Insights into the transcriptome of the marine copepod Calanus helgolandicus feeding on the oxylipin-producing diatom Skeletonema marinoi.

Diatoms dominate productive regions in the oceans and have traditionally been regarded as sustaining the marine food chain to top consumers and fisheries. However, many of these unicellular algae produce cytotoxic oxylipins that impair reproductive and developmental processes in their main grazers, crustacean copepods. The molecular mode of action of diatoms and diatom oxylipins on copepods is still unclear. In the present study we generated two Expressed Sequence Tags (ESTs) libraries of the copepod Calanus helgolandicus feeding on the oxylipin-producing diatom Skeletonema marinoi and the cryptophyte Rhodomonas baltica as a control, using suppression subtractive hybridization (SSH). Our aim was to investigate differences in the transcriptome between females fed toxic and non-toxic food and identify differentially expressed genes and biological processes targeted by this diatom. We produced 947 high quality ESTs from both libraries, 475 of which were functionally annotated and deposited in GenBank. Clustering and assembling of ESTs resulted in 376 unique transcripts, 200 of which were functionally annotated. Functional enirchment analysis between the two SSH libraries showed that ESTs belonging to biological processes such as response to stimuli, signal transduction, and protein folding were significantly over-expressed in the S. marinoi-fed C. helgolandicus compared to R. baltica-fed C. helgolandicus library. These findings were confirmed by RT-qPCR analysis. In summary, 2 days of feeding on S. marinoi activated a generalized Cellular Stress Response (CSR) in C. helgolandicus, by over-expressing genes of molecular chaperones and signal transduction pathways that protect the copepod from the immediate effects of the diatom diet. Our results provide insights into the response of copepods to a harmful diatom diet at the transcriptome level, supporting the hypothesis that diatom oxylipins elicit a stress response in the receiving organism. They also increase the genomic resources for this copepod species, whose importance could become ever more relevant for pelagic ecosystem functioning in European waters due to global warming.