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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.    Electronic supplementary material  The online version of this article (10.1186/s12867-019-0124-0) contains supplementary material, which is available to authorized users.

bmc molecular biology

Transcriptomic responses to grazing reveal the metabolic pathway leading to the biosynthesis of domoic acid and highlight different defense strategies in diatoms