## 3 Discussion
### Signal Transduction of Blue Light Receptors  
PHOT is activated by blue light to participate in physiological processes such as chloroplast movement and plant phototropism [71]. PHOT contains both the serine/threonine kinase binding domains at the C-terminal and two light, oxygen, and voltage (LOV) domains at the N-terminal [72]. Exposure of LOV to blue light causes cysteine residues in the protein to covalently bind to the isopyrazine ring to generate conduction signal [73]. In _Arabidopsis_, PHOT regulates the transcripts in response to high-intensity blue light, though PHOT is not largely involved in transcriptional regulation [74; 75]. However, PHOT causes significant changes in expression of genes involved in chlorophyll and carotenoid biosynthesis in _C. reinhardtii_, whereas the low-intensity blue light leads to a significant increase in the expression of _pds_[71]. Furthermore, the expression of _pds_ is inhibited as the expression of _phot_ is inhibited [71]. In our study, the expression of _phot_ was decreased, but not significantly, under blue light, compared with white light. In the chlorophyll synthesis pathway (Figure 8), most genes were significantly down-regulated on the second day but not on day 7 of the treatment with blue light irradiation. In the astaxanthin synthesis pathway (Figure 6), most genes were significantly down-regulated on day 2 of the treatment with blue light irradiation, while no significant changes were observed on day 7. The C-terminal domain of CRY is responsible for providing the interaction sites for DNA or downstream signal proteins, while the function of the N-terminal domain is to combine the second chromophore as an optical collection antenna [76]. Based on knockdown mutants, studies have shown that CRY regulated the transcriptional levels of various genes under blue light, caused by its oxidized fad chromophore. Under the blue light irradiation, the C-terminal domain interacts directly with downstream signal proteins, e.g., ubiquitin ligase. The photolyase homologous region (PHR) of cryptochrome can form homodimers, which are also essential for signal transduction [77]. The cryptochrome (CPH1) protein of _C. reinhardtii_ undergoes photoinduced degradation under white, red, and blue lights [48]. Similar results were also observed in our study, showing that the blue light irradiation of high intensity significantly reduced the expression of _cry_ in _H. pluvialis_. Furthermore, CPH1 also functions as a transcription regulator to regulate gene expression and induce the expression of both chlorophylls a and b binding protein and _hemL_[48]. Our results showed that the expression of _hemL_ was significantly down-regulated on the second day of the treatment with blue light irradiation, while no significant down-regulation was observed on day 7, and _cry_ was not significantly down-regulated under high-intensity blue light. Plants respond to light through photoreceptors. Our study showed that the expression of blue light photoreceptors (i.e., PHOT and CRY) were not significantly down-regulated under blue and white light irradiations of high intensity, suggesting that in _H. pluvialis_, the metabolic pathways are not affected by the content of photoreceptors but by the signal transduction of photoreceptors. For example, ROS at high concentrations causes damage in cells but functions as signal molecules at low concentrations. Activated CRY can produce trace ROS as signal molecules in response to light. Furthermore, the high content of CRY causes the increased production of ROS in response to light in _Arabidopsis_[78]. Our results showed that, compared with normal light intensity, high-intensity blue light significantly inhibited the expression of CRYs, while the expression of PHOTs was decreased on day 7 of the treatment of high-intensity irradiation.