Gärdes, Astrid
Lade...
Preferred name
Gärdes, Astrid
Official Name
Gärdes, Astrid
5 Ergebnisse
Gerade angezeigt 1 - 5 von 5
- Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Effects of anthropogenic changes on nitrogen cycling microbes of coral holobionts(2022-07-29); ; ; Corals have co-evolved in symbiotic relationships with diverse eukaryotic and prokaryotic microbes, collectively termed “the coral holobiont”. The efficient nutrient exchange between the coral host and photosynthetic Symbiodiniaceae supports the high productivity of corals in oligotrophic waters, yet critically depends on nitrogen limitation within the holobiont. Nitrogen cycling prokaryotes (bacteria and archaea) are thus key players in the functioning of coral holobionts. Octocorals are important benthic reef organisms, yet little is known about their-associated nitrogen cycling prokaryotes under environmental change. Denitrifiers (i.e., prokaryotes capable of reducing nitrate/nitrite to N2) could help maintain nitrogen limitation in holobionts, yet their community structure and (a)biotic controls are still unknown. This thesis aims to extend the current knowledge of nitrogen cycling prokaryotes in coral holobionts by focusing on the following research objectives: 1) investigating the effects of dissolved organic carbon (DOC) and concomitant heat stress on the abundance of diazotrophs (i.e., prokaryotes capable of converting N2 into ammonia) and denitrifiers in two octocoral species with distinct trophic strategies: the more mixotrophic soft coral Xenia umbellata and the highly autotrophic gorgonian Pinnigorgia flava; 2) investigating the respective contributions of host and algal identities to the structuring of denitrifier communities in cnidarian holobionts; 3) characterizing denitrifier communities in cnidarian holobionts and identifying their driving factors. The experimental work consisted of a multidisciplinary approach combing physiological, molecular, and bioinformatical tools and executed over the course of two aquarium-based experiments: 1) a 45-day experiment consisting of 21 days of DOC enrichment at ambient 26 oC followed by 24 days of DOC enrichment under concomitant heat stress (32 oC) and 2) an experimental manipulation of algal communities in the coral model Aiptasia to generate distinct holobiont combinations of host and algal strains as well as aposymbiotic (i.e., algal-free) hosts. Our findings revealed that excess DOC alone had no effect on the diazotroph and denitrifier abundance in octocorals at ambient temperature. However, excess DOC and concomitant heat stress resulted in a contrasting microbiome response between diazotrophs (increased abundance) and denitrifiers (host-specific unaltered or decreased abundance) in octocorals. Such increased diazotroph abundance without equivalent increased denitrifier abundance in octocoral holobionts may disrupt the nitrogen limitation required for maintaining the symbiotic relationship. The presence of algal symbionts increased denitrifier abundance by up to 22-fold in Aiptasia holobionts. As increase of denitrifier abundances aligned with the superior photosynthetic performance of the inoculated algae and the identification of mostly heterotrophic denitrifiers in photosymbiotic Aiptasia, the photosynthetic carbon released by Symbiodiniaceae may be an important energy source for denitrifier communities. Thereby, I propose a positive feedback loop of photosynthetic Carbon, Denitrifier, Nitrogen limitation, and Symbiosis (PDNS) in the photosymbiotic holobiont, which may effectively contribute to maintaining the cnidarian-algal symbiosis in the unperturbed holobiont. This thesis extends our understanding on the ecological significance of nitrogen cycling microbes in cnidarian holobionts and emphasizes the importance of algae-prokaryote interactions during this process. For a better understanding on the function of nitrogen cyclers underlying the cnidarian-algal symbiosis, future research should combine microbiome manipulations with functional gene knockouts in selected denitrifier isolates. At an ecological scale, comparing dynamics of nitrogen cycling prokaryotes in a range of marine cnidarian hosts, including octocorals, under environmental change may give new insights into the ecological drivers of novel reef ecosystems in which reef-building corals are largely replaced as dominant ecosystem engineers.Dissertation526 514 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Bacterial Response to Elevated Dissolved Organic Carbon in Coral Reef Ecosystems(2017-01-12); ; ; Coastal pollution and algal cover are increasing in many coral reefs, resulting in higher dissolved organic carbon (DOC) concentrations. High DOC concentrations are shown to have highly detrimental effects on coral reefs through the stimulation of bacterial growth and the accumulation of suspended particles (i.e., aggregate formation). In this thesis, aggregate formation processes and gene expression of planktonic coral reef microbial populations under elevated concentrations of naturally abundant monosaccharides (glucose, galactose,mannose, and xylose) in algal exudates and sewage inputs, were investigated. The results of this thesis show evidence of the detrimental effects of high DOC concentrations on coral reefs by promoting aggregate formation and microbial activity. Furthermore, this thesis shows that elevated DOC concentrations mediate the expression of virulence factors involved in invasion, iron chelation, accumulation of toxic substances, suggesting to be important for feeding the positive loop of coral reef degradation.Dissertation361 157 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Particulate organic matter : Vector for carbon export, indicator for aquaculture impact and microbial hotspot(2019-03-07); ; ; Marine particulate organic matter (POM) can serve as vector for carbon (C) export, indicator for aquaculture impact and microbial hotspot: Off NW Africa, upwelling stimulates primary production. Despite the importance of bacteria for the biological C pump (BCP), little is known about bacterial communities (BCC) off NW Africa. Hence, the first objective is the description of the free-living (FL) and particle-attached (PA) BCC off NW Africa and potential key players in C degradation. Results reveal high relative abundances of C degrading bacteria, suggesting their key role in POM turnover. Aquacultures in the tropics are often accompanied by eutrophication and high POM loads. In Bolinao, Philippines, recurring hypoxia leads to major fish kills. So far POM characteristics have not been quantified. Results indicate the aquaculture activities in POM characteristics, rendering them ideal