Marine battlefields:toxic diatoms and their copepod grazers

• Main Author: Harðardóttir, Sara
• Format: Book
• Language: English
• Published: Natural History Museum of Denmark, Faculty of Science, University of Copenhagen 2017
• Subjects: Calanus finmarchicus; Copepods
• Online Access: https://curis.ku.dk/portal/da/publications/marine-battlefields(f4a39e7c-58e8-40d5-8732-d89e7e65709c).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122539977605763

Phytoplankton species are photosynthetic organisms found in most aquatic habitats. In the ocean,phytoplankton are tremendously important because they produce the energy that forms the baseof the marine food web. Zooplankton feed on phytoplankton and mediate the energy to highertrophic levels as they are an important food source for various marine animals. For both phytoandzooplankton predation is a major cause of mortality, and strategies for protection oravoidance are important for survival. Diatoms of the genera Nitzschia and Pseudo-nitzschia areknown to produce a neuro-toxin, domoic acid (DA). Despite the toxin production, Pseudonitzschiacells are never the less consumed by e.g. copepods that seem resistant to the toxin andcan further vector the toxin into the food web.The aim of the studies reported in this thesis is to examine the interaction between toxicdiatoms and their copepod grazers. By exposing toxic and potentially toxic diatoms to copepods,Pseudo-nitzschia species were found to increase their toxin production when specific predatorsare present, indicating that DA-production is an induced defense mechanism. Grazer-induction isone of the most powerful known triggers for DA production in laboratory experiments. After thetrigger is removed, it can take several weeks for the cellular toxin content in the diatoms to revertto initial levels. The triggering compounds are chemical cues that the copepods excrete to theirsurroundings, and they were confirmed to be polar lipids called copepodamides. Not all diatomspecies reacted by producing toxin when exposed to grazing copepods, suggesting that the trait isspecific to certain diatoms. When exposed to a carnivore copepod that does not pose a threat tothe diatom cells, because it does not prey upon diatoms, the toxic Pseudo-nitzschia seriata doesnot elicit DA, in contrary to what it does when exposed to a herbivore copepod, indicating thatthe trait is grazer specific and needs a reliable threat.In all the experiments conducted for this thesis, there was a total lack of evidence ofgrazing reduction when the copepods were exposed to the toxin. But it became evident whenexamining the gene expression in the copepod Calanus finmarchicus after ingestion of DA, thatthe copepod is affected by the intake of DA. Multiple genes are repressed after toxin exposure.Behavioral changes will increase the mortality of copepods, and by testing escape responses intwo species of Calanus it became evident that intoxicated copepods suffer reduced escape jumps,suggesting they will become an easier prey for their predatorsAnother aim for this work is to explore the biosynthetic pathway of DA, using predatorcues to trigger DA production in P. seriata in order to reveal regulation of genes involved in DAsynthesis. Over a 1000 genes were differently expressed and most importantly it is proposed thatthe C10 isoprenoid product for biosynthesis of DA arises from the methylerythriol phosphatemetabolic pathway.In conclusion: Interaction between toxic diatoms and their grazers affects both opponents, andthe interaction results in toxin progressively being mediated in the food web.