Harder, Tilmann
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Preferred name
Harder, Tilmann
Official Name
Harder, Tilmann
Alternative Name
Harder, T.
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ORCID
5 Ergebnisse
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Item-typ:Veröffentlichung, Mass spectrometric characterisation and chemometric evaluation of dissolved organic matter(2025-01-23); ; 40 53 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Biogeography, diversity and risk potential of toxigenic Amphidomataceae (Dinophyceae) in the North Sea and adjacent areas(2020-10-14); ; ; This PhD considerably increased the knowledge on (toxigenic) Amphidomataceae and AZAs in the North Sea and adjacent areas, including a reliable qPCR assay for toxigenic Am. languida, the description of new amphidomatacean species, strains, AZA variants, toxin profiles and AZA cell quota variability as well as the comprehensive data set on biogeography, seasonality and vertical distribution of the three toxigenic representatives in the North Sea. Thus, this doctoral thesis study provides a highly valuable baseline for official monitoring and future studies on toxigenic Amphidomataceae.Dissertation771 225 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The diatom holobiont in a changing Arctic Ocean(2024-12-19); ; ; ; Interactions between diatoms and bacteria are crucial for marine ecosystem functioning and the global climate as they largely govern primary production in marine systems and drive global biogeochemical cycles. Zooming in on the immediate surroundings of a diatom cell, a microhabitat for a diverse and overall mutualistic bacterial community can be found that enables reciprocal exchanges of metabolites benefiting both the diatom and its associated bacteria, which together form a discrete ecological unit – the holobiont. This thesis aims to improve our understanding of how diatom holobionts adapted to occupy Arctic or temperate niches and to which extent diatom-associated bacterial microbiomes help to adapt their host under environmental conditions relevant to climate change and associated climate change-mediated poleward range shifts. The main objectives are to (I) experimentally quantify the response of Arctic and temperate diatoms to abiotic factors that characterize their biogeographic separation and thereby identify potential bottlenecks for adaptation of Arctic diatoms and poleward range shifts of their temperate relatives; (II) determine the net effect of the bacterial microbiome on Arctic and temperate diatom growth under multi-driver settings of these factors to understand its role in host adaptation; (III) unravel how abiotic conditions affect diatom-associated bacterial microbiome community composition and the underlying hostmicrobiome interactions (IV) develop a methodological strategy to understand diatommicrobiome community dynamics on the single- (host-) cell level to enable necessary speciesspecific (in-situ) information of natural diatom microbiomes in the future.Dissertation150 157 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Development of novel titanium dioxide based solid phase extraction for the selective isolation and identification of siderophores(2023-01-20); ; ; Im Mittelpunkt dieser Arbeit stand die Vereinfachung der Identifizierung organischer Fe(III)-Liganden, so genannter Siderophore, durch einen neuen affinitätschromatographischen Ansatz mit Titandioxid (TiO2). Siderophore werden im Allgemeinen von Mikroorganismen als Reaktion auf eisenlimitierende Wachstumsbedingungen gebildet. Ziel dieser Arbeit war es daher, einen neuen Ansatz zu entwickeln, der den Nachweis von Siderophoren in natürlichen Rohproben erleichtert, um die natürliche Eisenligandenzusammensetzung widerzuspiegeln und somit Licht in die Eisenspeziation in Lebensräumen wie den Weltmeeren zu bringen.Dissertation251 199 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Molecular and optical characterization of dissolved organic matter in the Central Arctic Ocean(2022-11-30); ; ; ; Dissolved organic matter (DOM) in the ocean is a complex mixture of molecules derived from autochthonous (marine) or allochthonous (terrestrial) origins. DOM plays an important role in marine biogeochemical cycles by attenuating light available for primary production, serving as an energy and nutrient source for heterotrophic communities, regulating the ultraviolet and visible light absorption, undergoing photochemical processing, and acting as a trace metal ligand. DOM in the Central Arctic Ocean (CAO) is influenced by increased freshwater input and associated terrestrial materials in recent decades due to rapid climate change. The quantification of DOM sources (terrestrial versus marine) in the water column of the CAO is not well constrained. Few studies have systematically investigated the seasonality and spatial variability of DOM by combining optical and molecular-level analytical techniques in the CAO, especially during winter. State of the art chemical characterization of DOM is subject to major challenges: Solid phase extraction (SPE) that is often used to desalt and pre-concentrate marine DOM introduces chemical fractionation effects, which limits the comparability between analytical results for original samples and those carried out for SPE-DOM. There is no specific method to quantify fractionation effects, nor specific guidelines to avoid fractionation. Using mass spectrometry, quantitative DOM analyses is challenged by selective ionization of molecules and the large number of unresolved structural isomers that prevent classical external calibration. In the first part of this thesis, a method was developed to quantitatively track optical or chemical fractionation during SPE and investigate the potential mechanisms. We found a decrease in extraction efficiency of dissolved organic carbon (DOC), fluorescence and absorbance, and polar organic substances with increasing carbon loading on the SPE column. As the surface loading of the solid-phase increased, the dominant extraction mechanism shifted from PPL physisorption to increased DOM self-assembly, resulting in optical and chemical fractionation. The relative DOC loading (DOCload) was used to assess the carbon loading during SPE, and a double sigmoid model was applied to our online permeate fluorescence data as a function of DOCload, which allowed us to assess the degree of variability induced by DOCload. This finding has ample implications for the future processing and previous interpretation of chemical characteristics in SPE-DOM of aquatic organic matter. For the second part of the thesis, original water samples were acquired from the “Multidisciplinary Drifting Observatory for the Study of Arctic Climate” (MOSAiC) expedition. The water column samples covered a full year (2019 / 2020) and included the regions Amundsen Basin, western Nansen Basin and Yermak Plateau and Fram Strait. Samples were analyzed using optical spectroscopy to determine chromophoric DOM (CDOM) and fluorescent DOM (FDOM). In addition, a new method was applied that used Fourier transform ion cyclotron resonance mass spectrometry hyphenated to high performance liquid chromatography (LC-FTMS). The method allowed DOM analysis in original filtered water and thus avoided the chemical fractionation introduced by SPE. During the MOSAiC expedition, DOC concentrations and CDOM characteristics in the water column were primarily influenced by regional differences. These differences were largely dependent on terrestrially-derived DOM (tDOM) input by the transpolar drift (TPD) as indicative of average 136% and 45% higher aCDOM(350) and DOC concentration, respectively, in the Amundsen compared to the western Nansen Basin and Yermak Plateau, and slightly modified by seasonal changes. Despite the convenient identification of tDOM, optical spectroscopy was not suitable to quantify the contribution of tDOM to bulk DOC or to track sea ice derived DOM in the water column. In contrast, using LC-FTMS, we found quantitative linear correlation between the summed mass peak magnitudes for each sample (intsum) and DOC concentration. By combing LC-FTMS and source identification with optical parameters, we were able to quantify DOM sources (terrestrial versus marine) in the water column: 83% of the summed peak magnitude of all samples could be related to marine or terrestrial sources. tDOM contributed ∼17% (or 8 µmol kg-1) to deep DOC (~2000 m) in the CAO and was more refractory and had a higher state of unsaturation compared to marine DOM. The quantitative characterization of DOM in original seawater from different origin is a major step in the field of research. It provides a unique and new insight into the molecular changes in marine DOM composition and an improved understanding of the terrestrial DOM distribution in the CAO.Dissertation444 317