# Caroenoid Production from Microalgae: Biosynthesis, Salinity Responses and Novel Biotechnologies
## 2 Health-Promoting Carotenoids from Microalgae and Their Biofunctions
### Health-Promoting Carotenoids and Their Production from Microalgae
#### 2.2.4 Zeaxanthin  
Zeaxanthin is a xanthophyll pigment existing in human eyes and skin. Together with lutein, zeaxanthin accumulates in the cornea as a macular pigment, protecting the retinal membrane from blue light and improving visual acuity [24]. Zeaxanthin also has potentials as an antioxidant, anti-inflammatory and for preventing neurological disease [20]. Maize, orange peppers and cooked scallions are good choices for dietary intake, while their contents are relatively as low at 16.3, 16.7 and 24.9 \(\mu\)g/g, respectively [13; 22].  
Currently, the marigold flower is applied for commercial production of zeaxanthin. Zeaxanthin content in different species of marigold petals ranges from 10 to 300 \(\mu\)g/g [61]. As the global number of people suffering from AMD [13] increases, the demand for natural sources with higher productivity of zeaxanthin is surging. The marine green alga _Chlorella ellipsoidea_ produces zeaxanthin (4.26 mg/g DW) nine-fold over that produced by red pepper [44]. Additionally, microalgae-derived zeaxanthin partly exists in free form, rather than as mono-/di- esters in flowers and fruits of plants. Thus, microalgae are more valuable sources for processing zeaxanthin with higher productivity and bioavailability.  
The production of zeaxanthin from microalgae is attractive and varies depending on microalgae species and cultivation factors. The favorable light condition for zeaxanthin varies in microalgae species. The zeaxanthin content of _C. zofingiensis bkt1_ (5.69 \(\pm\) 0.45 mg/g DW) was strongly enhanced by high light as compared with that of low light control (0.34 \(\pm\) 0.04 mg/g DW) [33]. Under high light, combined with N-starvation, C. _zofingiensis bkt1_ can accumulate 7.00 \(\pm\) 0.82 mg/g DW. However, high light is not always preferred for microalgae to accumulate zeaxanthin. A mutant _D. salinaze1_ can accumulate 5.9 mg/g DW of zeaxanthin under continuous low light (100 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\)), higher than that transferred to high light (4.18 mg/g DW) [43]. As _D. salina_ grows well in hypersaline medium, cultivation of this strain with seawater is potentially profitable. Results of satisfactory zeaxanthin production from microalgae are summarized in Table 2 with details of contents and productivities.  
Additionally, downstream processing of zeaxanthin needs more attention, such as extraction and separation. Solvent systems have different efficiencies in zeaxanthin extraction from microalgae, and the maximum extraction of zeaxanthin (30.2 mg/g DW) was in chloroform: methanol (1:2) [41]. Pressurized liquid extraction was also applied to extract zeaxanthin from _C. ellipsoidea_[41]. More importantly, zeaxanthin shares similar chemical structures to lutein. [PERSON] et al. developed an efficient method for the separation of zeaxanthin and lutein by ultra-high performance liquid chromatography (UHPLC) equipped with a Waters YMC Carotenoid C30 column [33].