indicators for aquaculture impact. Overall, the results advance our understanding of the BCP and biogeochemical processes on aquaculture-derived POM. Furthermore, they indicate how processes occurring on the microscale are relevant for current global challengesDissertation451 255 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Milkfish (Chanos chanos) under stress: Contributing to fish welfare in tropical aquaculture by identifying and quantifying potential stressors(2019-12-19); ; ; Due to an ever-growing world population, the demand for aquatic food resources has increased constantly in the last decades and will continue to do so in the future. In order to meet the growing demand, aquaculture practices have intensified tremendously over the last decades. Current intensification of management practices is considered as a potentially severe threat to fish welfare and a sustainable production. Besides potential stressors due to the lack of optimization of management procedures, outdoor-based aquaculture systems are additionally prone to daily and seasonal environmental fluctuations. Therefore, cultured fish have to be able to cope with environmental fluctuations, such as changes in temperature and/or dissolved oxygen concentration, which could potentially cause stress. Stressful stimuli initiate an endocrine response in fish. For teleost fish, this includes among others, the release of cortisol via the hypothalamic-pituitary-interrenal axis. Furthermore, stress induces glucocorticoid-mediated changes in the fish’s energy metabolism (e.g. reallocation of energy resources) to provide energy in order to restore the homeostatic state, and subsequently to cope with the stressor. Several Asian countries, involving also several (sub-) tropical countries, are dominating the list of top producers in global aquaculture production. Thereby, milkfish (Chanos chanos) represents an important aquaculture species for some of these (sub-) tropical countries. As observed globally, milkfish aquaculture practices have been intensified markedly over the last decades. In particular, for Philippine milkfish aquaculture, it has been reported that these practices have led to associated negative consequences, such as a remarkable deterioration in water quality, which was linked, among others, to massive fish kill events. Surprisingly, until now, research on (chronic) stress or stress response of milkfish to different potential stressors under aquaculture conditions (environmental or management related) has received comparatively little attention. Therefore, the aim of this thesis was to identify and quantify potential environmental and management related stressors for milkfish, hereby intending to contribute to fish welfare in tropical aquaculture. For this purpose, different tools, from scale cortisol, (enzymatic) biomarkers to respiratory measurements, were used to assess the chronic stress response as well as metabolic implications for juvenile milkfish. Therefore, the thesis was divided into two parts: (i) the assessment of the stress-physiological response of juvenile milkfish when exposed to thermal and/or hypoxic stress under controlled laboratory conditions (Chapter II and III); and (ii) the quantification of chronic stress levels of milkfish cultured in commercial marine cage systems, thereby intending to identify prevailing stressors (Chapter IV). The outcomes of this thesis showed that even a comparatively mild increase in water temperature led to a significant chronic stress response in milkfish (increased scale cortisol; Chapter II). Furthermore, higher water temperature also significantly affected the metabolism of juvenile milkfish, demonstrated by enhanced metabolic rates and indications of reduced energy resources (Chapter III). Additionally, we could demonstrate that milkfish might be more susceptible to further stressors, such as decreasing dissolved oxygen concentrations, when exposed to thermal stress (Chapter III). Based on our results and considering anthropogenic induced climate change and warming as well as the strong temperature dependence of oxygen solubility, increasing water temperatures and hypoxic conditions should be considered as a future challenge for cultured milkfish. During our fieldwork in the most productive milkfish mariculture area of the Philippines, we could quantify chronic stress levels (scale cortisol) of milkfish cultured in commercial mariculture systems for the first time (Chapter IV). Thereby, we could report that milkfish experienced stressful conditions in commercial cage systems and are most likely exposed not only to a single but to a variety of chronically stressful stimuli over time (Chapter II vs Chapter IV). The outcomes of this thesis contribute to the foundational understanding and identification of prevailing stressors in milkfish mariculture. By aiming to pinpoint potential chronic stressors for milkfish, including changes in environmental conditions (e.g. increase in water temperature) as well as different aspects of typically applied management strategies (e.g. stocking density), the results and applied methods of this thesis can be used as an approach to optimize current culture conditions. This can consequently contribute to improve fish welfare and sustainability in milkfish mariculture as well as for other species cultured in (tropical) mariculture.Dissertation467 1333 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Gradients of anthropogenic stress in Indonesian coral reefs Implications for bacterial community composition and aggregate formation(2016-09-30); ; ; The Indonesian Spermonde Archipelago, like other coral reefs located in close vicinity to urban areas, is confronted with a multitude of local anthropogenic stressors. A combination of experimental and observational studies was conducted at several fringing reefs and in the water column of the Archipelago during two sampling campaigns. Water quality parameters were not gradually decreasing with increasing distance from Makassar. Instead, there were two contrasting regions, one eutrophic inshore zone and the remaining, oligotrophic shelf. Bacterial community compositions were very distinct between those two described zones. Additionally, significantly higher total aggregated volume and sedimentation rates were observed in the eutrophic zone compared sampling within the oligotrophic outer shelf zone. Data presented in this thesis conclusively showed that dense human populations on the mainland and on the individual islands exerted a clear and measurable influence on the investigated water quality parameters, but that influence was restricted to a narrow zone close to the coast.Dissertation327 145