## 1 Introduction  
The genus _Dunaliella_ is composed of 24 unicellular species, uninucleate [1], enclosed by a glycocalyx [2] and without a rigid polysaccharide wall, allowing a rapid response to hypo or hyperosmotic conditions [3]. The halotolerant _Dunaliella salina_, the holotype of this genus, is characterized by an exceptional capacity to survive under some environmental stress by producing added value compounds as glycerol, polyunsaturated fatty acids, and a high percentage of \(\beta\)-carotene reaching 10% to 14% of dry weight [2]. The majority of carotenoids are C\({}_{40}\) isoprenoids with color ranging from yellow to red and are divided into carotenes and xanthophylls [4]. The \(\beta\)-carotene molecule is an orange compound, comprising a long conjugated chain of eight isoprene units with a center of symmetry and a \(\beta\)-cyclohexene ring at each end [5]. It can be found in four possible stereoisomers: _9-cis_\(\beta\)-carotene,all-trans_\(\beta\)-carotene, _13-cis_\(\beta\)-carotene, and _15-cis_\(\beta\)-carotene. The amount of _9-cis_ isoform could reach 60% of the total carotenes in _Dunaliella_ cells ([PERSON], 2017). Carotenoids are extremely beneficial for plants and human health. In photosynthetic organisms, this polyene compound is part of the light-harvesting antenna structure, Photosystem I (PS I) and Photosystem II (PS II), and cytochrome _b6f_ complex and maintains the integrity of cell architecture ([PERSON], 2017). It also protects the photosynthetic apparatus from an injury occurring during photo-inhibition, photo-oxidation ([PERSON], 2017), and high temperature by thermal dissipation ([PERSON], 2017). They have a wide range of applications, mainly thanks to their antioxidative properties. In the medical field, they are claimed to prevent or cure many pathologies ([PERSON], 2017; [PERSON], 2018) as cataract, cardiovascular diseases, and some cancers ([PERSON], 2018; [PERSON], 2018; [PERSON], 2018). In addition, their impressive color allows their exploitation in many food and cosmetic industrial applications as dietary supplements or colorants ([PERSON], 2018). _D. salina_ produces primarily the \(\beta\)-carotene that is the precursor of vitamin A and other carotenoids like zeaxanthin, cryptoxanthin, neoxanthin, ([PERSON], 2018), phytoene, phytofluene, and lutein. The carotenogenesis pathway in _Dunaliella_ could be divided into three steps ([PERSON], 2017), which are the geranylgeranyl pyrophosphate (GGPP) biosynthesis, lycopene biosynthesis, and the generation of different carotenoids with cyclohexane rings. Lycopene \(\beta\)-cyclase introduces two \(\beta\)-cyclohex rings in each extremity of lycopene to generate \(\beta\)-carotene ([PERSON], 2017). This key enzyme, responsible for the biosynthesis of \(\beta\)-carotene, seems to have the same common ancestor in plants, algae, and cyanobacteria. It exists under two types, epsilon and beta, resulting from an ancient gene duplication event or gene transfer proceeded by endosymbiosis ([PERSON], 2017). _D. salina_ possesses one copy of the _LCYB_ gene in the beta form with 11 exons and 10 introns ([PERSON], 2018), while epsilon form is supposed to be lost in evolution ([PERSON], 2018). The functioning of this enzyme can be inhibited by many factors, such as nicotine ([PERSON], 2018). _D. salina_ LCYB (lycopene \(\beta\)-cyclase) was identified by [PERSON] et al. ([PERSON], 2018) and [PERSON] et al. ([PERSON] et al., 2018). LCYB isolated by [PERSON] et al. is a 584 amino acid protein of 64.5 kDa (ACA34344.1). The findings of [PERSON] et al. ([PERSON] et al., 2018) suggested that LCYB might be found as a multi-enzyme complex with another lycopene desaturase. This biogenesis pathway of \(\beta\)-carotene is supposed to be regulated both at transcriptional and posttranscriptional levels ([PERSON] et al., 2018) as it is influenced by many conditions. Many researchers reviewed and tried to pose the possible mechanism involved in its accumulation.