Description: Background A major cause of phytoplankton mortality is predation by zooplankton. Strategies to avoid grazers have probably played a major role in the evolution of phytoplankton and impacted bloom dynamics and trophic energy transport. Certain species of the genus Pseudo-nitzschia produce the neurotoxin, domoic acid (DA), as a response to the presence of copepod grazers, suggesting that DA is a defense compound. The biosynthesis of DA comprises fusion of two precursors, a C10 isoprenoid geranyl pyrophosphate and l-glutamate. Geranyl pyrophosphate (GPP) may derive from the mevalonate isoprenoid (MEV) pathway in the cytosol or from the methyl-erythritol phosphate (MEP) pathway in the plastid. l-glutamate is suggested to derive from the citric acid cycle. Fragilariopsis, a phylogenetically related but nontoxic genus of diatoms, does not appear to possess a similar defense mechanism. We acquired information on genes involved in biosynthesis, precursor pathways and regulatory functions for DA production in the toxigenic Pseudo-nitzschia seriata, as well as genes involved in responses to grazers to resolve common responses for defense strategies in diatoms. Results Several genes are expressed in cells of Pseudo-nitzschia when these are exposed to predator cues. No genes are expressed in Fragilariopsis when treated similarly, indicating that the two taxa have evolved different strategies to avoid predation. Genes involved in signal transduction indicate that Pseudo-nitzschia cells receive signals from copepods that transduce cascading molecular precursors leading to the formation of DA. Five out of seven genes in the MEP pathway for synthesis of GPP are upregulated, but none in the conventional MEV pathway. Five genes with known or suggested functions in later steps of DA formation are upregulated. We conclude that no gene regulation supports that l-glutamate derives from the citric acid cycle, and we suggest the proline metabolism to be a downstream precursor. Conclusions Pseudo-nitzschia cells, but not Fragilariopsis, receive and respond to copepod cues. The cellular route for the C10 isoprenoid product for biosynthesis of DA arises from the MEP metabolic pathway and we suggest proline metabolism to be a downstream precursor for l-glutamate. We suggest 13 genes with unknown function to be involved in diatom responses to grazers.

Description: In costal ecosystems, copepods coexist with toxin‐producing phytoplankton. The presence of copepods can amplify the phytoplankton toxin production and thereby increase the overall toxicity of a bloom. Copepods are not always affected by the toxins and can vector the toxins to higher trophic levels. To investigate the interactions between toxin producers and their grazers, we determined the kinetics of grazer‐induced increases in toxin production and the subsequent toxin reduction in a domoic acid (DA)‐producing diatom, Pseudo‐nitzschia seriata. The cellular DA level of the diatom was within the range of in situ measurements. Ten days after removal of the copepods, 28% ± 8% of the cellular DA still remained in the cells. Simultaneously, we monitored the toxicokinetics of DA in two grazers; Calanus finmarchicus and Calanus glacialis. After 144 h of grazing on the toxic diet, the copepods accumulated and retained high concentrations of DA. Nine hours after exposure to the toxic diet was terminated, the copepods had depurated 70% ± 10% of the DA. The depuration lasted 4 ± 2 d and was independent of Calanus species and treatment. We explored the possible physiological responses in copepods after feeding on a purely toxic diet from gene expression profiles of C. finmarchicus. Expression of genes regulating several major metabolic and cellular processes was reduced in copepods feeding on DA‐containing diatoms, and we hypothesize that this is because of exposure to DA.