PhD students

PhD students within the research area Oceanography.

Sei SusukiSei Suzuki

Title of PhD project

Predatory behavior in heterotrophic nanoflagellates

Supervisors

Thomas Kiørboe and Anders Peter Andersen

Background of project

The ocean hosts a great diversity of single celled microorganisms that are characterized by their flagella: a flexible fine appendix that serves for motility and for capturing and handling food particles such as bacteria and phytoplankton. These unicellular flagellates play a key role in the oceanic food chains and in the biogeochemical cycles of marine ecosystems. Despite of their importance in these marine biological processes, the mechanisms of flagellate feeding and their associated costs in mortality still remain widely unknown.

About the project

This study will focus on heterotrophic nanoflagellates: very small flagellates (2-20μm) that exclusively feed on other organisms. At this small scale, aquatic environments become as viscous as a thick syrup and present a challenge for prey capture. First, I aim to understand how nanoflagellates overcome the impeding effects of viscosity by creating currents with their flagella to draw the prey towards them. I will describe these events and study the different types of feeding currents for several species with highspeed video recordings. I will also perform experiments to quantify the rate in which the flagellates graze upon their prey by culturing them together, and I will compare the results with the calculations of computed models of the feeding currents. And secondly, I will investigate potential defense mechanisms: how can the nanoflagellates themselves avoid or reduce the chance of being eaten while they search for food.

Perspective

The overarching aims of this PhD are to describe and to understand prey encounter mechanisms in important marine heterotrophic nanoflagellates. The results of my project will illustrate the evolution of different prey-capture strategies and will establish their potential trade-offs. Studying the feeding mechanisms of these small organisms is important for a better overall understanding of the predator-prey interactions that take place at the small scale.

Anders Dalhoff Bruhn JensenAnders Dalhoff Bruhn Jensen 

Title of PhD project

Terrestrial and coloured dissolved organic matter in Arctic waters: Towards in situ sensor based monitoring of Arctic-Atlantic organic carbon exchange at major Arctic gateway

Supervisors

Colin Stedmon and Christopher Lee Osburn

Background of project

Terrestrial dissolved organic matter from river upland and permafrost erosion can be found throughout the Arctic Ocean. Terrestrial dissolved organic matter can affect ocean chemistry, carbon cycling and in the end marine ecosystems. With global warming the release of terrestrial dissolved organic matter into the Arctic Ocean will increase simultaneously. It is therefore becoming more and more urgent to understand the fate of this dissolved organic matter.  

About the project

This project will study how to use lignin phenols as a terrestrial plant biomarker and thereby be able to understand and map the fate of terrestrial dissolved organic matter in the ocean, particularly the Arctic Ocean. To separate the lignin phenols I will apply reversed phase High Pressure Liquid Chromatography. Since the lignin phenols hold different spectral fingerprints, absorbance and fluorescence spectroscopy can be used for detection. Finally, chemometric decomposition methods such as Parallel Factor Analysis will be used to analyze the data. Once the method is developed, seawater across the Fram Strait will be sampled and analyzed for lignin. From these results, we can hopefully deepen our understanding of how the terrestrial dissolved organic matter is exported into the Atlantic Ocean. For the future, I hope to establish a lignin method that can be applied in situ.

Perspective

From understanding the fate of terrestrial dissolved organic matter in the Arctic Ocean and its export through the Fram Strait, we can hopefully begin to predict how future climate change will affect ocean chemistry and carbon cycling. Will the terrestrial dissolved organic matter be taking up by living organisms, be stored in the deep ocean for millennia or be respired into the atmosphere as carbon dioxide?

Kristian MaarKristian Maar

Title of PhD project

Ecology and fluid dynamics of aquatic suspension feeders

Supervisors

Thomas Kiørboe, Anders Peter Andersen and Uri Shavit

Background of project

Aquatic suspension feeders span from unicellular organisms to the blue whale and are characterized by variou mechanisms of filtration, which enable them to separate and retain particles of food from the water. The ocean is nutritionally dilute and marine suspension feeders must therefore be highly efficient in order to successfully capture enough food to grow and reproduce. The biomechanical adaptations suspension feeders have evolved to solve this problem are as diverse as the community of suspension feeders themselves and is fundamentally constrained by physical properties e.g. the size of the filter feeder and the type and size of particle they capture. The flow generated by active suspension feeders also affects their local environment and is theorized to facilitate the aggregation and sinking of marine snow.

About the project

The first part of my project focuses on the fluid dynamics of suspension feeding in sessile barnacles. To determine the flow field generated by barnacle suspension feeding I will use high-speed video and Particle Image Velocimetry (PIV). The second part of my project focuses on the impact of colonization of microscopic suspension feeders on the formation and sinking of marine snow. I will quantify this phenomenon by conducting experiments comparing aggregation and settling of marine snow with and without active suspension feeders.

