Directly observing predator-prey interactions is necessary to establish a mechanistic understanding of phytoplankton defenses. Photo and graphic work: Fredrik Ryderheim.

PhD defence about predator-induced phytoplankton defenses

Monday 27 Sep 21



Thomas Kiørboe
DTU Aqua
+45 35 88 34 01

On 7 October 2021, Fredrik Ryderheim will defend his PhD thesis. The defence is held at DTU in Lyngby and can be watched online.

Phytoplankton are small, unicellular photosynthetic organisms that form the base of the food web in aquatic environments and are responsible for roughly half of the planet’s primary production. To protect themselves from predation, they have—like terrestrial plants—evolved a wide variety of be physiological, morphological and behavioral defensive characteristics.

The general consensus is that the defenses must have associated trade-offs. If there were no costs, all phytoplankton would evolve defenses, but this is not the case. In addition, many defense mechanisms are only induced in the presence of predators, which further suggests that the costs are substantial. Nevertheless, these costs are largely still unknown, which may complicate the process of accurately describing ecosystems and predicting changes in the oceans. Also, it is poorly understood how the defenses actually work and affect the interactions between predator and prey.

In his PhD thesis, Fredrik Ryderheim, DTU Aqua has explored the benefits and trade-offs of three common phytoplankton defenses—toxin production, colony formation, and structural changes in the cell wall—by using a combination of tested and novel experimental approaches in the laboratory and directly observing predator behavior in the presence of phytoplankton with different types and levels of defense.

The results show that the mechanism by which adult copepod predators select prey is similar between the type of defense. Phytoplankton are caught and individually handled by the copepods before they choose to ingest or reject the prey. The rate at which phytoplankton are caught or rejected is closely tied to the level of the defense. Fredrik Ryderheim also demonstrates that both the trade-offs and benefits of the defenses vary with predator community composition and predator abundance. Additionally, he finds that some trade-offs are direct, i.e., in terms of reduced growth rate, while others are likely ecological that will not manifest in laboratory conditions.

About the PhD defence

Frederik Ryderheim will defend his PhD thesis "Opening the black box on predator-induced phytoplankton defenses: mechanisms, traits, and trade-offs” on Thursday 7 October 2021 at 1:00 pm (CEST) via Zoom (find link below) and at DTU, Anker Engelunds Vej 101, 2800 Lyngby, building 101, room S01.


  • Principal Supervisor: Professor Thomas Kiørboe, DTU Aqua
  • Co-supervisor: Professor Ken Haste Andersen, DTU Aqua
  • Co-supervisor: Professor Per Juel Hansen, University of Copenhagen
  • Co-supervisor: Senior Lecturer Erik Selander, University of Gothenburg, Sweden


  • Associate Professor Marja Koski, DTU Aqua
  • Professor Kam Tang, Department of Biosciences, Swansee University, UK
  • Professor Karin Rengefors, Lund University, Sweden

Chairperson at defence

  • Professor André Visser, DTU Aqua
Learn more

A popular science summary of the thesis can be downloaded here

A copy of the thesis is available by e-mail on request. Please contact Karin Stubgaard,

Link to Zoom

It is possible to attend Fredrik Ryderheim’s defence on Zoom using this link:

Please, enter the meeting 10 minutes prior to the defence proceedings are scheduled to start. All participants are muted per default, but we ask you to double check that your microphone is turned off. There will be instructions regarding the proceedings in the beginning of the defence.

It's also possible to attend the defence at DTU, Anker Engelunds Vej 101, 2800 Kgs. Lyngby, building 101, room S01.