Anna Junker Olesen defends her thesis at the National History Museum of Denmark

PhD defence: Anna Junker Olesen

Toxic Marine Diatoms in Polar Regions
Inducible Defence Mechanisms and their Effect

Supervisors:

Associate Professor Nina Lundholm, NHMD
Professor Thomas Kiørboe, DTU
Professor Per Juel Hansen, BIO

Assessment Committee:

Professor Karin Rengefors, Lund University – Sweden
Professor Hans G. Dam, University of Connecticut – USA
Associate Professor Peter Rask Møller (chair), NHMD
.

Abstract
In this thesis, two key research questions are investigated: Why and how do toxic marine diatoms defend themselves, and what does it cost them? The second question is which potentially toxic marine diatom species are present in polar regions. The organisms in focus are species of the genus Pseudo-nitzschia, comprising widely distributed and frequently recorded marine pennate diatoms. For over three decades, investigations have expanded our understanding of the genus, its toxic potential, and harmful algal bloom capabilities.
The questions addressed in this thesis arise from significant knowledge gaps in combination with a rapidly changing environment, altering water chemistry and conditions for aquatic organisms. Diatoms, especially diatoms like Pseudo-nitzschia, are crucial for photosynthesis and energy transfer in the polar regions of the ocean. Still, toxin development simultaneously poses a risk for animals and humans higher up in the food chain. Polar ecosystems are at greater risk as they rely almost exclusively on marine primary production. The potential for ecosystem risks is heightened with shorter and more intense bloom periods and lower species diversity.
Knowing the variation among conspecific strains can help us improve our interpretation of results and design experiments on diatoms, specifically Pseudo-nitzschia. This phenotypic plasticity was investigated on 40 strains of P. seriata and exceeded our expectations. What is not surprising, but alarming is the increasing toxic potential with increasing water temperatures. There may be multiple reasons why Pseudo-nitzschia produces the neurotoxin domoic acid. However, as domoic acid production is induced by their primary predators, the copepods, or cues from them, it is reasonable to imagine it has a defensive or deterring nature.
The absence of direct evolutionary beneficial evidence to produce the potentially expensive toxin allowed us to ask two questions about domoic acid production: why and at what cost? We addressed this by direct observations of copepods-provided strains of P. seriata induced by predator cues to contain high amounts of domoic acid. We found an enhanced survival probability when the cells contained domoic acid. Theoretically, all ABSTRACT iv inducible defences come with a cost. We found decreasing growth rates related to increases in domoic acid content, indicating a cost of the toxin production. This was confirmed in five additional strains of P. seriata, this time containing unprecedentedly high amounts of DA in the presence of predator cues, even when light availability was limited. In summary, this thesis investigates defence mechanisms, distribution, and ecological implications, focusing on the widely distributed genus Pseudo-nitzschia.
Through direct observations, we investigated the nature of their production of the neurotoxin domoic acid, shedding light on its defensive role and associated costs. Further, these organisms had unexpected intraspecific variation and increasing toxic potential correlated with increasing water temperatures. Collectively, we are now one step further in understanding the nature of predator-prey interactions and the occurrence and toxic potential of Pseudo-nitzschia in polar regions.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@snm.ku.dk