Perspective

Elucidating the mechanics of suspension feeding provides novel insight into predatorprey relationships and specific solutions to complex fluid dynamic problems. Biomimetic efforts inspired by marine suspension feeders have already yielded technological advancements in industrial filter technology and is currently being discussed as potential solutions to microplastic in the ocean. Understanding the processes of marine snow formation will also increase the predictive power of carbon pump models and contribute to the detailed understanding of sequestration of carbon in the deep ocean.

Anton AlmgrenAnton Vergod Almgren

Title of PhD project

A trait based approach to the biological pump 

Supervisors

Andre Visser and Ken Haste Andersen

Background of project

The biological pump is the vertical transport of carbon, bound in organic matter through photosynthesis, from the surface ocean to the deep ocean. Together with the solubility pump (the transfer of atmospheric CO2 into the ocean), the biological pump play an important role in the global climate system, as it removes CO2 from the atmosphere and transport it to the deep ocean where it stays for hundreds or thousands of years. A major part of the vertical transport of carbon happens through the sinking of marine snow (particulate organic matter), and the fraction of the marine snow that is not re-mineralized before it reaches the sequestration depth is buried in the deep ocean.

About the project

Throughout my PhD, I will develop a model that is able to describe the degradation and remineralization, as well as the aggregation, of marine snow particles. This will be put into a global context, using trait-based models of marine ecosystems, to provide input in terms of particulate organic matter. The aim is to use this modelling approach for providing a realistic estimate of the efficiency of the biological pump on both global and regional scales.

Perspective

The efficiency of the biological pump in terms of carbon sequestration is difficult to estimate, and current estimates based on e.g. sediment traps, provide a wide range of carbon sequestration rates. Further, the current estimates say little about the mechanistic processes involved in the biological pump. By modelling the processes of particle degradation and sinking, we will both get a more realistic estimate of the efficiency of the biological pump, as well as a framework that may be used for future climate scenarios.

Gunaalan KuddithambyGunaalan Kuddithamby

Title of PhD project

Risk assessment of microplastic pollution in marine ecosystems 

Supervisors

Torkel Gissel Nielsen, Rodrigo Almeda and Jes Vollertsen

Background of project

Microplastics (1 μm - 5 mm size) are ubiquitous in the marine environment, making them a major environmental concern. Despite of the increased scientific interest in microplastics pollution, many questions on their fate and toxicity remain and their ecological impact is still under debate. Therefore, comprehensive research studies need to fill the gaps in our scientific knowledge on distribution of microplastics  in the marine environment. It is essential to understand the distribution of microplastics since they play a crucial role as vectors of different potential contaminants to enter the marine food web. Microplastics can be accidently ingested by zooplankton and egested as part of their fecal pellets. 

About the project

My PhD project aims on the ecological risk of microplastics pollution on the marine environment and understand the links between oceanographic processes, environmental distribution of microplastics, and their impacts. This project comprises three major interconnected work packages to assess the “risks” of microplastics pollution. Investigate the “abundance” of microplastics and their characterization; the “fate” of microplastics and assess the “impacts” of weathered plastic, plastic leachates and additives by examining ecological and physiological changes in marine copepods at different spheres along a gradient of marine environment from water column to seafloor.

Perspective

The project will assess the abundance, composition and sizes of microplastics in Danish fjords and coastal waters (Kattegat strait) in relation to hydrography. The outcomes of work packages probably lead to develop a model for risk assessment for microplastics in Danish waters.  

Delove AsieduDelove Abraham Asiedu 

Title of PhD project

Effect of anthropogenic stressors on the planktonic food webs in the Arctic seas 

Supervisors

Marja Koski and Sigrún Jónasdóttir

Background of project

Recent reports suggest that marine ecosystems worldwide are increasingly exposed to overexploitation and pollution from heavy metals, petroleum, plastics and persistent organic pollutants. In addition, the UN's Intergovernmental Panel on Climate Change has reported that the temperature of the ocean is likely to increase by ~1-6˚C by the end of this century, with fastest-warming occurring in the Arctic areas. We, however, lack understanding of how key organisms (both native and non-native), from primary producers to higher trophic levels, are impacted by climatic and non-climatic changes and how the individual responses of species influence trophic interactions and community structure. This is particularly urgent for Arctic marine ecosystems because they are highly vulnerable to warming due to ice acting as the vital ecosystem element. 

About the project

This PhD project will combine field and experimental studies to quantify the zooplankton community composition and production in the Arctic and their sensitivity to the combined effects of climate change (e.g., increasing sea surface temperature, decreasing salinity and turbidity) and pollution. The zooplankton will consist of both native and non-native species, thus including aspects of the potentially different environmental tolerance of these groups. Also, special focus will be on small (≤ 1 mm) under-studied copepod species that dominate the abundance of Arctic zooplankton at many locations and seasons.

