Data_Sheet_1_Transcription of biochemical defenses by the harmful brown tide pelagophyte, Aureococcus anophagefferens, in response to the protozoan grazer, Oxyrrhis marina.docx

Aureococcus anophagefferens is a small marine pelagophyte that forms recurrent harmful brown tides blooms with adverse ecological and economic impacts. During blooms, A. anophagefferens 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 A. anophagefferens when exposed to the protozooplankton, Oxyrrhis marina, and assessed whether this response involved chemical cues. Transcriptomes were generated from A. anophagefferens populations grown at high (1×106 cells mL−1) and low (5×105 cells mL−1) cell densities incubated directly with O. marina or receiving only filtrate from co-cultures of A. anophagefferens and O. marina 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 A. anophagefferens 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 (p < 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 O. marina 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 A. anophagefferens can induce biochemical pathways that reduce grazing mortality and support blooms.

棕潮藻（Aureococcus anophagefferens）是一种小型海洋淡色藻，可周期性引发有害褐潮水华，对海洋生态系统与区域经济均造成显著负面影响。在水华暴发阶段，棕潮藻的浮游动物牧食致死率显著低于其他浮游植物，这一现象可能与其合成抗牧食化合物相关——其中包括胞外多糖。本研究解析了棕潮藻在暴露于浮游原生动物海洋锐滴虫（Oxyrrhis marina）时的转录组响应，并评估了该响应是否由化学信号介导。实验以两种细胞密度（高密：1×10^6 细胞·mL^-1、低密：5×10^5 细胞·mL^-1）培养的棕潮藻种群为材料，分别设置直接与海洋锐滴虫共培养、仅添加棕潮藻与海洋锐滴虫共培养滤液的两组处理，以明确化学信号的调控作用，并由此获取对应的转录组数据。分析结果显示，低细胞密度组中因牧食诱导的差异表达基因数量显著高于高细胞密度组；相较于滤液处理组，直接牧食处理组的差异表达基因数量同样更多。京都基因与基因组百科全书（KEGG）通路分析表明，直接暴露于牧食者可显著上调次级代谢产物合成相关基因的转录水平（p < 0.001）。转录组数据提供了充分的广谱证据，证实浮游动物牧食可诱导聚酮类化合物与固醇的生物合成通路——这类物质可对海洋生物产生毒性损伤。此外，暴露于海洋锐滴虫还可改变碳水化合物代谢相关转录本的丰度，这一变化有助于胞外多糖基质的形成，其中涵盖糖蛋白合成与碳水化合物转运相关的功能基因。综上，本研究结果支持如下假说：棕潮藻可通过诱导特定生化通路降低牧食致死率，进而促进褐潮水华的形成与维持。