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

Abstract

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.

Keywords: Brown tide; gene express; harmful algal bloom; induced defense; zooplankton grazing.

Figure 1

Diagram of treatments used in the experiment.

Figure 2

Cellular net growth rate (per day) of A. anophagefferens in the (A) high cell density treatments and (B) low cell density treatments. Bars are means while error bars show standard deviation of the mean of the three replicates in each treatment. Letters indicate significantly different mean growth rates (Tukey HSD).

Figure 3

(A) Cellular net growth rates (per day) of Aureococcus anophagefferens (Direct, high) and Tisochrysis lutea (Grazer control, high). Significance by Welch’s t-test. (B) Cellular net growth rates (per day) of Aureococcus anophagefferens (Direct, low) and Tisochrysis lutea (Grazer control, low). Significance by Welch’s t-test.

Figure 4

Principal-component analysis (PCA) of normalized read counts for each replicate. Percentages on each axis represent the percent of variance explained.

Figure 5

Significantly upregulated KEGG pathways with a p value of less than 0.05 for each treatment. Gene count represents the number of significant genes in each pathway for each treatment.

Figure 6

Mean expression vs. log fold change (MA plot) compared to the control treatment for (A). Direct grazing, high cell density, (B) Indirect grazing, low cell density, (C) Direct grazing, low cell density, (D) Indirect grazing, low cell density. Significant genes (FDR = 0.05) shown in blue, non-significant in gray.

Figure 7

Significantly differentially expressed genes with a log-fold change threshold of ±0.6 in the direct, high treatment from the secondary metabolite KEGG pathway.

Figure 8

Venn diagram showing (A). Genes with significantly increased transcript abundance and (B). Genes with significantly decreased transcript abundance for each treatment and across treatments.