Abstract
Alexandrium minutum is a typical marine toxic dinoflagellate responsible for producing paralytic shellfish poisoning (PSP) toxins. Until now, we know little about the genomic information of A. minutum, so a transcriptome study was conducted to clarify the physiological adaptations related to nutritional deficiency. Here, we performed RNA-Seq analysis to assess the gene expression patterns of A. minutum under N and P deficient conditions for 0 (control), 6, and 72 h. Main differences between the control and experimental groups were observed in hydrolase activity and fatty acid, lipid, protein, and P metabolism. Activities of photosystem I (PSI) and PSII were significantly down-regulated, and the endocytosis pathway (clathrin-dependent endocytosis) was significantly enriched under N and P stress compared with the control, indicating that A. minutum shifts its trophy pattern under N and P stress. We also identified several unigenes related to the process of sexual reproduction, including sex determination, sperm-egg recognition, sex differentiation, mating, and fertilization. Approximately 50% of the successfully annotated unigenes were differentially expressed between the short-term stimulated sample (6 h) and control (R). However, the expression level of most unigenes returned to normal levels after 72 h, indicating that N and P stress plays a limited role in the induction of sexual reproduction. Furthermore, the quantitative real-time PCR (qRT-PCR) results of the five representative sex-related unigenes were consistent with sequencing data, which confirmed the authenticity of transcriptomic analysis. Also, qRT-PCR analysis showed that the long and short form transcripts of the saxitoxin biosynthesis gene (sxtA) were down-regulated under the nutrient deficient condition compared with the control, indicating that N and P stress regulates sxtA expression. Overall, transcriptome analysis of A. minutum revealed that N and P deficiency induced responses associated with stress response, photosynthetic efficiency, toxin biosynthesis, and sexual reproduction. Our data indicate that algae change their trophic modes (to facultative mixotrophy) and related physiological reactions under stress conditions; this possibly represents an ecological adaption strategy in the algal life cycle.
Highlights
Harmful algal bloom (HAB) is an ecological phenomenon caused by the explosive growth and accumulation of phytoplanktons, protozoa, or bacteria in seawater (Hallegraeff, 1993; Granéli and Turner, 2006b; Zhang et al, 2018a)
The aim of this study was to: (1) investigate the expression patterns of genes involved in photosynthesis and endocytosis pathways, (2) identify and analyze differences in expression levels of genes related to sexual reproduction between control and nutrient deficient conditions, and (3) understand the other molecular mechanisms underlying the response of A. minutum to nutrient deficiency, and collect evidence to prove that A. minutum is not a purely photosynthetic organism
We describe the transcriptomic profiles of the toxinproducing algae of A. minutum under normal and nutrient (N and P) deficient conditions
Summary
Harmful algal bloom (HAB) is an ecological phenomenon caused by the explosive growth and accumulation of phytoplanktons, protozoa, or bacteria in seawater (Hallegraeff, 1993; Granéli and Turner, 2006b; Zhang et al, 2018a). Dinoflagellates, a representative HAB species, are haploid eukaryotic microalgae with diverse modes of growth and can be found in most aquatic habitats worldwide. Some dinoflagellates, such as Alexandrium minutum, produce toxins [for example, paralytic shellfish poisoning (PSP) toxins] that threaten the health of human beings. The deposition of resting cysts in sediments potentially contributes to the formation and maintenance of A. minutum blooms. Based on their characteristics, resting cysts are considered to form the seed bed of HAB (Dale, 1983). The concentration of NH4 has been shown to promote cyst formation in Scrippsiella trochoidea (Wang et al, 2014)
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