Shells of different species of diatoms.  collected, cleaned, and arranged under the microscope by British biologist Klaus Kemp (section of original picture).

PhD defence about defence mechanisms in diatoms

Monday 09 May 22


Thomas Kiørboe
DTU Aqua

On 19 May 2022, Josephine Grønning will defend her PhD thesis. It is possible to attend the defence at DTU in Lyngby or online.

Marine phytoplankton forms the base of the marine food web and account for roughly half of the planet’s primary production. About 40 percent of this is accounted for by diatoms – a group of phytoplankton characterized by e.g., the cell being encapsulated in a glass shell.

Defence plays an important role in shaping the planktonic food web and the interaction between predation and defence is key in driving and maintaining species diversity. A wide array of defence mechanisms has evolved in phytoplankton. For diatoms, these includes, e.g., tough cell walls, colony-formation, toxin production and production of resting stages. 

Many of the defence mechanisms are inducible, i.e., they are harnessed or intensified when predators are present. Inducible defences are believed to evolve when the defences are costly, however, the trade-offs to defences have been notoriously difficult to demonstrate and quantify.

In her thesis, Josephine Grønning, DTU Aqua, explores the inducibility and trade-offs of defence mechanisms in diatoms. Specifically, she demonstrates how different species of diatoms are capable of increasing the thickness of their shell when chemical cues from predators (copepods) are present. This reduces the mortality rate, but the trade-off is lowered growth rate and cell size. 

During the work, the group made the new discovery that some diatom species aggregate heavily by increasing the stickiness of the cell, when predator cues are present. The aggregates sink to deeper waters where predation pressure is lower, and Josephine Grønning argues that aggregation may be a defence mechanism and a way of escaping predation.

About the PhD defence

Josephine Grønning will defend her PhD thesis "Defence in diatoms: mechanisms and trade-offs" on Thursday 19 May 2022, 13:00 on Zoom and at DTU, 2800 Lyngby in building 101, meeting room S01.


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


  • Associate Professor Marja Koski, DTU Aqua (chair)
  • Professsor Victor Smetacek, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany
  • Professor Angela Wulff, Department of Biological & Environmental Sciences, University of Gothenburg, Sweden

Chairperson at defence

  • Professor Torkel Gissel Nielsen, DTU Aqua  
Learn more

Download a popular science summary of the thesis

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

Registration for online defence

To attend Josephine Grønning’s defence on Zoom, you must register at Karin Stubgaard, no later than on the day before the defence. You'll then receive an e-mail with a link to Zoom.

The physical defence takes place at DTU, Anker Engelunds Vej 101, 2800 Kgs. Lyngby, building 101, room S01. 

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