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

Abstract

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.

Domoic acid biosynthesis in the red alga Chondria armata suggests a complex evolutionary history for toxin production

“…Previous studies have demonstrated limited trace metal chelation by DA, affording a potential selective advantage for diatoms thriving in iron-limited coastal regimes ( 58 , 59 ). Other work has suggested a role for DA in grazer defense, particularly with respect to copepod grazing in the diatom system ( 60 ). However, no studies to date have provided a concrete ecological mechanism beyond reasonable doubt in both diatoms and red algae.…”

Toxicity of the Diatom Genus Pseudo-nitzschia (Bacillariophyceae): Insights from Toxicity Tests and Genetic Screening in the Northern Adriatic Sea

“…Horizontal gene transfer in protists is perhaps a neglected phenomenon and may be exacerbated by widespread viral infections [47]. For this hypothesis to be examined, a clear evolutionary role of DA should be established, which is at the moment not the case since proposed roles range from grazer deterrence [10,11,48] to metal chelation [4][5][6], while strains and species are all known to fare well in the absence of DA production. On the other hand, the competitive advantage of DA production may be clearer when local environmental conditions are considered.…”

“…At the moment, however, the design of suitable probes is hampered by the lack of sequences from different species and could perhaps only be developed for P. multiseries and now with our data for P. multistriata. We acknowledge that there are other genes involved in the biosynthesis of DA [9,11] that we did not examine in this work and that may prove to be more conserved and thus better targets for such efforts. However, the focus of this work was on the toxicity profiles of NE Adriatic strains, and we hope that this work will open new opportunities for the study of gene expression and the discovery of genes involved in the synthesis of DA.…”

“…Nonetheless, the understanding of toxin regulation and physiology has improved significantly recently, particularly with the discovery of the DA biosynthetic pathway, where four enzymes (DabA-D) coded by four genes (dabA-D) were discovered [9]. Furthermore, it was established that DA production is induced by copepod grazer cues [10,11] although an increase in DA concentration did not significantly affect grazing in these studies. Recently, however, new evidence has been presented for the deterrent function of DA against grazers [12].…”

The impact of urea on toxic diatoms – Potential effects of fertilizer silo breakdown on a Pseudo-nitzschia bloom

“…The toxin is a simple neurotoxic amino acid called domoic acid (DA). The chemical compounds, metabolic precursors, and the penultimate enzymatic steps for the biosynthesis of DA were recently described (Maeno et al, 2018;Brunson et al, 2018;Harðardóttir et al, 2019) but the final isomerization reaction, sources, and compartmentalization are yet to be elucidated.…”