Transcription of biochemical defenses by the harmful brown tide pelagophyte, , in response to the protozoan grazer, .

is a small marine pelagophyte that forms recurrent harmful brown tides blooms with adverse ecological and economic impacts. During blooms, experiences lower zooplankton grazing mortality than other phytoplankton potentially due to the synthesis of anti-predator compounds including extracellular polysaccharides. This study characterized the transcriptomic response of when exposed to the protozooplankton, , and assessed whether this response involved chemical cues. Transcriptomes were generated from populations grown at high (1×10 cells mL) and low (5×10 cells mL) cell densities incubated directly with or receiving only filtrate from co-cultures of and to evaluate the role of chemical cues. There were a greater number of genes differentially expressed in response to grazing in the lower concentration of compared to the high concentration treatment and in response to direct grazing compared to filtrate. KEGG pathway analysis revealed that direct grazer exposure led to a significant increase in transcripts of genes encoding secondary metabolite production ( < 0.001). There was broad transcriptional evidence indicating the induction of biosynthetic pathways for polyketides and sterols in response to zooplankton grazers, compounds associated with damage to marine organisms. In addition, exposure to elicited changes in the abundance of transcripts associated with carbohydrate metabolism that could support the formation of an extracellular polysaccharide matrix including genes related to glycoprotein synthesis and carbohydrate transport. Collectively, these findings support the hypothesis that can induce biochemical pathways that reduce grazing mortality and support blooms.