Identification and expression analysis of blue light receptor aureochrome in the harmful alga Heterosigma akashiwo (raphidophyceae)

The raphidophyte Heterosigma akashiwois a bloom-forming phytoplankter and responsible for mass fish kills. Previous studies have indicated the importance of light on bloom formation of raphidophytes, but information on their light receptors is limited. Aureochrome (AUREO) is a blue light receptor conserved in photosynthetic stramenopile algae but has not been studied in H. akashiwo. In this study, four full-length cDNA sequences of aureo (named Haaureo1, Haaureo2, Haaureo3 and Haaureo4) were isolated from H. akashiwo. A basic region/leucine zipper (bZIP) domain and a light-oxygen-voltage (LOV) domain were found in each of the four genes. Quantitative reverse-transcription PCR assays revealed different expression patterns among the Haaureo genes in the light: dark (LD) cycle; only Haaureo1 exhibited a clear diel rhythm, with the highest and the lowest transcript abundances occurring at dawn and dusk, respectively. When the cultures were transferred from LD cycles to continuous dark (DD), the diel rhythm of Haaureo1 mRNA disappeared, indicating that Haaureo1 transcription is not under circadian clock control. At transitions from darkness to various wavelength light (red, white or blue spectrum), Haaureo1 transcript decreased, most markedly under blue light. These results suggest that HaAUREO1 is a blue light-sensitive negative gene regulator, which is probably driven by dark-to-light transitional cues and in turn regulates expression of other genes in the light dark cycle. This study provides a basis on which to elucidate the molecular network governing AUREO-mediated biological responses, and gain understanding on the role of light in H. akashiwo bloom formation.