## 4 Discussion
The same profile was found by [PERSON] et al., where under hypersaline stress (3 M), the expression level of ENO protein and its activity decreased after 3 h of stress among _D. salina_ and re-increased at 6 h [62]. The authors explained that the down-regulation of _ENO_ inhibited the transformation of 2-PGA to pyruvate, which decreased the amount of available carbon for the tricarboxylic acid cycle and the formation of PEP. _Dunaliella_ is indeed capable of surviving under hyperosmotic stress, thanks to the rapid accumulation of osmoprotectants like glycerol. In this condition, starch reserved in the chloroplast will be degraded into glucose to be converted to glycerol [2]. As a result, 2-PGA and then DHAP (dihydroxyacetone phosphate) will be accumulated in the middle of the glycolysis pathway. The DHAP is then transformed via GPDH (glycerol-3-phosphate dehydrogenase) and GPP (glycerol-3-phosphate phosphatase) to glycerol. The restabilization of _ENO_ gene expression after 72 h allowed a sufficient supply of carbon to PEP synthesis and, subsequently, of the pyruvate, essential for the synthesis of isoprenoids. This hypothesis was in accordance with that of [PERSON] et al. [2], supposing that response to salt stress goes through two stages where, in the first step, it accumulates glycerol and glycine betaine, and in a later time, it produces neutral lipid and carotenoid. The pyruvate and G3P (glyceraldehyde-3-phosphate) will be converted to DXP (deoxyxylulose 5-phosphate) by DXS, catalyzing the first reaction in MEP (methylerythmitol 4-phosphate) pathway [1]. Therefore, it is implicated in the synthesis of isoprenoids, such as carotenoids, chlorophylls, volatiles [64], and squalene [65]. The regulation of the MEP pathway required more clarification, resulting in the disagreement for the regulation of _DXS_. [PERSON] et al. [21] found that the transcript level of _DXS_ increased under nitrogen deprivation to provide more substrate to caroten synthesis. While [PERSON] et al. [56] reported that the mRNA level of _DXS_ decreased under nitrogen limitation on account of growth decrease, low photosynthetic activity, and chloroplast development. Our results showed a _DXS_ increase at 24 h, but it trended to stabilize after 72 h with lower expression level than non-stressed cells. The discrepancy in result between publications suggested the possibility of the existence of more than one isoform of _DXS_ having different expression profiles and functions like among the microalgae, _Botryococcus braunii_, and plants (tomato, Arabidopsis, rice, maize, citrus fruit) [64; 66]. Besides, the expression of _DXS_ is subject to translational, posttranslational, and posttranscriptional modulations [67; 68]. In fact, it was demonstrated that _DXS_ expression was regulated by feedback exercised by isopentenyl diphosphate (IDP) and Dimethylallyl diphosphate (DMADP) [67; 69]. It is important to recall the assumption that _D. salina_ lost the mevalonate (MVA) pathway and uses only the MEP pathway to provide isoprenoids precursors [2]. Also, [PERSON] and his collaborators supposed that carotenogenesis, dependent on the regulation of enzymes, is located in the final step of the MEP pathway [56]. This supposition was confirmed by the significant rise at 72 h of the expression level of _LCYB_, which is involved in the cyclization of linear lycopene to \(\beta\)-carotene. Thus, the accumulation of this isoprene is directly related to the activity of this enzyme and the molecular regulation and control of its gene [20]. Our result was in accordance with those of [PERSON] et al. [18]. In _D. bardawil_, _LCYB_ was regulated in salt stress, thanks to two salt regulated elements (SRE), having GT1 GMSCAM4 sequence in the promoter and a GT-rich region in its first intron [70]. The promotor sequence of _LCYB_ also contains a light-inducible element (GATABOX, CGCGBOXAT, SORLIP1 AT) and W-boxes, a binding domain for the WRKY transcriptional factor [71].