## 1 Introduction
Furthermore, studies have shown that SA regulates the astaxanthin synthesis of _H. pluvialis_ by scavenging free radicals [27]. Under stressed conditions, a large amount of triacylglycerglycerol (TAG) is also accumulated in _H. pluvialis_[28]. Astaxanthin mostly exists in the form of esterification, while an astaxanthin monoester joins an astaxanthin diester to form liposomes in the cytoplasm [29]. Moreover, under stresses, most of the de novo fatty acids are assembled into TAGs, which are esterified into cytosolic liposomes, along with astaxanthin esters through an unknown mechanism [30]. Therefore, it is important and beneficial to further explore the synthesis of fatty acids in order to identify novel experimental strategies to improve the accumulation of astaxanthin in microalgae. As a key biological technology, RNA-seq is an important tool which is applied to investigate differential gene expression in various model organisms. For example, RNA-seq technology provides de novo gene sequence and annotation analyses [31]. With the complete genome of _H. pluvialis_ recently sequenced [32], the molecular mechanism regulating the accumulation of astaxanthin in _H. pluvialis_ could be further elucidated based on RNA-seq analysis. Previous studies based on transcriptome sequencing have investigated the molecular metabolism of _H. pluvialis_ in different environments, e.g., strong light [33], different light qualities [10], salt ions [3; 34], CO\({}_{2}\)[35], and trisodium citrate [36]. Furthermore, studies showed that the accumulation of astaxanthin in _H. pluvialis_ was affected by pyruvate metabolism [37]. To date, transcriptome sequencing has become an essential method for studying the molecular metabolism of _H. pluvialis_. Furthermore,proteomic analysis has been commonly used to characterize the functions of various types of proteins in the development of plants and microalgae. Due to the large amount of protein sequence information available and the application of mass spectrometry in sensitive rapid protein identification, proteomic sequencing provides a novel approach for the analysis of complex functions in plants and microalgae. In this study, we first investigated the response of _H. pluvialis_, also known as _H. lacustris_[38], to blue light, and further explored the synergistic effects of both blue light and SA stresses on the molecular response of _H. pluvialis_. We then identified and analyzed the differentially expressed genes (DEGs) of _H. pluvialis_ under blue and white lights to explore the response and adaptation of _H. pluvialis_ to the change of light wavelength. RNA-seq analyses were performed based on samples treated with white light (day 0) and blue light with or without the addition of SA (day 2 and day 7). Our results indicated that combined analysis of transcriptome and proteome sequencing was efficient in clarifying the metabolic mechanisms of astaxanthin production by _H. pluvialis_. Our study clearly demonstrated that the combined analysis of RNA-seq and proteomic sequencing comprehensively revealed the variations in the molecular mechanisms of _H. pluvialis_ in response to different wavelengths of light and SA, further providing essential insights into the metabolic regulation involved in the synthesis of astaxanthin at molecular level. We noted that our study was unique, based on the following: firstly, we applied the combination of both the transcriptomic and proteomic analyses in our investigation of the molecular response of _H. pluvialis_ to both blue light and SA. Furthermore, we applied a high-intensity blue light, whereas previous studies generally utilized either a low-intensity blue light or a white light of high intensity with SA to treat _H. pluvialis_, or different species, e.g., _Ulva prolifera_[23; 39; 40]. Moreover, we reported, for the first time, the differentially expressed genes (DEGs) and proteins (DEPs) involved in the heme pathway at the genomic level.