Perspective

The project will provide novel knowledge on the mechanistic impacts of stressor combinations on native, non-native and small under-studied zooplankton species at different ontogenic stages of development. This will contribute to the general understanding of how individual tolerances accumulate to community-level stressor responses, which is essential knowledge to be able to predict the effects of environmental change in the vulnerable Arctic ecosystem.

Federica MianoFederica Miano

Title of PhD project

Fluid dynamics, ecology, and evolution of flagellate foraging

Supervisors

Thomas Kiørboe, Seyed Saeed Asadzadeh and Anders Andersen

Background of the PhD project

Flagellates represent highly relevant species among eukaryotes both from evolutionary and ecological perspectives. They are found among all the branches of the eukaryotic tree of life, with highly diverse flagellar arrangements and resource acquisition modes. Also, they play a crucial role in the biogeochemical cycles of the global ocean. Their key position in the microbial food web is governed by their feeding on bacteria and other picoplankton, by their photosynthetic activity, and by themselves being grazed by predators. Their degree of success in eating without being eaten is the key to understand the functioning of predatory flagellates. Their feeding activity dangerously ex-poses them to rheotactic predators that are sensitive to flow disturbances. Therefore, flagellates have evolutionarily developed singular behaviors in terms of feeding modes and predator avoidance, to find an equilibrium between resource acquisition and predation risk. These trade-offs are still largely unexplored among flagellates.

About the project

During my PhD, I will study representative flagellate species belonging to different branches of the eukaryotic tree of life to look at their behaviors both as predators and prey. Firstly, I will investigate escape responses from predators feeding currents to understand their propulsion mechanism that leads to very fast and long jumps, and characterize the fluid signals that elicit them. Secondly, I will quantitatively investigate the kinematics and 3-dimensional beat patterns of diverse flagellar arrangements and use them as input to CFD models to quantify foraging-predation risk trade-offs.

Perspective

My PhD project aims at describing these trade-offs quantitatively and at understanding how they are differently optimized among flagellate species. This is crucial because the diversity of eukaryotic microbial communities is determined by such trade-offs in concert with environmental constraints and microbial diversity in turn governs the functionality and “services” of microbial communities and so their role in ocean biogeochemistry. 

Caroline GjelstrupCaroline Gjelstrup 

Title of PhD project

Changing oceanographic conditions of East Greenland and its link to regional fisheries

Supervisors

Colin Stedmon, André Visser and Jesper Boje 

Background of PhD project

Knowledge of oceanographic conditions and their variability is essential for assessment of environmental impacts on biological communities, ecosystem services and regional climate variability. East Greenland is a region of both climatic and ecological importance, providing a connection between the Arctic and Atlantic oceans as well as eco-system services such as carbon sequestration and fisheries production. The region is influenced by cold fresh waters from the Arctic and warm saline waters from the Atlantic divided by a continuous front extending along the shelf-break. Oceanographic fronts are often associated with elevated plankton production due to entrainment of nutrients enhancing phytoplankton growth and zooplankton grazing, which supports pelagic and demersal fish. Ongoing Arctic climate change, including diminishing sea-ice cover, increasing discharge from the Greenland ice sheet and anomalous warm water pulses of subtropical origin propagating through the region, alter the physical environment. 

About the project

This PhD project aims to improve our understanding of variability in oceanographic conditions in East Greenland, and how this relates to ecosystem change and fisheries productivity. A combination of in-situ and remotely sensed observational data will be used to characterize oceanographic conditions and resolve underlying mechanisms responsible for variability herein. Eventually, a trait-based model will be applied to understand how changes in environmental conditions influence ecosystem function.

Perspective

By gaining insights as to how the spatiotemporal distribution of water masses in the East Greenland region are changing, and what that change implies for nutrient availability and plankton dynamics we can begin to foresee how East Greenland will respond to future change.

Previous PhD student (since 2000)


Josephine Grønning

Defence in diatoms: mechanisms and trade-offs

Rocío Rodríguez Torres

Ingestion and effects of microplastics on marine planktonic food webs

Mads Rode

Physics of microbial feeding. Studies of feeding flows near surfaces, ciliate filtration, and non-intrusive tethering of microswimmers
Go to DTU Orbit to download the thesis

Frederik Ryderheim

Opening the black box on predator-induced phytoplankton defenses: mechanisms, traits, and trade-offs
Go to DTU Orbit to download the thesis

Jérôme Pinti
Vertical Migration: Structure and function of pelagic ecosystems
Go to DTU Orbit to download the thesis