## 2 Results
### DEGs and DEPs Associated with the Lipid Metabolic Pathway  
When _H. pluvialis_ is cultured under stressed environments, astaxanthin accumulation is usually accompanied by the synthesis of a large amount of fatty acids. TAGs are usually synthesized de novo from fatty acids and combined with astaxanthin to form liposomes [29]. As the precursor of fatty acid synthesis, NADPH is derived from the pentose phosphate pathway (PPP) with the involvements of both glucose-6-phosphate dehydrogenase (G6 PD) and 6-phosphogluconate dehydrogenase (6 PGD) [42]. Our results showed that two and three unigenes were identified encoding 6 PGD and G6 PD, respectively (Figure 4A). Transcriptome analysis showed that on the second day of the treatment with blue light irradiation, the expression of _6 pgd_ was up-regulated, while the two transcripts of _g6 pd_ showed opposite expression patterns with one significantly up-regulated and the other significantly down-regulated. On day 7, the expressions of both 6 PGD and G6 PD were significantly up-regulated (Figure 4B). Compared with white light, no significant changes of both 6 PGD and G6 PD were observed under blue light irradiation on day 7. The addition of SA caused the down-regulation of the expression of both _6 pgd_ and _g6 pd_ on day 7. However, no significant changes caused by SA were observed at the protein level (Figure 4B). Figure 2: Expression profiles of the seven genes (i.e., _dxr_, _dxs_, _ispD_, _ispE_, _ispF_, _ispG_, and _ispH_) involved in MEP pathway based on transcriptomic analysis of pairwise comparisons in _Haematococcus pluvialis_. *—indicates significant down-regulation (_p_ < 0.05 and Log2 FC \(\leq-\)1). In _H. pluvialis_, the fat acid synthesis is initiated with acetoyl-COA, which is catalyzed by acetyl-CoA carboxylase (ACC1) to synthesize malonyl-COA. The malonyl-COA reacts with acyl carrier protein (ACP) catalyzed by malonyl-CoA:ACP transacylase (FabD) to produce malonyl-[ACP] (Figure 5). After multiple reactions catalyzed by enzymes of beta-ketoacyl-[acyl-carrier-protein] synthase II (FabF), 3-oxoacyl-[acyl-carrier-protein] reductase (MabA), 3-hydroxyacyl-[acyl-carrier-protein] dehydratase (FabZ), and FabI, the C18:0-[ACP] is finally formed. Then, C18:0-[ACP] is catalyzed by fatty-acid synthase (FASN) to generate stearic acid [33]. Figure 3: Expressions profiles of genes and proteins involved in astaxanthin synthesis pathway revealed in the pairwise comparisons in _Haematococcus pluvialis_ based on transcriptomic and proteomic analyses. Compared with white light, on the second day of the treatment with blue light irradiation, five of the seven genes (i.e., _acc1_, _fabF_, _fabI_, _fabZ_, and _fasn_) related to fatty acid synthesis were significantly down-regulated, while the other two genes (i.e., _fabD_ and _mabA_) showed transcripts either significantly up-regulated or down-regulated. On day 7 of the treatment with blue light irradiation, both _fabD_ and _mabA_ were significantly up-regulated. No significant changes were revealed in the early stage (day 2) of the treatment with SA,  
Figure 4: Expression profiles of genes _6 pgd_ and _g6 pd_ (**A**) and proteins 6 PGD and G6 PD (**B**) involved in the NADPH metabolism revealed in the pairwise comparisons of _Haematococcus pluvialis_ based on transcriptomic and proteomic analyses, respectively. *—indicates significant up-regulation (_p_ < 0.05 and Log2 FC \(\geq\) 1; \(p\) < 0.05 and FC \(\geq\) 1.5) or significant down-regulation (_p_ < 0.05 and Log2 FC \(\leq\) \(-\)1). Figure 5: Fatty acid synthesis pathway in _Haematococcus pluvialis_. whereas both _fabD_ and _mabA_ were significantly down-regulated on day 7 (Figure 6). At the protein level, ACC1 and FabD were significantly down-regulated on the second day of the treatment with blue light irradiation, while only MabA was significantly up-regulated on day 7 (Figure 7). No significant changes were observed on these proteins with the addition of SA in _H. pluvialis_.