs synaptogenic requirements. Differential Mmp regulation of heparan sulfate proteoglycan (HSPG), Dally-like protein (Dlp)-dependent intercellular Wnt signaling was found to restrict structural and functional NMJ development. A reciprocal Dlp-Mmp1 interaction was identified that localizes Mmp1 function in the synaptomatrix. This identified mechanism also causes aberrant Mmp1 upregulation in the FXS disease state which is thought to be causal for synaptogenic defects characterizing FXS.